1.1. Introduction - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/6436/6/06...It involves the modification of chemical substituents groups, the physiochemical properties or prodrug

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THESIS GYAN VIHAR UNIVERSITY

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1.1. Introduction

The oral route is most preferred for chronic drug therapy in human beings. Probably at

least 90% of all drugs are administered by oral route. Oral delivery of 50% of the drug

compounds is facing problems because of the high lipophilicity (Low hydrophilicity) and low

permeability of the drug itself. The solubility issues complicating the delivery of new drugs, and

also affects the delivery of many existing drugs. Relative to highly soluble compounds, low drug

solubility often manifests itself in a host of in vivo consequences, including decreased

bioavailability, increased chance of food effect, more frequent incomplete release from the

dosage form and higher inter-patient variability. Poorly soluble compounds also present many in

vitro formulation obstacles, such as severely limited choices of delivery technologies and

increasingly complex dissolution testing with limited or poor correlation to the in vivo

absorption. Poor oral bioavailability has the consequence of more variability and poorly

controlled plasma concentration and drug effects to the patients. In recent years, much attention

has been focused on lipid based formulations for delivering, Biopharmaceutics Classification

System (BCS) class II and Biopharmaceutics Classification System (BCS) class IV drug

candidates which suffer limited oral bioavailability, high intra- and intersubject variability and

lack of dose proportionality (Gursoy and Benita, 2004). Thus, for such compounds, the

absorption rate from the gastrointestinal (GI) lumen is controlled by dissolution (Amidon et al.,

1995). The prospects in delivering poorly soluble drugs will grow in significance in the coming

years as innovator companies rely upon NCEs for a larger share of the revenue within the

pharmaceutical market. Poorly soluble compounds also present poor correlation for in-vitro

studies and the in-vivo absorption. These in vivo and in vitro characteristics and the difficulties

in achieving predictable and reproducible in vivo / in vitro correlations are often sufficiently

formidable to halt development on many newly synthesized compounds due to solubility issues.

Of equal challenges which the development candidates; despite possessing ideal pharmacological

characteristics are to be left aside due to insufficient oral bioavailability. Poor oral bioavailability

has the consequence of more variable and poorly controlled plasma concentration and drug

effects.

Transposition of a drug from an oral dosage form into the blood circulation can be described in

four-steps:

i. Disintegration of the drug product (if the drug product is solid).

ii. Dissolution of the drug in the fluids at the absorption site.



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iii. Movement of the dissolved drug through the membranes of the GI Tract.

iv. Movement of the drug away from the site of absorption into the blood circulation.

1.2. Biopharmaceutical Classification System

For the effective formulation and to sort out the drug related problem, one should have to

be very familiar with the Biopharmaceutical Classification System (BCS). The introduction of

the Biopharmaceutical Classification System (BCS) in FDA guidelines represents a major step

forward in the regulation of oral drug products. The guidelines tried to classify drug substances

into four categories according to their solubility and permeability properties given in Table 1.1.

Table 1.1: Biopharmaceutical Classification System of Drugs

Class Solubility Permeability

I High High

II Low High

III High Low

IV Low Low

Knowledge of BCS help the formulation scientists to develop a dosage form based on

mechanism, not on the empirical, approaches. The BCS groups poorly soluble compounds as

Class II and IV drugs, feature poor solubility and high permeability, and poor solubility and poor

permeability, respectively. According to BCS, drug substances are considered highly soluble

when the largest dose of a compound is soluble in <250 mL water over a range of pH from 1.0 to

7.5; highly permeable compounds are classified as those compounds that demonstrate >90

percent absorption of the administered dose (Aungst, B.J., 1993; Raimar and Gordon, 2000).

In contrast, compounds with solubility below 0.1mg/mL face significant solubilisation obstacles,

and even compounds with solubility below 10mg/mL present difficulties related to solubilisation

during formulation. Class I drugs do not pose any problem in absorption (though its systemic

availability may be low due to first pass metabolism) when solubility or permeability are

considered, therefore efforts are made to change the properties of Class II, III, IV drugs with

respect to dissolution and permeability in order to resemble Class I. Figure 1.1, shows the

possibilities of shifting the solubility-dissolution characteristics of a Class II, III and IV drug to

resemble Class I characteristics.



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Figure 1.1: Possibilities of shifting the Solubility-Dissolution Characteristics of BCS class-II, III and IV

drug nature to the BCS class-I drug nature

1.3. Bioavailability Enhancement

Poor bioavailability by the per-oral route can be due to poor solubility, degradation in GI

lumen, poor membrane permeation, premucosal clearance and presystemic elimination. Any of

the approaches, which can alter these characteristics, should help in improving the bioavailability

of the drugs. Generally, four major approaches are followed for overcoming the poor

bioavailability

The Formulations approach

It involves the modification of the formulation, manufacturing process or the physiochemical

properties of the drug without changing the chemical structure. The dissolution rate, solubility

and/or permeability are generally modified by this method.

The Chemical approach

It involves the modification of chemical substituents groups, the physiochemical properties or

prodrug approaches, which leads to increase drug solubility and permeability.



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The Pharmacokinetic approach

It involves the modification of chemical structure, drug combinations, dose and regiments to

alter the pharmacokinetic behaviour of the drug.

The Biologic approach

It involves the change in the route and rate of administration of the drug to the human tissue. This

can be achieved by many ways, one of them is the simultaneous administration of the two or

more drug which alter the human physiology by many ways, like Basic Metabolic Rate (BMR)

changing, enzymatic induction or inhibition etc which ultimately change the pharmacokinetics of

the desired administered drug.

Modification of the physicochemical properties, such as salt formation and particle size

reduction of the compound may be one approach to improve the dissolution rate of the drug.

However, these methods have its own limitations. For instance, salt formation of neutral

compounds is not feasible and the synthesis of weak acid and weak base salts may not always be

practical (Serajuddin, A.T.M., 1999). Particle size reduction may not be desirable in situations

where handling difficulties and poor wettability are experienced for very fine powders

(Serajuddin, A.T.M., 1999). To overcome these drawbacks, various other formulation strategies

have been adopted including the use of cyclodextrins complexes, nanoparticles, solid

dispersions,solid lipid Nanoparticles and permeation enhancers etc (Robinson, J.R., 1996,

Aungst, B.J., 1993). Indeed, in some selected cases, these approaches have been successful.

Some of the approaches are shown in Figure 1.2.



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Figure 1.2: Formulation strategies to improve solubility and bioavailability of the drug substances

(Schwarz, J., 2003).

1.4. Formulations Strategies Based On Lipids

In recent years, much attention has focused on lipid-based formulations to improve the

oral bioavailability of poorly water soluble drug compounds. In fact, the most popular

approaches is the incorporation of the active lipophillic component into inert lipid vehicles such

as oils, surfactant dispersions, nanoemulsions, microemulsions, self-emulsifying formulations,

self nano or microemulsifying formulations, emulsions and liposomes (Serajuddin et al., 1988;

Myers et al., 1992; Stella et al., 1978; Palin et al., 1986; Wakerly et al., 1986; Craig et al.,

1993; Aungst et al., 1994; Shah et al., 1994; Schwendener et al., 1996; Burcham et al., 1997;

Kang et al., 2004; Charman et al., 1992). Most of them increase surface area of the drugs to

improve solubilisation behaviour, as well as permeation. From the viewpoint of oral drug

delivery, lipids are studied as components of various oily liquids and dispersions that are

designed to increase solubility and bioavailability of drugs belonging to the class II, III and IV of

the biopharmaceutical drug classification system (Amidon et al., 1995). There are various

reasons for increasing interest in lipid-based systems (Stuchlík and Zak., 2001) viz:

Formulations strategies to

improve the solubility and

bioavailability

Natural and synthetic

surfactants

Lipid based

formulation

Mixed micelles

Co-solvents

Nano- and Micro-

suspensions

Solid dispersions

Complexation

techniques

Nano- and

Microemulsion



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– an improved understanding of the manner in which lipids enhance oral bioavailability

and reduce plasma profile variability,

– better characterization of lipidic excipients,

– formulation versatility and the choice of different drug delivery systems.

Lipids are ubiquitously distributed compounds that play fundamental roles in the

architecture and functionality of all living cells and therefore, are most commonly studied as

components of foodstuff and important energy source in the enteral nutrition. Lipids also serve as

a source of three families of unsaturated fatty acids represented by its parent member, α-linolenic,

linoleic and oleic acid. They have numerous interrelationships, which have considerable clinical

impact. Ingested food containing lipids can significantly alter postprandial drug absorption and

its bioavailability (Fleischer et al., 1999; Charman et al., 1997).

Oils administered as a drug vehicle may, for instance, exacerbate changes in hepatic

cytochrome P 450 mediated xenobiotic metabolism that are often reported following ingestion

of dosage form. Vegetable and animal oils may have differential effects on specific hepatic

CYP isoforms and may add to the variability in metabolism when xenobiotics are administered

in lipid based vehicle (Brunner et al., 2000).

Self-emulsifying system (SES) is one of the oily based most popular and commercially

viable approaches for the delivery of such solubility problem drugs that exhibit dissolution-rate-

limited absorption. SES is ideally an isotropic mixture of oils and surfactants and sometimes

cosolvents, which emulsifies spontaneously to produce fine oil-in-water (water-in-oil) emulsion

when introduced into aqueous phase under gentle agitation. Upon peroral administration, these

systems form fine (micro or nano) emulsions in the gastrointestinal tract (GIT) with mild

agitation provided by gastric mobility (Charman et al., 1992. Shah et al., 1994, Pouton, C.W.,

1997). Conventionally, SES is contained in hard or soft gelatin capsules for ease of

administration. However, certain problems such as leaking, leaching of components from the

capsule shell, and interaction of SES with capsule shell components are quite common. Sorption

and permeation by the capsule shells has also been observed for such liquid-filled capsules. To

sort out above listed problems, now a days SE system is being formulated and dispense in the

form of solid and semi solid dosage forms. Before starting with the solid and semi-solid self-

nanoemulsifying drug delivery system, it is worthwhile to set familiar with these including

microemulsion, nanoemulsion, self-nanoemulsifying mixture and phase behaviors etc.



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1.5. Microemulsion

One of the promising technologies is lipid based nanoemulsion or microemulsion drug

delivery system, which is being applied to enhance the oral bioavailability of the poorly soluble

drugs. It was Hoar and Schulman who generated a clear single-phase solution by titrating a milky

emulsion with medium chain alcohol such as hexanol andtherefore introduced the microemulsion

concept as early as 1940s. Schulman and coworkers subsequently in 1959 coined the term

microemulsion as the droplet sizes (100-600 nm) were much smaller than those of milky ordinary

emulsions (Schulman et al., 1959) and has then been defined and redefined on many occasions.

The term microemulsion implies a close relationship to ordinary emulsions (macroemulsions).

This is misleading because the microemulsion state embraces a number of different

microstructures, most of which has little in common with classical two-phase emulsions.

Microemulsions are readily distinguished from normal emulsions by their transparency, low

viscosity, and more fundamentally by their thermodynamic stability. In contrast, the preparation

of two phase emulsions usually require a considerable input of energy, both thermal and

mechanical, and on storage such dispersions attempt to revert back to separate oil and water

phases via the distinct processes of flocculation, creaming, coalescence and Oswald ripening.

Although, systems containing low concentrations of disperse phase (less than 25%) may like

emulsions, be composed of droplets of either oil dispersed in water (o/w) or water dispersed in

oil (w/o), they are essentially stable, single phase swollen micellar solutions rather than unstable

two phase dispersions. The confusion over the years has meant that the terminology adopted is

often related to the specific area of interest. For example in biotechnology it is called as

―nonaqueous media or reverse micelles‖ rather than microemulsion (Eccleston, J., 1994) and

sometimes are referred as swollen micellar solutions, miceller solutions, miniemulsions,

submicron emulsions etc. Danielsson and Lindman described microemulsions as optically

isotropic and thermodynamically stable liquid solutions of oil, water and amphiphile (Danielsson

and Lindman, 1981). Although some authors still define microemulsions in the wider sense to

include both thermodynamically, stable systems and kinetically stable emulsions of fine droplet

size. Nanotechnology is a new concept developed recently according to which systems associated

with physical dimensions of less than 100 nm can be called as nano delivery system (Lee, V.H.,

2004) whereas those with more than 100 to 1000 nm can be called as micro particulate delivery

system. The word microemulsion is a misnomer and should not be used for the system where

droplet size is less than 100 nm. As suggested by Shinoda and Kunieda, a dispersed system

containing microdroplets, which is not thermodynamically stable, may be called a



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microemulsion. The system which is thermodynamically stable and the droplet size is between

10-100 nm may be called as swollen micellar solution (Shinoda and Kunieda, 1973). Lawrence

and Rees in their exhaustive review pointed out that distinguishing between swollen micelles

and microemulsion droplet is largely a semantic exercise, but it is recognized that as the ratio of

dispersed phase to surfactant increases, the physiochemical properties approach those of the pure

solvent (Lawrence and Rees, 2000).

1.6. Nanoemulsion

The nanoemulsions can thus be defined as thermodynamically stable, transparent (or

translucent) dispersions of oil and water stabilized by an interfacial film of surfactant molecules

having the droplet size less than 100 nm. The observed transparency of these systems is due the

fact that the maximum size of nanoemulsion droplets is less than the one-fourth of the

wavelength of visible light (approximately 150 nm). Droplet size in thermodynamically stable

nanoemulsions is usually 10-100 nm (Sinto and Shaprio, 2004). Bouchemal et al., 2004, has

defined nanoemulsions as kinetically stable system but approaching thermodynamic stability due

to its long term physical stability (with no flocculation or coalescence) covering the size range

between 100 - 600 nm. The homogeneous systems that can be prepared over a wide range of

surfactant concentrations and oil to water ratios (20-80%) are all fluids of low viscosity.

Nanoemulsion provides ultra low interfacial tensions and large o/w interfacial areas.

Nanoemulsions being colloidal nanodispersions of oil in water (or water in oil),

thermodynamically stabilized by an interfacial film of surfactant(s) and co-surfactant(s) have

revealed tremendous potential in nanoengineering of various inorganic materials (Date and

Patravale, 2004). Nanoemulsions have a higher solubilization capacity than simple micellar

solutions and their thermodynamic stability offers advantages over unstable dispersions, such as

emulsions and suspensions, because they can be manufactured with little energy input (Low

energy emulsification techniques/heat or mixing) and has a long shelf life. The nanosized

droplets leading to enormous interfacial areas associated with nanoemulsions would influence the

transport properties of the drug, an important factor in sustained and targeted drug delivery

(Lawrence and Rees, 2000; Eccleston, J., 1994). The attraction of o/w nanoemulsion systems

lies in their ability to incorporate hydrophobic drugs into the oil phase thereby enhancing their

solubility (Lawrence and Rees, 2000). Nanoemulsions have been reported to make the plasma

concentration profiles and bioavailability of drugs more reproducible (Kommuru et al., 2001;

Kawakami et al., 2002a,b; Constantinides, P.P., 1995; Lawrence and Rees, 2000).



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1.7. Self -nanoemulsifying drug delivery system (SNEDDS)

SNEDDS is defined as isotropic mixtures of oil and surfactant and cosurfactants that form

o/w nanoemulsion upon mild agitation followed by dilution in aqueous media, such as GI tract.

Self-microemulsifying formulations spread readily in the GI tract, and the digestive motility of

the stomach and the intestine provide the agitation necessary for self-emulsification (Charman

et al., 1992; Shah et al., 1994; Constantinides et al., 1995; Wakerly et al., 1986; Craig et al.,

1993). When SNEDDS are administered orally, the droplet size might be smaller than those in

vitro, because bile salts would be incorporated into the surfactant layers of the emulsion droplets.

Thus SNEDDS are related to nanoemulsions except that they are concentrate of oil and

surfactant, and form nanoemulsion when diluted by GI fluids in vivo. Thus, for lipophillic drug

compounds that exhibit dissolution rate limited absorption, these systems may offer an

improvement in the rate and extent of absorption.

The most notable example of a SNEDDS relates to the oral delivery of cyclosporin A, in

particular the commercial Neoral® formulation of Cyclosporin A. The original Sandimmune

formulation was based on a solution of cyclosporin A in vegetable oil.

In a study carried out on perfused rats, the administration of cyclosporine A in positively

charged SEDDS led to higher blood levels as compared to the corresponding negatively charged

formulation (Gershanik et al., 2000). This is because positively charged oil droplets interact

with the negatively charged surface components of the GI lumen. It is interesting to note that

larger droplets in sizes of a few microns are less neutralized by mucin than smaller droplets in

submicron sizes formed by the same formulation (Gershanik et al., 1998), that gives a a clear

advantages of nanosizing of oils droplets.

It has also been shown that the amount of free drug and extent of absorption were affected by

micellar solubilization of lipophilic drugs with high concentrations of surfactants in the

formulation. In addition, in vitro permeability studies conducted utilizing the Caco-2 cell line

demonstrated a decrease in permeability of cyclosporine A in the presence of surfactants, such as

Cremophor EL or RH40 and TPGS at concentrations above 0.02% w/v, which was attributed to

micellar solubilization (Chiu et al., 2003).

So self-nanoemulsifying mixture is some what advanced over the nanoemulsions drug

delivery system, because in former case water is not used, so the drug leaching from the nano-

sized oily droplets to the aqueous medium of the formulation during storage is minimized. It was



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also observed that self-nanoemulsifying mixture upon exposure to the GI-fluid gives smaller

nanoemulsion as compared to the in-vitro nanoemulsion because of the bile salt, which itself acts

as a surfactant. Basically self-emulsifying mixtures are broadly classified according to their sizes

as:

1. Self-emulsifying drug delivery system (SEDDS): Droplets of emulsion size is more than 600

nm.

2. Self-microemulsifying drug delivery system (SMEDDS): Droplets of micron sizes i.e. lies

between 100-150 nm.

3. Self-nanoemulsifying drug delivery system (SNEDDS): Droplets of nanosized i.e. lies

between 10-100 nm.

1.8. Theories of Nanoemulsification

Many approaches have been used to explore the mechanisms of nanoemulsion formation and its

stability. Some emphasize the formation of an interfacial film and the production of ultra low

interfacial tension (mixed film theories); others emphasize the monophasic nature of many

nanoemulsions (solubilization theories). Thermodynamic theories consider the free energy of

formation of the nanoemulsions and the bending elasticity of the film. The different approaches

have been reviewed in the literature and are briefly summarized here (Eccleston, J., 1994) but no

one approach alone covers all aspects of nanoemulsion structure and stability and all have a place

in the overall understanding of nano or microemulsions.

1.8.1 Mixed film Theories

The early scientific treatment of nanoemulsions was developed by Schulman‘s school and

emphasized the importance of the interfacial film and ultralow interfacial tension (Prince, L.M.,

1967; Schulman et al., 1959). The spontaneous formation of nanoemulsion droplets was

considered due to the formation of a complex film at the oil water interface by the surfactant and

co surfactant. This caused a reduction in the oil-water interfacial tension to a very low value

(from close to zero to negative).

The mixed interfacial film in equilibrium with both oil and water was considered to be

liquid and duplex in nature (i.e., showing different properties at the oil and water sides) with a

two dimensional spreading pressure, πi, which determined the interfacial tension γi by equation



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Where γo/w represents the oil/water interfacial tension without the film present. When large

amounts of surfactant and cosurfactants are adsorbed to form the interface, the spreading

pressure, πi, may become larger than γo/w. A negative interfacial tension results and energy is

available to increase the interfacial area, effectively reducing droplet sizes. This negative

interfacial tension produced by the mixing of the components is a transient phenomenon, and at

equilibrium, it becomes zero or a very small positive value.

A major drawback to Schulman‘s concept was the high value of the spreading pressure,

necessary to give the transient negative interfacial tension. Prince (Prince, L.M., 1967) later

postulated that the negative interfacial tension could be a result of the depression of γo/w, rather

than the unrealistically high initial pressure in the original model. The alcohol co-surfactant

partitions between the oil phase and the interface, with the fraction in the oil phase able to

significantly depress the γo/w from its normal value of approximately 50 mN/m to a new value

(γo/w)a of around 15 mN/m.

The interfacial film must be curved to form small droplets, and the concept duplex film

was used to explain both the stability of the system and the bending of the interface. A flat

duplex film would be under stress because of the different tensions and spreading pressures on

either side of it. The reduction of this tension gradient by equalizing the two surface pressures

and tensions is the driving force for the film curvature. Both sides of the interface expand

spontaneously with penetration of oil and co-surfactant until the two pressures become equal.

The side with the higher tension would be concave and would envelope the liquid on that side,

making it the internal phase. The pressure gradients, and hence the type of nanoemulsion, are

influenced by the molecular structures of the oil, surfactant and cosurfactant and the

concentrations of each. Since it is generally easier to expand the oil side of an interface (by

penetration of the oil or cosurfactant into the hydrocarbon chain area) than the water side, it is

easier to form w/o rather than o/w nanoemulsion (Tadros, T.F., 1984). A short to medium chain

length cosurfactant ensures that the film is flexible enough to readily deform around the droplets.

1.8.2. Solubilization Theories

The group of Shinoda (Shinoda and Friberg, 1975;Shinoda and Kunieda, 1973) and

Friberg (Friberg and Burasczenska, 1978) considered nanoemulsions to be thermodynamically

γi = γo/w - πi



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SDSWater

Pentanol

P-xylene

stable monophasic solutions of water-swollen (w/o) or oil-swollen (o/w) spherical micelles. The

inverse miceller region of the ternary system water, pentanol, and sodium dodecyl sulphate

(SDS) is shown as the base triangle of Figure 1.3. The region is composed of water solubilized

in reverse micelles of SDS in pentanol. The addition of up to 50% p-xylene gives rise to

transparent w/o regions containing a maximum of 28% water with 16% pentanol and 6%

surfactant (i.e., nanoemulsion). The quaternary phase diagram constructed on addition of

hydrocarbon clearly shows the relationship of these areas to the isotropic inverse micellar phase.

These four-component systems could be prepared by adding hydrocarbon directly to the inverse

micellar phase or by the titration method of Schulmann and co-workers. Thus these systems were

identical to Schulman‘s microemulsions and since they were an extension of the inverse micellar

region rather than small emulsion droplets ( Shinoda and Kunieda, 1973).

Similar diagrams were presented to explain the relationship between o/w nanoemulsions

and the isotropic aqueous micellar region. The solubilization of oil in normal micelles is small

and the molecular characteristics and concentration of all the components are critical for an

aqueous micelle to solubilize large amounts of hydrocarbon and swell directly into an oil droplet

without forming a large number of intermediate structures of low curvature.

Figure. 1.3: Phase diagram illustrating that the w/o microemulsion region containing p-xylene is a direct

continuation of the inverse micellar solution of the three structure-forming elements in the base triangle,

water, surfactant, SDS and cosurfactant pentanol.

1.8.3. Thermodynamic Treatments

The free energy of nanoemulsion formation can be considered to depend on the extent to

which surfactant lowers the surface tension of the oil-water interface and the change in entropy



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of the system (Overbeek, J.T.G., 1978; Ruckenstein and Krishnan, 1980; Ruckenstein and

Chi, 1978) such that:

Where ΔGf is the free energy of formation

γ is the surface tension of the oil water interface

ΔA is the change in interfacial area on nanoemulsification.

T is the temperature

ΔS is the change in the entropy of the system which is effectively the

dispersion entropy.

When a nanoemulsion is formed the change is in ΔA is very large due to the large number of

nanosized droplets formed. Originally, workers proposed that in order for a nanoemulsion to be

formed, a (transient) negative value of γ was required, but it is now recognized that while value

of γ is positive at all times, it is very small (of the order of fractions of mN/m), and is offset by

the entropic component. The dominant favourable entropic contribution is the very large

dispersion entropy arising from the mixing of one phase in the other in the form of large numbers

of small droplets. However, there are also expected to be favourable entropic contributions

arising from other dynamic processes such as surfactant diffusion in the interfacial layer and

monomer-micelle surfactant exchange. Thus, a negative free energy of formation is achieved

when large reduction in surface tension is accompanied by significant favourable entropic

change. In such cases, nanoemulsification is spontaneous and the resulting dispersion is

thermodynamically stable.

The role of the cosurfactant cannot be entirely reconciled to its effect on packing. A highly

flexible film is required to form small droplets (Overbeek, J.T.G., 1978). The bending of an

interface requires work against both interfacial tension and the bending stress of the interface.

The bending stress, which is particularly important for very low interfacial tensions and highly

curved interfaces, is represented by K, the rigidity (i.e., elastic) constant. The interplay between

bending and thermal energies play an important role in these systems, because thermal

fluctuations produce large undulations in surfactant layers when their elastic energy is

comparable to the thermal energy. This interplay is expressed in terms of persistence length,

which represents the average length of the straight part of the film. The persistence length

increases exponentially with K, in such a way that a small reduction of K would drastically

decrease the persistence length of the film toward a very curved phase. A large value of K

represents a rigid interface for which large energy is required to bend the interface, and a lamellar

ΔGf = γ ΔA – T ΔS



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birefringent phase often forms near the nanoemulsion region of the phase diagram. The rigidity

constant is lowered by a cosurfactant and can cause a transition from lamellar phases to isotropic

nanoemulsions phases. A small value of K represents a fluid interface for which little energy is

necessary for bending, and the interface can become extremely wrinkled to give bicontinuous

structures.

1.9. Phase Behaviour

The phase behavior of simple nanoemulsion systems comprising oil, water and surfactant can be

studied with the aid of ternary phase diagram in which each corner of the diagram represents

100% of that particular component. In the case where four or more components are investigated,

pseudo-ternary phase diagrams are used where a corner will typically represent a binary mixture

of two components such as surfactant/cosurfactant, water/drug or oil/drug. The number of

different phases present for a particular mixture can be visually assessed. It should be noted that

not every combination of components produce nanoemulsions over the whole range of possible

compositions, in some instances the extent of nanoemulsion formation may be very limited.

Constructing phase diagrams is time consuming, particularly when the aim is to

accurately delineate a phase boundary, as the time taken for the system to equilibrate can be

greatly increased as the phase boundary is approached. Heat and sonication are often used,

particularly with systems containing nonionic surfactants, to speed up the process. The procedure

most often employed is to prepare a series of (pseudo) binary compositions and titrate with the

third component, evaluating the mixture after each addition. Care must be taken to ensure not

only that the temperature is precisely and accurately controlled, but also that observations are not

made on metastable systems. Clearly, however, time constraints impose a physical limit on the

systems that can be left to equilibrate and consequently to eliminate the metastable states which

are difficult to ensure in practice, although centrifugation can be useful to speed up any

separation (Shafiq et al., 2007).

The relationship between the phase behaviour of a mixture and its composition can be

captured with the aid of a phase diagram. Compositional variables can also be studied as a

function of temperature and pressure, the large majority of systems are studied under conditions

of ambient pressure. The phase behaviour of simple nanoemulsion systems comprising oil, water

and surfactant can be studied with the aid of ternary phase diagram in which each corner of the

diagram represents 100% of that particular component. More commonly, however, and usually in



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case of nanoemulsions in pharmaceutical applications, the nanoemulsion will contain additional

components such as a cosurfactant and drug. The cosurfactant is also amphiphilic with an affinity

for both the oil and aqueous phases and partitions to an appreciable extent into the surfactant

interfacial monolayer present at the oil–water interface. The cosurfactant need not necessarily be

capable of forming association structures in its own right.

The number of different phases present for a particular mixture can be visually assessed.

A highly schematic (pseudo) ternary phase diagram illustrating these features is presented in

Figure 1.4. It should be noted that not every combination of components produce nanoemulsions

over the whole range of possible compositions, in some instances the extent of nanoemulsion

formation may be very limited.

At low surfactant concentration multiple phases may exist. Within this region and

other multiphase regions of the ternary phase diagram, nanoemulsion can exist in equilibrium

with the excess water or oil phase, these phases are referred to as Winsor phases (Ghosh and

Murthy, 2006; Eccleston, J., 1994; Lawrence and Rees, 2000). They are

1. Winsor I: With two phases, the lower (o/w) nanoemulsion phase in equilibrium

with the upper excess oil.

2. Winsor II: With two phases, the upper (w/o) nanoemulsion phase in equilibrium

with the lower excess water.

3. Winsor III: With three phases, middle Nanoemulsion phase (o/w plus w/o, called

bicontinuous) in equilibrium with upper excess oil and lower excess water.

4. Winsor IV: In single phase, with oil, water and surfactant homogeneously mixed.

In the Winsor classification, the one phase nanoemulsions that are generally explored as drug

delivery systems are known as Winsor IV systems.

Nanoemulsions stabilized by non-ionic surfactants, especially those based on polyoxyethylene,

are very susceptible to temperature because a decrease in surfactant solubility occurs with

increasing temperature as the polyoxyethylene group is dehydrated with increasing temperature

thus making it more lipophillic at higher temperature (Eccleston, J., 1994; Ghosh and Murthy,

2006; Lawrence and Rees, 2000). In contrast, nanoemulsions stabilized by ionic surfactants

have little or no sensitivity to temperature. The formation and stability of nanoemulsions

consisting of non-ionic components is not affected by the pH and or ionic strength of the aqueous

phase in the pH range between 3 and 10. This property can be beneficial for drugs and other



16

molecules exhibiting higher solubility and/or stability at low or high pH (Constantinides, P.P.,

1995). The commercially available lipid based formulation are listed in the Table 1.2.

1.10. Advantages of nanoemulsion as a delivery system

They are thermodynamically stable.

They act as supersolvents (acts as a solvents for both the hydrophobic and hydrophilic

drugs), improving the solubility and thermodynamic activity of the drug.

The small particle size of the microemulsion as well as both the hydrophilic and

lipophillic domains of microemulsion enhances the oral and percutaneous uptake of the

drugs.

They act as potential reservoir of the drugs through which pseudo-zero order kinetics can

be obtained.

The small size of the droplets give large interfacial area from which drug can be quickly

released, improving the oral absorption of poorly water soluble drugs.

Ease of preparation with no significant energy contribution.

They can improve the efficacy of a drug allowing the dose reduction and side effect

minimization.

This delivery system by-pass the hepatic degradation because of lymphatic absorption. As

we know that lymphatic systems provides a route for absorption of nutrients; it gathers

fats, excess fluid, body wastes and other materials, removing them from the cell spaces

and carries them to the blood for eventual elimination.

Absorption from this delivery system are independent of GI physiology like pH,ions and

enzymes.

They protect hydrolysis and oxidation of the drug when the drug is in oil phase (Vyas

and Khar, 2002).

They can be used for both kind of drugs like hydrophobic and hydrophilic

They can also be used for steroidal, proteinous and enzymatic delivery purposes.

As it is oil based drug delivery system it also acts as a P-glycoprotein efflux inhibitor.

1.11. Disadvantages of nanoemulsion as a delivery system

Limitation of the lipid excipients

Stability solely dependent on the lipid stability

For the formulation stability, one should have to select only synthetic oils



17

Delivery system needs large amount of surfactants, which ultimately leads to toxic effect

on the GI mucosa

It faces the dispensing problems

Table 1.2: Commercial lipid-based formulations exhibiting enhanced bioavailability

Product Formulati

on type

Strength Dose Bioavailability

enhancement

Company

Neoral®

(cyclosporine)

Liquid

filled soft

gelatin

capsule

25 and

100 mg

Up to 1g/day (10

capsules a day)

20-50%

compared to

Sandimmune®

Novartis

Pharmaceuticals

Norvir®

(ritonavir)

Semi-solid

filled hard

gelatin

capsules

100 mg 600 mg bid (12

capsules/ day)

Similar to an oral

solution

Abbott

Laboratories

Fortovase®

(Saquinavir)

Liquid

filled soft

gelatin

capsule

200 mg 1200 mg tid (18

capsules/ day)

AUC increase 3.3

fold compared to

Invirase®

Roche

Pharmaceuticals

Agenarase®

(amprenavir)

Liquid

filled soft

gelatin

capsule

150 mg 1200 mg bid (16

capsules / day)

Conventional oral

formulation gave

no detectable

blood levels

Glaxo Wellcome.

1.12. Classification of self-nanoemulsifying drug delivery system (SNEDDS)

The self-nanoemulsifying drug delivery system is classified into following types:

1.12.1. Liquid Self-nanoemulsifying drug delivery system

1.12.2. Semisolid self-nanoemulsifying drug delivery system

1.12.3. Solid self-nanoemulsifying drug delivery system (SSNEDDS)

1.12.1. Liquid Self-nanoemulsifying drug delivery system

SES is normally prepared as liquid dosage forms that can be administrated in soft gelatin

capsules or hard gelatin capsules for ease of administration. It is basically the mixture of

optimized ratio of surfactant, cosurfactant and the oils but not the water, obtained from the phase

diagram, which is prepared during the aqueous titration method. When this mixture goes into the



18

GI-tract, it get mixed with the water and is changed into nanoemulsion. Liquid Self-

nanoemulsifying drug delivery system is (SNEDDS) only used for the oral purpos of oral

delivery.

1.12.2. Semisolid self-nanoemulsifying drug delivery system

In this delivery system, the self-nanoemulsifying mixture gets solidified with the help of cooling

chambers or freeze solidified using different moulds, which upon ingestion by oral route, get

converted to nanoemulsion at the body temperature. For this purpose is commonly used Gellucire

as a main excipients.

Recently some worker (Patil et al., 2004) reported the gelled self-emulsifying drug delivery

system. Gelled self-emulsifying KPF (Ketoprofen) formulation consisted of diesters of

caprylic/capric acids (Captex 200), C8/C10 mono-/diglycerides (Capmul MCM),

polyoxyethylene 20 sorbitan monooleate (Tween 80), and colloidal silicon oxide (A 200).

Gelling agent was incorporated with the intention that gelled SES may require lesser excipients

to convert in solid dosage forms such as tablets and capsules and may retard the drug release as

well. After ingestion of such formulations, the gelling agents swell in GI fluid and form a gel like

structure. It eas also observed that the addition of colloidal silicon dioxide caused an increase in

the viscosity of the liquid crystal phase, which in turn increased the average droplet size of the

emulsion formed and slowed the drug release. Increasing the amount of cosurfactant was found

to increase the drug release.

1.12.3. Solid self-nanoemulsifying drug delivery system (SSNEDDS)

Conventionally, SES is normally prepared as liquid dosage forms that can be

administrated in soft gelatine capsules or hard gelatin capsules for ease of administration, which

have some disadvantages especially in the manufacturing process (in-process controls), with

consequent high production costs. Other than this, it also posses certain problems such as

leaking, leaching of components from the capsule shell, and interaction of SES with capsule shell

components are often observed for such liquid-filled capsules. Solidification of liquid systems

has been a challenge that has attracted wide attention due to handling difficulties and

machinability and stability problems that are often encountered with liquids. An alternative

method, which is currently investigated by several authors, is the incorporation of liquid self-

emulsifying ingredients (oil/surfactant/water mixture) into a powder in order to create a solid

dosage form (pellets, tablets, capsules). Various attempts have been reported in literature to



19

transform liquids into solids self-emulsifying systms. Many reports on producing Bliquisolids,

based on the concept of blending liquid systems with selected powder excipients to produce free

flowing, readily compressible powders have been documented in the literature (Liao and

Jarowski, 1984; Spireas et al., 1992 Spireas et al., 1998; Yang et al., 1979). Examples of such

solid systems are pellets produced by extrusion/spheronisation, which can finally be incorporated

into hard gelatin capsules (Newton et al., 2001) or the inclusion in micro porous or cross-linked

polymeric carriers (Chiellini et al., 2003). However, this process required cheap and inert

solidifying aids such as cellulose, lactose, and silicates etc. They can be formulated in different

forms like.

Tablets (Attama et al., 2003; Serajuddin, A.T., 1999; Breitenbach, J., 2002; Nazzal and

Mansoor, 2006; Patil et al., 2004)

Pellets (Abdalla and Mader, 2007; Taleu et al., 2004; Newton et al., 2001; Serajuddin,

A.T., 1999; Schwarz, J., 2003; Vojnovic et al., 1993; Newton et al., 2007; Serratoni et al.,

2007; Chambin and Jannin, 2005; Newton et al., 2005; Franceschinis et al., 2005)

Granules (Chambin and Jannin, 2005; Chambin et al., 2004; Breitenbach, J., 2002)

Microsphere loaded with self-nanoemulsifying system (You et al., 2006)

Porous polystyrene beads loaded self-nanoemulsifying system (Patil and Paradkar,

2006)

Nazzal et al, formulated eutectic-based solid self-nanoemulsifying drug delivery systems

(SSNEDDS) using interaction between ubiquinone and oils that formed wax-like paste, which

was further mixed with copolyvidone, maltodextrin, and microcrystalline cellulose to obtain

tablets. Solid SES comprising goat fat and Tween 65 were formulated for delivery of diclofenac

(Attama et al., 2003). Booth et al., 2003 formulated solid SES using an extrusion spheronization

technique, wherein lactose and microcrystalline cellulose were used as solidifying aids. Schwarz

reported transformation of SES in solid dosage forms by addition of large amounts of solidifying

excipients (adsorbents and polymers) (Schwarz, J., 2003). Recently, gelled SES containing

ketoprofen has been formulated with the view that such a gelled system may serve as an

intermediate for further transformation into semisolid or solid dosage forms (Patil et al., 2004).

In an attempt to transform SES into a solid form with minimum amounts of solidifying aids, so as

to avoid leaking and leaching problems of conventional liquid SES formulations, alternatively it

was hypothesized that using capillary forces SES can be loaded into the microchannels of

preformed porous polystyrene beads (PPB) typically produced by copolymerizing styrene and

divinyl benzene. Many more literature reports are published regarding the delivery of self-



20

emulsifying mixture in the form of pellets and, on the various parameters based upon it (Newton

et al., 2007; Abdalla and Madar, 2007; Newton et al., 2006; Serratoni et al., 2007;Chambin

and Jannin, 2005; Chambin et al., 2004; Newton et al., 2005; Franceschinis et al., 2005;

Tuleu et al., 2004; Breitenbach, J., 2002; Newton et al., 2001).

1.13. Rationale For Pelletization

In pharmaceutical application, an agglomeration process that results in agglomerates of a

rather wide size distribution within the range of 0.1 – 2.0 mm, with a high intra agglomerate

porosity (about 20–50 %) is named a granulation process, and the agglomerates are called

granules. If the final agglomerates are spherical, free flowing, and of a narrow size distribution in

the size range of 0.5 – 2.0 mm, and a low intra-agglomerate porosity (about 10 %), the process is

often referred to as pelletisation process, and the agglomerates are called pellets.

Pellets are of great interest to the pharmaceutical industry for a variety of reasons. Pelletized

products besides providing flexibility in dosage form design also improve the safety and efficacy

of bioactive agents. However, the single most important factor responsible for the wide use of

pelletized product is the popularity of conventional delivery system having large surface area and

uniform size, shape.

Pellets containing the active ingredients administered in vivo in the form of suspension, capsules

or disintegrating tablets, offer significant therapeutic advantages over single unit dosage form.

Because, pellets disperse freely in the gastrointestinal tract, they invariably maximize drug

absorption, reduce peak plasma fluctuations and minimize the potential side effects without

appreciably lowering drug bioavailability. Pellets also reduce variations in gastric emptying rate

and overall transit time. Thus, intra and inter subject variability of plasma profiles, which are

common with single unit regimens are minimized. Another advantage that pellets provide over

single unit dosage form is that high local concentrations of bioactive agents, which may

inherently be irritative or anesthetic, can be avoided. Pellets are less susceptible to dose dumping

than the reservoir type formulation.

Pellets provide the pharmaceutical scientist with tremendous flexibility during the

development of oral dosage form. For instance, pellets composed of different drug entities can be

blended and formulated in a single dosage form. Such an approach has numerous advantages. It

allows the combined delivery of two or more bioactive agents, which may or may not be

chemically compatible, at the same site or at the different sites with in the gastrointestinal tract. It



21

also permits the combination of pellets of different release rates of the same drug in a single

dosage form. Because pellets flow and pack freely, it is not difficult to obtain uniform and

reproducible fill weights in capsules, provided that the size and densities of pellets as favourable.

Due to various shades of colour that can easily be imparted to them during the manufacturing

process, attractiveness of pellets can also be increased (Ghevre-Sellassie., 1989).

1.13.1. Self-emulsifying Pellets

Pelletization is also referred to as size enlargement process and if the final agglomerates are

spherical, free flowing, and of a narrow size distribution in the size range of 0.5 – 2.0 mm

(Schaefer et al., 1992) and a low intra agglomerate porosity (about 10 %), the process is often

referred to as pelletization process, and the agglomerates are called pellets. The

extrusion/spheronization process is an accepted method of producing pellets. This process

consists of five unit operations—blending, wet massing, extrusion, spheronization and drying

resulting in the formation of spherical pellets showing a homogeneous surface (Otsuka et al.,

1994). The process of extrusion/spheronization to produce pharmaceutically acceptable pellets

containing 30–40% of a self-emulsifying system has been extended to include other types of

systems. For these to function, water must be present in the wet mass used in the process. It was

found that as the proportion of water present in the wet mass increases, the mechanical strength

and the disintegration time of the final pellets increase.

The self-emulsifying pellets are resembled to the normal pellets in texture and method of

preparation. In this drug delivery system the self-emulsifying mixture is loaded into the pellets in

many ways, which ultimately emulsify into the GI- tract. Recently a work has shown that it is

possible to prepare pellets containing a high proportion of a self-emulsifying system as a

potential method to convert a liquid into a solid dosage form (Schwarz, J., 2003[patent-

20030072798]; Booth et al., 2003;Patent-6,630,150).When a poorly water soluble drug was

formulated in such a system, the product was equally bioavailable compared to nanoemulsion

when administered to dogs ( Tuleu et al., 2004 ).

1.13.2. Desirable properties of pellets

Ideally the pellets should have:

Uniform spherical shape

Uniform size



22

Good flow properties

Reproducible packing (into hard gelatin capsules)

High strength

Low friability

Low dust

Smooth surface

1.13.3. Advantages of pelletization

Particle size enlargement by pelletisation is often desirable for several reasons:

Prevention of segregation of co-agglomerated components, resulting in an improvement

of the content uniformity.

It bypasses the disintegration steps of drug absorption

Provides great increase in the drug release profile due to increase surface area.

Prevention of dust formation resulting in an improvement of the process safety, as fine

powders can cause dust explosions and the inspiration of fines can cause health problems.

Increasing bulk density and decreasing bulk volume.

The defined shape and weight may improve the appearance of the product.

Improvement of the handling properties due to the free-flowing properties of pellets.

Controlled release application of pellets due to the ideal low surface area-to-volume ratio

that provides an ideal shape for the application of film coatings.

Lower risk of side effects due to dose dumping (Bechagaard and Hegermann, 1978)

Less irritation of gastro-intestinal tract (Bechagaard and Hegermann, 1978)

Pellets show better flow properties (Reynolds, A.D., 1970)

Pellets are less friable in nature (Reynolds, A.D., 1970)

Pellets show narrow particle size distribution, ease of coating and uniform packing

(Reynolds, A.D., 1970)

Pellets show reproducibility of the drug plasma profile (Bechagaard and Hegermann,

1978; Eskilson, C., 1985)

1.13.4. Disadvantages of pelletization

Typical disadvantages of pellets and pellet production are:



23

Often pellets cannot be pressed into tablets because they are too rigid. In that case, pellets

have to be encapsulated into capsules.

The production of pellets is often an expensive process and/or requires highly specialised

equipment.

The control of the production process is difficult e.g. the amount of water to be added is

very critical and over wetting occurs easily.

1.14. Pharmaceutical Pelletization Technology

The most widely used Pelletization process as in the pharmaceutical industry are extrusion

spheronization, solution / suspension layering, and powder layering. Other processes with limited

application in the development of pharmaceutical pelletized product include globulation, balling

and compression as shown in the flow chart (Figure 1.5) (Ghevre-Sellassie., 1989).

Figure. 1.5: Flow Chart of Classification for Pelletization processes.

Balling describes a pelletization process in which finely divided particles are converted,

upon the addition of appropriate quantity of liquid, to spherical particles by a continuous rolling

and tumbling motion.

Compression is a pelletization process in which mixtures or blends of active ingredients

and excipients are compacted under pressure to generate pellets of defined shapes and sizes.

These pellets are small enough to be filled into capsules.

Agitation Compaction Layering Globulation

Spray

congealing

Spray

drying

Solution /

Suspension

Powdering

Extrusion

Spheronisation

Compression Balling

Pelletization



24

Drug layering involves the deposition of successive layers of drug entities from solution,

suspension, or dry powder on preformed nuclei, which may be crystals or granules of the same

material or inert starter seed.

Globulation or droplet formation describes the two related processes of spray drying and

sprays congealing. During spray drying, drug entities in solution or in suspension form are

sprayed, with or without excipients, into a hot-air stream to generate dry and highly spherical

particles. Though the technique is suitable for the development for the controlled-release pellets,

it is generally employed to improve the dissolution rates and hence, bioavailability of poorly

soluble drugs.

Spray congealing/Freeze pelletization (Figure 1.7 and 1.8) is a process in which a drug is

allowed to melt, disperse, or dissolve in hot melts of gums, waxes, fatty acids, etc., and is sprayed

into an air chamber (Cooling column) where the temperature is below the melting points of the

formulation components, to provide, under appropriate processing conditions, to congealed

pellets.

In solution and suspension layering, the drug particles are dissolved or suspended in the

binding liquid. Once the formulation is sprayed, the droplets, which owe their existence to the

surface tension of liquid, spread out on the nuclei and drying follows. Spreading depends on the

droplet wetting characteristics, the wettability of material, and droplet dynamics. As the liquid

evaporates, the dissolved substances crystallize out. The strength of solid bond depends on the

properties of the binder, other additives and the active ingredient. In suspension layering,

however, the particles have little solubility and are bonded, for the most part, by the solid bridges

formed from the hardening binder. Powder layering process involves the layering of drug

powder onto nonpareils using syrup as the adhesive solution. The resulting pellets are then coated

with a mixture of glyceryl monostearate and hygroscopic beeswax to provide a slow- dissolution

profile. In all the above mentioned process of pelletization, a powder mass gradually changes to

the granular/pellets mass, which encompasses many mechanism of ball growth that lead to

changing the powder mass to pellets shown in the Figure 1.6.



25

Figure 1.6: Mechanism of ball grouth and breaking during pelletization (Eliasen et al., 1999; Newitt and

Conway, 1958; Capes and Danckwerts, 1965; Sastry and Fuerstenau, 1977; Kapur and Fuerstenau, 1966)

Freeze pelletization

Figure 1.7: Freezing pelletization (A) Figure 1.8: Freezing pelletization (B)



26

Description of Figure

1. Carrier solids (hydrophilic or hydrophobic) are melted with either a water bath or an electric

heater

2. The actives and excipients are added to the molten mass and mixed with a stirrer to form a

uniform solution or dispersion.

3. This molten matrix is introduced as droplets into the liquid in the column

4. This molten matrix is introduced as droplets into the liquid in the column

5. Needles or nozzles

6. Liquid droplets

7. Solidified hard pellets

8. Pellete collector

9. Valves

10. Collectors.

1.15. Extrusion And Spheronisation Technology

Smith Kline provoked interest in pellets in early 1950s with the introduction of Spansule

and French, Inc. Recent activity in the area of novel drug delivery systems has resulted in

resurgence in the methodology of preparing spherical particles. There are many processing

methods available to prepare pellets; however, one popular spheronization method involves the

use of an extruder and spheronizer.

The spheronization process based on extrusion and spheronization has been outlined in a

processing flow chart as shown in Figure 1.9. The overall process begins with a blending

operation for dry powders to prepare a uniform, heterogeneous mixture prior to wet– granulation

operation. The wetted mass is passed through the extruder to form rods. The granulation solvent

or solution serves as the binding agent to form the granules as well as the lubricant during the

extrusion operation. The wet extrudate is then processed in the spheronizer to form pellets, and

the resulting pellets are dried.

The extrusion operation can be considered to be a specialized wet- granulation technique as well

as an integral part of the overall spheronization process. In general, the extrusion operation is the

major contributing factor in the final particle size of the pellets. The diameter of extruder- screen

openings directly controls the diameter of the extrudate, which is related to the mean particle size



27

of the final pellets. However, the mean pellet diameter also depends on the formulation. The

formulation must not adhere to the extruder screws or screens, nor should it block the extruder

screen during processing. The granulation operation must provide a cohesive plastic mass that

has the necessary lubricant properties for extrusion without generating an excessive amount of

heat. The heat generated during extrusion can result in premature drying of the extrudates

(Figure 1.11) and subsequent damage to the screen. The premature drying of extrudates can also

lead to a poor quality of pellets. The diagramme of the extrusion assembley is shown in Figure

1.10. The extruded granulation must have the combined characteristics of cohesiveness, firmness

and plasticity shown in Figure 1.11 and 1.12. The spheronization operation,shown in Figure

1.13, has been divided into three stages:

(1) Breaking of cylindrical segments or extrudates,

(2) Agglomeration of the broken segments, and

(3) Smoothing of the particles.

The breaking stage has been attributed to the interaction of the extrudate with the rotating

plate, the stationary wall, and other extrudate particles. Agglomeration occurs when the larger

granules pick up the small fragments produced during the breaking stage during smoothing. The

smoothing stage creates spherical pellets by generating rotational motion of each granule

(Ghevre-Sellassie., 1989).



28

Figure. 1.9: Processing flow chart for the spheronization process using the extruder and spheronizer,

indicating the individual processing variables.

GRANULATION OPERATION

- Equipment type

- Solvent or solution type

EXTRUSION OPERATION

- Extruder type

- Screw speed

- Screen opening size

SPHERONIZATION OPERATION

- Spheronizer type

- Plate speed

- Residence time

DRYING OPERATION

- Equipment type

- Drying temperature

BLENDING OPERATION

- Equipment type

- Mixing time



29

Figure 1.11: Extrudates Figure 1.12: Spheronized pellets

Cylindrical ►Spherical rods ►Dumb bell ►Hemisphere ►Spherical

Figure 1.13: Mechanism of pelletization by extrusion-spheronisation

1.15.1. Parameters influencing the final pellets quality: They are

1. The moisture contents of the htanulated mass

2. The type of granulation liquid

3. The physical properties of the starting materials

4. The type of extruders

5. The extrusion speed

6. The properties of the extrusion screen

7. The extrusion temperature

8. The spheronisation speed

9. The spheronisation time



30

10. The spheronisation load

11 Drying method

1.16. Formulation and preparation of solid self-nanoemulsifying drug delivery system

(SSNEDDS)

The formulation of nanoemulsion usually involves three to five components i.e. oil phase;

aqueous phase and a primary surfactant & in many cases a secondary surfactant (cosurfactant)

and sometimes an electrolyte. Their formation is highly specific process involving spontaneous

interaction among the constituent molecules.

1.16.1. Choice of components

Although there are no strict rules for choosing the appropriate microemulsion components, still

choosing surfactant is a crucial step. The general characteristics of different components are:

The drug dose should be low for the ease of formulation

The drug must be strongly hydrophobic in nature

The drug selected must have logP value 4, for the o/w microemulsion.

The oil selected must have sufficients intrinsic solubility of the drug, to avoid any

drug precipitations.

For the immediate drug absorption from the nanoemulsion, the oil selected must be of

monoglyceridic in nature, and on the contrary for prolonged release, oil should be of di

and triglyceride in nature.

The surfactant(s) chosen must lower interfacial tension to a very small value to aid

dispersion.

The surfactant(s) chosen must provide a flexible film that can readily deform round small

droplets.

The surfactant must be of appropriate HLB character to provide the correct curvature at

the interfacial region for the desired microemulsion type.

The co-surfactants chosen must be able to lower the HLB value which leads to flexibility

and transient negative interfacial tension

1). Surfactant

The surfactant chosen must be able to lower interfacial tension to a very small value to aid

dispersion process during the preparation of the nanoemulsion. Provide a flexible film that can

readily deform around droplets and be of the appropriate lipophillic character to provide the



31

correct curvature at the interfacial region for the desired nanoemulsion type i.e., for o/w, w/o or

bicontinuous.

The surfactant used to stabilize the nanoemulsion may be nonionic, cationic, anionic or

zwitterionic. Combination of anionic or cationic surfactants of high HLB with a cosurfactant of

lower HLB, a doubled chained surfactant of the appropriate molecular composition or a single

chained nonionic surfactant of the polyethylene glycol alkyl ether type at appropriate temperature

are generally used for the formulation of nanoemulsion and are effective in increasing the extent

of nanoemulsion region. Examples of non-ionic include polyoxyethylene surfactants such as Brig

35 and sugar esters such as sorbitan mono-oleate (Span 80). Phospholipids are notable example

of zwitterionic surfactants and exhibit excellent biocompatibility. Lecithin preparations from a

variety of sources including soybean and egg are available commercially and contain

diacylphosphatidylcholine as its major constituents (Aboofazeli et al., 1994; Shinoda et al.,

1991; Attwood et al., 1992; Aboofazeli and Lawrence, 1993). Quaternary ammonium alkyl

salts form one of the best known classes of cationic surfactants with

hexadecyltrimethylammonium bromide (CTAB) (Mehta and Kawaljit, 1998), and the twin

tailed surfactant didodcecylammonium bromide (DDAB) are amongst the most well known

(Barnes et al., 1988; Skodvin et al., 1993). The most widely studied anionic surfactant is

probably sodium bis-2-ethylhexylsulphosuccinate (AOT) which is twin tailed and is a

particularly effective stabilizer of w/o nanoemulsions (Osborne et al., 1988; Barnes et al.,

1988; Bergenholtz et al., 1995; Trotta et al., 1990). Besides these, many other surfactants have

been used like (Kawakami et al., 2002; Kommuru et al., 2001)

Polyoxyethylene (40) hydrogenated castor oil (HCO-40®)

Polyoxyethylene (60) hydrogenated castor oil (HCO-60®)

Polyoxyethylene (10) monolauric ester (MYL-10®

)

Polyoxyethylene (25) monostearic ester (MYS-25®)

Polyoxyethylene (20) sorbitan monooleic ester (Tween 80; HLB = 15)

Polyoxyethylene (20) sorbitan monolauric ester (Tween 20; HLB = 16.7)

Polyoxyethylene (77)polyoxypropylene(29)polyoxyethylene(77)copolymer(PluronicF68®)

Polyoxyethylene (19)polyoxypropylene(43)polyoxyethylene(19)copolymer(PluronicP84®)

Various literatures also suggest the use of saturated polyglycolised C8-C10 glycerides

(Kawakami et al., 2002; Kommuru et al., 2001) as surfactants.



32

Polyoxy-35-castor oil (Cremophor-EL®

) (HLB = 14-16)

Caprylocaproyal macrogol-8-glyceride (Labrasol®)

Polyoxyethylene ester of stearic acid (Tagat S®)

Oleoyl macrogol glyceride (Labrafil M1944CS®)

Propylene glycol laurate (Laurogylcol®)

Attempts have been made to rationalize surfactant behaviour in terms of the hydrophile–

lipophile balance (HLB), as well as the critical packing parameter (CPP). The HLB takes into

account the relative contribution of hydrophilic and hydrophobic fragments of the surfactant

molecule. It is generally accepted that low HLB (3–6) surfactants are favoured for the formation

of w/o nanoemulsions whereas surfactants with high HLB (8–18) are preferred for the formation

of o/w nanoemulsion systems according to Bankroft rule (Bankroft, W.D., 1913). Ionic

surfactants such as sodium dodecyl sulphate that have HLB greater than 20, often require the

presence of a cosurfactant to reduce their effective HLB to a value within the range required for

nanoemulsion formation. The CPP relates the ability of surfactants to form particular aggregates

to the geometry of the molecule itself (Ghosh and Murthy, 2006; Israelachvilli et al., 1976;

Lawrence and Rees, 2000). The CPP can be calculated using the following equation:

υ = Partial molar volume of the hydrophobic portion of surfactant.

a = Optimal head group area

l c = Length of surfactant tail (Critical length of the hydrophobic chain) generally assumed to

be 70 to 80% of its full extended length (Lawrence and Rees, 2000). The CPP is the measure if

the preferred geometry adopted by the surfactant and consequently is predictive of the type of

aggregate that is likely to form.

If CPP = 1/3 – Globular structure of surfactant

If CPP = 1/2 - Cylindrical structure of surfactant

If CPP = 1 – Planner structures

Safety is a major determining factor in choosing a surfactant. Emulsifiers of natural origin

are preferred since they are considered to be safer than the synthetic surfactants

(Constantinides., 1995; Yuasa et al., 1994; Georgakopoulos et al., 1992; Hauss et al., 1998).

However, these excipients have a limited self-emulsification capacity. Non-ionic surfactants are

less toxic than ionic surfactants but they may lead to reversible changes in the permeability of the

CPP = υ / a. l c



33

intestinal lumen (Wakerly et al., 1986; Swenson et al., 1994). Usually the surfactant

concentration ranges between 30 and 60% w/w is used in order to form stable nanoemulsion. It

is very important to determine the surfactant concentration properly as large amounts of

surfactants may cause GI irritation.

The surfactant involved in the formulation of oral nanoemulsions should have a relatively

high HLB so that immediate formation of o/w droplets and/or rapid spreading of the formulation

in the aqueous media can be achieved. For an effective absorption, the precipitation of the drug

compound within the GI lumen should be prevented and the drug should be kept solubilized for a

prolonged period of time at the site of absorption (Shah et al., 1994; Serajuddin et al., 1988).

Surfactants are amphiphilic in nature and they can dissolve or solubilize relatively high amounts

of hydrophobic drug compounds. There is a relationship between the droplet size and the

concentration of the surfactant being used. In some cases, increasing the surfactant concentration

could lead to droplets with smaller mean droplet size such as in the case of a mixture of saturated

C8-C10 polyglycolized glycerides (Labrafac CM-10). This could be explained by the stabilization

of the oil droplets as a result of the localization of the surfactant molecules at the oil-water

interface (Levy and Benita, 1990). On the other hand, in some cases the mean droplet size may

increase with increasing surfactant concentrations (Craig et al., 1995; Kommuru et al., 2001;

Wakerly and Pouton, 1987). This phenomenon could be attributed to the interfacial disruption

elicited by enhanced water penetration into the oil droplets mediated by the increased surfactant

concentration and leading to ejection of oil droplets into the aqueous phase (Pouton, C.W.,

1997).

2). Co-surfactant

In most of the cases, single chain surfactants alone are unable to reduce the oil water interfacial

tension to sufficient level to enable a microemulsion to formation. Thus a cosurfactant which is

usually a medium chain fatty alcohol, acid or amine taken along with the surfactant to lower the

interfacial tension to a very small or even transient negative value. At this value fine droplets get

formed due to the interface expansion and more of surfactant/co-surfactant get adsorbed on the

surface until the bulk condition is depleted enough to make the interfacial tension positive again.

This process called the spontaneous emulsification forms the microemulsion. Cosurfactants have

an effect of further reducing the interfacial tension by the ‗dilution effect‘ whilst increasing the

fluidity of the interface by decreasing the rigidity constant K, thereby increasing the entropy of

the system.They also increase the mobility of the hydrocarbon tail and allow greater penetration



34

of the oil into this region. Any alcohol also influences the solubility properties of the aqueous and

oily phases due to its portioning between these phases.

Transient negative interfacial tension is rarely achieved by the use of single surfactant, usually

necessitating the addition of a co-surfactant. Fluid interfacial film is again achieved by the

addition of a co-surfactant. In the absence of a co-surfactant, a highly rigid film is formed by the

surfactant and thus produces nanoemulsion over only a very limited range of concentration. The

presence of co-surfactants allows the interfacial film sufficient flexibility to take up different

curvatures required to form nanoemulsion over a wide range of composition (Ghosh and

Murthy, 2006; Lawrence and Rees, 2000).

In most cases, single-chain surfactants alone are unable to reduce the oil/water interfacial

tension sufficiently to enable a nanoemulsion to form (Attwood, D., 1994; Eccleston., 1994;

Lawrence., 1996; Lawrence., 1994). Medium chain length alcohols, which are commonly added

as co-surfactants, have the effect of further reducing the interfacial tension, whilst increasing the

fluidity of the interface thereby increasing the entropy of the system (Attwood, D., 1994;

Lawrence., 1994; Eccleston., 1994). Medium chain length alcohols also increase the mobility of

the hydrocarbon tail and also allow greater penetration of the oil into this region. It has also been

suggested some oils like ethyl esters of fatty acids, also act as co-surfactants by penetrating the

hydrophobic chain region of the surfactant monolayer (Warisnoicharoen et al., 2000). All of the

aforementioned mechanisms are considered to facilitate nanoemulsion formation. In case of

nanoemulsions stabilized by ionic surfactants, the additions of alkanols also serve to reduce

repulsive interactions between the charged head groups.

A number of double chain surfactants such as Aerosol-OT (AOT) and didodecyl-

dimethyl-diammonium bromide (DDAB) are able to form nanoemulsion without the aid of co-

surfactants (D’Angelo et al., 1996; Osborne et al., 1988; Barnes et al., 1998). These surfactants

are characterized by having small head groups in comparison to their hydrocarbon tails.

Phosphatidylcholine or lecithin is also a twin-tailed surfactant, but in this case it is generally

necessary to include a cosurfactant in order to disrupt the lamellar structures, which characterize

its biological behaviour. Thus, medium chain alcohols have been successfully used as co-

surfactants for the formation of lecithin based nanoemulsion (Attwood et al., 1992; Aboofazeli

and Lawrence, 1993).



35

A wide variety of molecules can function as co-surfactants including non-ionic

surfactants (Osborne et al., 1988; Sagitani and Friberg, 1980), alcohols (Attwood et al., 1992;

Aboofazeli and Lawrence, 1993), alkanoic acids, alkanediols and alkyl amines (Aboofazeli et

al., 1994). The most appropriate cosurfactant is generally a small molecule, typically an alcohol

of short to medium chain length (C3-C8), which can diffuse rapidly between the bulk oil and

water phases and the interface (Ghosh and Murthy, 2006). Co-surfactant of short to medium

chain length alcohols also ensures that the interfacial film is flexible enough to deform readily

around droplets, as the intercalation between the primary surfactant molecules decreases both the

polar head group interactions and hydrocarbon chain interactions (Ghosh and Murthy, 2006).

Lamellar liquid crystalline phases rather than nanoemulsion phase often form with longer chain

length co-surfactant or in the absence of co-surfactant due to rigidity of the interfacial film

(Shinoda et al., 1991).

Although many researchers use medium chain alcohols like pentanol and hexanol, but

they are not of pharmaceutical grade and are not used in drug delivery due to their high irritation

potential as well as their volatile nature which can destabilize the system. Thus less irritant; non-

ionic surfactant (polyoxyethylene alcohol esters) was investigated by many researchers for use as

co-surfactant. Polyethylene glycol derivative of distearoyl phosphatidyl ethanolamine, ethanol,

fatty acid esters of propylene glycol, oleic esters of polyglycerol, ethyldiglycol and polyethylene

glycol were also evaluated as co-surfactants in micro and nanoemulsions drug delivery system

(Dalmora et al., 2001; Kawakami et al., 2002).

3). Co-solvent

Unlike the solubility enhancements caused by partitioning into micelles or microemulsions,

solubility enhancements caused by cosolvent addition generally occur because of changes in the

bulk properties of the isotropic solution. As with mixed surfactants, advantages are gained by

using mixed cosolvents. Short-chained linear alcohols are excellent in solubilizing small

chlorohydrocarbons, whereas, larger hydrocarbon cosolvents work best for larger and more

hydrophobic contaminants. Addition of short linear alcohols also enhances the solubility of the

larger cosolvents. In actual practice, the benefits of flushing with cosolvents may result from the

above mentioned increase in solubilization within the aqueous phase or through the more

efficient process of mobilization. Mechanisms responsible for mobilizing nonaqueous phase

liquid (NAPL) contaminants include: (1) creation of a single phase condition, (2) decrease in the

water-NAPL interfacial tension, and (3) swelling of the NAPL by solubilization of the cosolvents



36

within this phase. Mobilizing by creating a single phase is essentially the same as solubilizing the

entire NAPL. Decreasing the interfacial tension occurs due to the changes in the surface tension

of both phases (the alcohol dissolves in both phases making their chemical and physical

interfacial properties more similar. Similarly, swelling of the NAPL is accompanied by beneficial

changes in phase viscosity, density and other properties. As with surfactant systems, mobilizing,

rather than solubilizing, results in:

(1) use of less total material (surfactant or cosolvent),

(2) use of smaller volumes of material (i.e., fewer pore volumes), and

(3) shorter treatment times.

However, precise formulations are required, resulting in much more intensive laboratory bench

and pilot-scale work. Rather than using conventional surfactants or cosolvents, several

researchers have proposed the use of other solubilizing agents that associate or bind

contaminants. Mark Brusseau at the University of Arizona has proposed using cyclodextrins as

flushing agents that was conducted for the field study with cyclodextrins at Hill Air Force Base

in 1996. These compounds are generally linear chains of glucose molecules with the ends joined

to form a cyclic structure. Because the diameter of the ring can be controlled by the number of

(glucose) sugar molecules in the chain, specific cyclodextrins possible could be used for specific

contaminants (Chad, T.J., 1996).

Organic solvents are suitable for oral administration. Example are, ethanol, propylene glycol,

polyethylene glycol, which may help to dissolve large amounts of hydrophilic surfactants or

hydrophobic drug itself in the oils which is miscible in that co-solvent but not in liquid base.

Addition of an aqueous solvent such as Triacetin, (an acetylated derivative of glycerol) for

example glyceryl triacetate or other suitable solvents act as co-solvents. The production of an

optimum SEEDs required relatively high concentration of surfactants. Organic solvents such as

ethanol, propylene glycol (PG), and polyethylene glycol (PEG) are suitable for oral delivery, and

they enable the dissolution of large quantity of either the hydrophilic surfactants or the drug in

the lipid base. These solvents can even act as co-surfactants in microemulsion system. On the

other hands, alcohol and other volatile co-solvents have the disadvantages of evaporating into the

shell of soft gelatin or hard sealed gelatin capsule from conventional SEDDS leading to drug

precipitation.



37

4). Oils

Oils represents one of the most important excipients in the nanoemulsion formulation not only

because it can solubilize marked amounts of the lipophillic drug but also because it can increase

the fraction of lipophillic drug transported via the intestinal lymphatic system, thereby increasing

absorption from the GI tract depending on the molecular nature of the triglyceride (Charman

and Stella., 1991; Holm et al., 2002; Gershanik et al., 2000; Lindmark et al., 1995). Both

long and medium chain triglyceride oils with different degree of saturation have been used for

the design of self-emulsifying formulations. Further edible oils which could represent the logical

and preferred lipid excipients choice for the developments of SEDDS are not frequently selected

for their poor ability to dissolve large amounts of lipophillic drugs. Modified or hydrolyzed

vegetable oils have been widely used since these excipients form good emulsification system

with a large number of surfactants approved for oral administration and exhibit better drug

solubility properties (Constantinides, P.P., 1995; Hauss et al., 1998; Kimura et al., 1996).

They offer formulative and physiological advantages and their degradation products resemble the

natural end products of intestinal digestion. Novel semi-synthetic medium chain derivatives,

which can be defined as amphiphilic compounds with surfactants properties, are progressively

and effectively replacing the regular medium chain triglycerides oils in the self-emulsifying oily

formulations (Constantinides, P.P., 1995; Karim et al., 1994).

The oil component influences curvature by its ability to penetrate and hence swell the tail group

region of the surfactant monolayer. Short chain oils penetrate the tail group region to a greater

extent than long chain alkanes, and hence swell this region to a greater extent, resulting in an

increased negative curvature (and reduced effective HLB). Various long and medium chain

triglycerides like labrafac, luroglycol, labrafil M 1944CS and olive oil, has also been reported

(Cortesi and Nastruzzi., 1999; Nazzal et al., 2002). Novel semi synthetic medium chain

derivatives, which can be defined as amphiphilic compounds with surfactant properties, are

progressively and effectively replacing the regular medium chain triglyceride oils (Karim et al.,

1994; Constantinides, P.P., 1995). Solvent capacity for less hydrophobic drugs can be improved

by blending triglycerides with other oily excipients, which include mixed monoglycerides and

diglycerides. Since these are similar to the natural degradation products of triglycerides (the

difference being that their monoglyceride content is mostly 1-monoglyceride rather than 2-

monoglyceride) they do not prevent efficient digestion. As excipients with GRAS (Generally

Regarded as Safe) status, they have the same advantages as triglycerides, and are useful for

blending with triglycerides or for use as an alternative (Pouton, C.W., 1997). Thus most of the



38

drugs are not soluble in hydrocarbon oils, rather the polarity of the majority of the poorly water-

soluble drugs favor their solubilisation in small/medium molecular volume oils such as tributyrin,

medium chain triglycerides or mono or diglycerides (Lawrence and Rees, 2000).

Example of oils that has been used by many researchers are Olive oil, Castor oil, Corn oil,

Sunflower oil, Miglyol 812® (Fractionated coconut oil), Labrafac lipo

®, IPM, IPP, Oleic acid,

Safsol 218® (Propylene glycol monocaprylic ester), Sefsol-228

® (Propylene glycol dicaprylic

ester), Homotex PT® (Glycerol monocaprylic ester), Triacetin (Glycerol triacetate), Capryol 90

®,

Lauroglycol 90® (Polyglycolysed glycerides), Captex 355

® (C8/C10 triglycerides), Capmul

MCM® (C8/C10 mono-diglycerides).

5). Solid excipients

Microcrystalline cellulose (MCC) is maximally used as a pelletisation aid. Because not much

literature about this subject is available, and the high-shear pelletisation process has some aspects

in common with the extrusion and spheronisation process, some literature about this last field

will be discussed. Kleinebudde used a mixture of microcrystalline cellulose (50-70%, MCC),

low substituted hydroxypropylcellulose (0-20 %, L-HPC), and acetaminophen (30%) for the

preparation of pellets with the extrusion and spheronisation method and found a decreased water-

sensitivity of the process and good dissolution properties of acetaminophen from the pellets. But

still, more than half of the formulation existed used MCC. In comparison, the minimal amount of

MCC needed to form a continuous network, the so-called percolation threshold, is about 14 %

(Kuentz et al., 1999). At any volume concentration higher than 14 %, MCC has formed a

continuous network. So, in order to find other excipients than MCC (for example a mixture of

different materials) suitable for pelletisation, the amount of MCC in this mixture has to be below

14 %. Kleinebudde and Lindner reported a study using powdered cellulose as pelletisation aid

(Kleinebudde and Lindner, 1993). The pellets obtained with powdered celluloses showed

higher porosities and faster releasing properties compared to those made with MCC. Since this

study, a few more studies were reported concerning the search towards more products that could

be used as pelletisation aids. Chatlapalli and Rohera, 1998; prepared pellets containing

hydroxypropylmethylcellulose (HPMC) and hydroxyethylcellulose (HEC) and used isopropyl

alcohol as granulation liquid. Both HMPC and HEC were found to be suitable pelletisation aids.

Also a mixture of MCC (11 %) and -cyclodextrine (89%) was reported as a suitable extrusion /

spheronisation agent giving satisfactory products (Gazzaniga et al., 1998). With this

formulation, the amount of water needed to obtain good quality pellets was highly decreased. As



39

a conclusion it can be stated that questions like ―why is microcrystalline cellulose such a good

pelletisation agent‖, and ―is it possible to find other excipients except from microcrystalline

cellulose that can be used for the preparation of pellets‖ still are mostly unanswered. Using melt

pelletisation as an alternative for the high-shear pelletisation technique, i.e. using a meltable

binder, a series of pelletisation agents can be used, such as polyethylene glycol (PEG) (Knight,

P.C., 1993; Schæfer, T., 1996; Heng et al., 1999.), Gelucire (Montousse et al., 1999), glycerol

monostearate or stearic acid (Maejima et al., 1998). Because MCC is so frequently applied it has

also been used in the work described in this thesis.

Many findings (Abdalla and Mader, 2006; Yu et al., 2006; Chambin and Jannin, 2005;

Chambin et al., 2004; Newton et al., 2007; Tuleu et al., 2004; Breitenbach, J., 2002; Newton

et al., 2001; Newton et al., 2006; Serratoni et al., 2007; Newton et al., 2005; Franceschinis et

al., 2005) have been reported for the solid self- nanoemulsifying drug delivery system. Among

them most researchers have used the microcrystalline cellulose for the pelletization purposes due

to its maximum suitability to extrusion method. As we know that MCC has largest water holding

capacity and elasticity which aids in the extrusion process, which ultimately influences the

hardness and sphericity of the pellets. Some literature also had reported the lactose along with the

microcrystalline cellulose. Other solid excipients used are: K-carrageenan, sucrose, lactose,

dextrose, maltose, mannitol, maize starch, dicalciumphosphate dehydrate, porous polystyrene

beads, silicon, magnesium stearate, copolyvidon and pectinic acid ( Tho et al., 2001) etc.

Although microcrystalline cellulose (MCC) is commonly used as an excipient in

extrusion/spheronisation process. However, MCC owns several disadvantages as lack of

disintegration and drug adsorption (Thommes and Kleinebudde, 2006). Therefore, K-

carrageenan was investigated to substitute MCC in pelletising processes. Formulations with 20%

of pelletisation aid (K-carrageenan or MCC) and acetaminophen as a model drug have been

produced. Different fillers like, lactose, mannitol, maize starch and dicalciumphosphate dihydrate

at fractions of 0, 20, 40 and 80% have been evaluated and the properties of the resulted pellets

were determined (e.g. yield, aspect ratio, mean Feret diameter, 10% interval fraction, tensile

strength and release profile). K-Carrageenan has proven to be a suitable substitute as pellets with

sufficient quality were produced. Pectinic acid ( Tho et al., 2001) has also proven to be a

suitable substitute as pellets with sufficient quality were produced. The pellet batches of different

formulations were characterised by high yield, spherical pellet shape and narrow pellet size

distribution. The distinguished behaviour between K-carrageenan and MCC pellets was the lower

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16326085&query_hl=1&itool=pubmed_docsum



40

tensile strength and the faster release of K-carrageenan pellets. For the various types and

fractions of fillers only minor effects to the pelletisation process and pellet properties were

noticed. From the practical view these effects are neglectable indicating a robust formulation and

process (Thommes and Kleinebudde, 2006).

1.17. Characterization of self-emulsifying pellet

As formulation solid self-nanoemulsifying pellets consists of to steps i.e. formulation of

nanoemulsion and solidification of nanoemulsion into pellets. Therefore characterization of the

self-nanoemulsifying pellet divided into two major following classes

1.17.1. Characterization of the nanoemulsions

1.17.2. Characterization of the self-nanoemulsifying pellets

1.17.1. Characterization of the nanoemulsion

Nanoemulsion has been characterized using a wide variety of techniques. The characterization of

nanoemulsion is a difficult task due to their complexity, variety of structures and components

involved in these systems, as well as the limitation associated with each technique, but such

knowledge is essential for their successful commercial exploitation. The rate of release of sodium

salicylate from a lecithin-based nanoemulsion, is dependent upon their microstructure

(Khoshnevis et al., 1997).

Nanoemulsions have been evaluated using a wide range of different techniques over the years,

but a complementarity of methods is generally required in order to fully characterize these

systems. At the macroscopic level viscosity, conductivity and dielectric methods provide useful

information

1. Measurements of ξ -potential

The charge of the oil droplets of SEDDS is another property that should be assessed.The charge

of the oil droplets in conventional SEDDS is negative due to the presence of free fatty acids;

however, incorporation of a cationic lipid, such as oleylamine at a concentration range of 1.0-3%,

will yield cationic SEDDS. Thus, such systems have a positive ξ -potential value of about 35-45

mV (Gershanik and Benita, 1996; Gershanik et al., 1998; Gershanik et al., 2000). This

positive ξ -potential value is preserved following the incorporation of the drug compounds.



41

D = K T / 6 π η r

2. Phase Behaviour studies

As already discussed earlier in section 1.9 about the phase diagram showing the nanoemulsion

region, information about of different phases as a function of composition variables can be

analysed from vigorous study of the phase diagrams.

3. Viscosity measurements

These measurements can indicate the presence of rod like or worm like reverse micelle (Yu et

al., 1995; Angelico et al., 1998).

4. Conductivity measurement

This determines the type of nanoemulsion and also detect phase inversion phenomenon (Yu et

al., 1995; Mehta et al., 1994; Angelico et al., 1998).

5. Dielectric measurements

They are powerful means of probing both structural and dynamic features of nanoemulsions

systems (Angelico et al., 1998).

6. Nuclear Magnetic Resonance (NMR) studies

The structure and dynamics of nanoemulsions can be studied by NMR techniques. Self-diffusion

measurements using different tracer techniques, generally radio labeling, supply information on

the mobility and microenvironment of the components (Kahlweit et al., 1987; Parker et al.,

1993; Shinoda et al., 1991; Regev et al., 1996; Angelico et al., 1998). The fourier transform

pulsed-gradient spin-echo (FT-PGSE) techniques uses the magnetic gradient on the samples and

it allows simultaneous and rapid determination of the self diffusion coefficients of many

components. Self-diffusion coefficient (D) can be calculated using Stokes-Einstein equation

K = Boltzmann constant



42

T = Absolute Temperature

η = Viscosity of the medium

r = radius of the droplet

7. Electron microscopic studies

Freeze fracture transmission electron microscopy (FFTEM) and Transmission electron

microscopy (TEM) are the most important technique for the study of microstructures because it

directly produces images at high resolution and it can capture any coexistent structure and

microstructure transitions (Bolzinger et al., 1998; Vinson et al., 1991; Bolzinger et al., 1999).

However, extremely rapid cooling of the sample is required in order to maintain structure and

minimize the possibility of artifacts in FFTEM studies.

Recent developments in the cryofixation technique have overcome many problems associated

with artifact formation in early studies. A complementary technique is of direct imaging, in

which thin portions of the specimen are directly investigated in the frozen hydrated state by using

a cryostage in the TEM. The development of glass forming nanoemulsions that do not

breakdown during cooling and in which neither disperse nor matrix phase crystallizes during the

cooling process, has provided a way for direct studies of nanoemulsion and nanoemulsion

structures.

8. Scattering techniques

Scattering method that has been employed in the study of nanoemulsions and nanoemulsions

include small angle X-ray scattering (SAXS) (Hirai et al., 1999; Barnes et al., 1988; Regev et

al., 1996; Kahlweit et al., 1987). Small angle neutron scattering (SANS) (Regev et al., 1996

&1999; Kahlweit et al., 1987; Bergenholtz et al., 1995), and static light scattering and dynamic

light scattering or Photon correlation spectroscopy (PCS) (Constantinides and Scalart, 1997;

Patel et al., 1998; Kahlweit et al., 1987; Yu et al., 1995).

The lower limit of size that can be measured with these techniques is about 2 nm. The

upper limit is about 100 nm for SANS and SAXS and up to a few micrometers for light

scattering. These methods are very valuable for obtaining quantitative information in the size,

shape and dynamics of components. Nevertheless, successful determination has been carried out

using a dilution technique that maintains the identity of the droplets. Dynamic light scattering



43

photon correlation spectroscopy (PCS) is used to analyze the fluctuations in the intensity of the

scattering by the droplets due to Brownian motion.

9. Interfacial t ension measurement

The formation and properties of nanoemulsion can be studied by measuring the interfacial

tension. Spinning-drop apparatus can be used to measure the ultra low interfacial tension.

10. Droplet size distribution

Droplet size of the prepared nanoemulsion was determined by using photon correlation

spectroscopy which analyzes the fluctuations in light scattering due to Brownian movement of

the particles (Attwood et al., 1992).

11. Refrective index/ Isotropicity

Refractive index of nanoemulsions formulations was determined using an Abbe type

refractrometer It basically gives an idea about the isotropicity of the formulations.The isotropic

nature of microemulsions and their optical clarity makes their study by spectroscopic techniques

straightforward, particularly in comparison to conventional macroemulsions.

12. Stability

Stability tests are much simpler and needed less frequently for coarse dispersions, where droplet

sizes and phase changes must be followed. To overcome the problem of metastable formation

which are not thermodynamically stable and takes long time to separate, thermodynamic stability

tests are recommended. The formulations are subjected to different stresses such as heating

cooling cycle, centrifugation and freeze thaw cycle tests. If the nanoemulsion are stable over

these conditions and thus does not require frequent test on storage, unless of course chemical

reactions occur,which change the nature of the components and hence of the nano or

microemulsion.

1.17.2. Characterization of the self-nanoemulsifying pellets

1. Apparent pellet density



44

Samples are weighed and placed in the air pycnometer (air comparison pycnometer, model

Beckman 930, Beckman LTD, Irvine, CA, USA) using ambient air as a gaseous medium, and

their apparent density is determined in triplicate (Sausa et al., 2002).

2. Porosity

The porosity of the pellets is calculated from the ratio between the apparent pellet density as

determined by air pycnometer and the apparent particle density of the powder mixture (also

determined by the air pycnometer), according to the following relationship,

The apparent powder density of the powder blends is calculated from the value of the individual

powders, allowing for their proportion in the mixture. Both Harrison, P., (1983) and Chapman,

S.R., (1985) established that this ratio was equivalent to the value of the porosity obtained by

measuring the density of the pellets by a mercury intrusion pycnometer and far safer.

3. Sieve analysis (Size and size distribution of the pellets)

The size of the pellets is analyzed by using mechanical sieving (Test sieve shaker). One hundred

grams of each preparation is shaken for 10 min. The mesh diameter of the British Standard No.

BS 410 sieves followed a 2 progression between 500 and 2000 µm. Hundreds of grams of pellets

are weighed and put on the top of the sieve with a series of openings ranging from 1.41 mm

(sieve no. 14), 1.00 mm (sieve no. 18), 0.84 mm (sieve no. 20), 0.41mm (sieve no 40) to 0.20mm

(sieve no. 60). The results are reported as percentage of weight retained on each sieve size

(Umprayn et al., 1999).

4. Shape

The shape factor reported by Podezeck and Newton (1994) is used to evaluate the pellet shape.

Measurements is carried out using a Seescan Image Analyzer (Solitaire 512) attached to a black

and white camera (CCD-4 miniature video camera) connected to a zoom lens (18-108/ 2-5) and

analyzed for shape. To perform this analysis, 40 spheres is mounted on a surface previously

Apparent pellet density

Porosity = 1- ------------------------------

Apparent powder density



45

painted with non-reflective black ink. The pellets is illuminated from above using a twin cold

light source placed at 180° to the surface under conditions, which satisfies the requirements set

by Podezeck et al (1999).

5. Determination of the Pellet Size

For different batches of the pellets prepared, around 30-40 pellets are randomly selected, and

their diameters are measured using a digital vernier caliper. For pellets that are found to be non-

spherical in shape, the average weight of 30-40 pellets is determined, and their equivalent

spherical volume diameters are calculated by the formula given below

Where Mv is the average mass of a pellet.

6. Density of the pellets

System of particulate solids is the most complex physical system encountered in pharmacy. In

case of pellets, no two pellets are identical in nature and that‘s why its flow property cannot be

determined by observing the single pellets. Bulk properties of powder are determined in part by

the chemical and physical properties of their component solid and in part by the manner in which

the various components interact. So to predict the flow properties, we have to be familiar with the

various micromeratics parameters, which directly concern with the different pharmaceutical

densities as Bulk density, Tapped density, Granular density and True density.

a). Bulk density is a property of particulate materials. It is the mass of many particles of the

materials divided by the volume they occupy. The volume includes the space between particles

as well as the space inside the pores of individual particles.

b). Tapped density:Bulk density is not an intrinsic property of a material; it can change

depending on how the material is handled. For example, grain poured in cylinder will have a

particular bulk density; if the cylinder is disturbed, the grain particles will move and settle closer

together, resulting in a higher bulk density. For this reason, the bulk density of powders is usually

reported both as "freely settled" and "tap" density (where the tap density refers to the bulk

density of the powder after a specified compaction process, usually involving vibration of the

container.)

c). Porosity

http://en.wikipedia.org/wiki/Density



46

Porosity is a measure of the void spaces in a material, and is measured as a fraction, between 0–

1, or as a percent between 0–100%.

The porosity of a porous medium describes the fraction of void space in the material, where the

void may contain, for example, air or water. It is defined by the ratio

where VV is the volume of void-space (such as fluids) and VT is the total or bulk volume of

material, including the solid and void components. Both the mathematical symbols φ and n are

used to denote porosity.

The volume/density method is fast and surprisingly accurate (normally within 2 % of the actual

porosity). To do this method pour material into a beaker, cylinder or some other container of a

known volume. Weigh container to know its empty weight, then pour material into the container.

Tap the side of the container until it has finished settling and measure the volume in the

container. Then weigh container full of this material, so you can subtract the weight of the

container to know just the weight of material. So now both the volume and the weight of the

material are known. The weight of material divided by the density of material gives the volume

that material takes up, minus the pore volume. So, the pore volume is simply equal to the total

volume minus the material volume, or more directly.

Many techniques are employed to determined the porosity as given below:

i. Water saturation /Benzene saturation method is slightly harder to do, but is more

accurate and more direct. Again a known volume of material is taken and also a known volume

of water. (Make sure the beaker or container is large enough to hold both compenents) slowly

dump material into the water and allowwd to saturate. The beaker is then seal (with a piece of

parafilm tape or if you don't have parafilm tape a plastic bag tied around the beaker ) and is then

allowd to sit for a few hours to ensure complete saturation.Finally remove the unsaturated water

from the top of the beaker and measure its volume using the expression

Total volume

Pore volume = ---------------------

Material volume

http://en.wikipedia.org/wiki/Porous_medium

http://en.wikipedia.org/wiki/Ratio

http://en.wikipedia.org/wiki/Volume

http://en.wikipedia.org/wiki/Weight

http://en.wikipedia.org/wiki/Density

http://en.wikipedia.org/wiki/Parafilm



47

ii. Water evaporation method is the hardest to do, but is also the most accurate. A fully

saturated, known volume of the material with no excess water on top is taken.The container with

the material and water is then weighed and place into a heater to dry it out. Drying out the sample

may take several days depending on the heat applied and the volume of the sample. Then weigh

the dried sample. Since the density of water is 1 g/cm3, the difference of the weights of the

saturated versus the dried sample is equal to the volume of the water removed from the sample

(assuming its measuring in grams), which is exactly the pore volume.

iii. Mercury intrusion porosimetry requires the sample to be placed in a special filling

device that allows the sample to be evacuated followed by the introduction of liquid mercury.

The size of the mercury envelope is then measured as a function of increased applied pressure.

The greater the applied pressure, the smaller the pore entered by mercury. Typically this method

is used over the range of pores from 300 µm to 0.0035 µm.

iv. Nitrogen gas adsorption is used to determine fine porosity in materials such as charcoal.

In very small pores, nitrogen gas condenses on the pore walls less than 0.090 µm. This

condensation is measured either by volume or weight

7. Carr’s compressibility index

The tap density apparatus (Model SVM10, Copley/Erweka, Nottingham, UK; lift height 3

mm,tapping frequency 150 taps/min) can be used to determine Carr‘s compressibility index. The

tared 250-mL measuring cylinder iss filled with test material to 100 mL, and the volume and

weight of the material is noted. With minimal disturbance to the measuring cylinder, it is

transferred to the tap density apparatus. The method of determination recommended in USP-NF

7th Supplement (p. 3936/7) is used.

Total volume of water

Pore volume = -----------------------------

Unsaturated water

Weight of saturated sample in grams

Pore volume =

Weight of dried sample in grams



48

The flow properties of the powdered lipid formulations is determined by the Carr‘s flowability

index method. The following four measurements: (1) compressibility, (2) angle of repose, (3)

angle of spatula, and (4) uniformity coefficient or cohesion are carried out. The flowability index

(FI) is then calculated with the point scores as described (Carr, R.L., 1965).

7.1. % Compressibility

The granular powder (10 g) is poured lightly into a 25ml graduated cylinder. The powder is

tapped until no further change in volume is observed. Powder bulk density,

b (g/cm3), and powder tapped density, p (g/cm3) are calculated as the weight of the powder

divided by its volume before and after tapping, respectively. Percentage compressibility is

computed from the following equation: (Nazzal and Mansoor, 2006)

p = Tapped density, b =Bulk density

7.2. Angle of repose

Angle of repose is measured using a protractor for the heap of granules formed by passing 10 g

of the sample through a funnel at a height of 8 cm from the horizontal surface (Woodruff and

Nuessle, 1972; Hellen et al., 1993b).

Θ = angle of repose

7.3. Angle of spatula

Angle of spatula is measured using a protractor and a steel spatula with a 2 in.×1 in. blade. The

spatula is inserted to the bottom of the heap that is carefully built by dropping the material

through a funnel at a height of 8 cm from the horizontal surface. The spatula is then withdrawn

b

Compressibility = p - ----- x 100

p

Vertical height of heap

Tan θ = ---------------------------

Radius of heap base



49

vertically and the angle of the heap formed on the spatula is measured as the angle of spatula

(Nazzal and Mansoor, 2006).

Θ = Angle of spatula

7.4. Uniformity coefficient

Uniformity coefficient is obtained by sieve analysis of 10 g of the powdered material using a

Retsch® sieve shaker type AS200 (F. Kurt Retsch GmbH, German). The sieve shaker is fitted

with eight U.S. standard sieves (Dual mfg. Co., Chicago, IL) ranging in size from 0.075 to 1.7

mm, and vibrated at a setting of 80 for 120 second. Uniformity coefficient is measured as the

numerical value arrived at by dividing the width of the sieve opening that will pass 60% of the

sample by the width of sieve opening that will pass 10% of the sample (Nazzal and Mansoor.,

2006).

8. Hausners ratio

Pellets (100g) are placed in a 10 ml volumetric cylinder and their volume is determined. The bulk

density is calculated as g/cm3.The cylinder was then tapped 1250 times and the volume is

determined again afterward to calculate the tapped density (Steckel and Nogly, 2004).

9. Sphericity

One of the most important characteristics of a pellet is its roundness. Several methods exist to

determine the roundness: visual inspection of the pellets and classification into a group (Hellen

et al., 1992; Hileman et al., 1993; Ku et al., 1993); one-plane-critical-stability (OPCS), being

Vertical height of heap

Tan θ = ---------------------------

Radius of spatula

Tapped Density

Hausner ratio = -------------------

Bulk Density



50

the angle to whicha plane has to be tilted before a particle begins to roll (Rowe, R.C., 1985;

Fielden et al., 1992a, 1992b, 1993; Newton et al., 1992; Pinto et al., 1993); the ratio of the

largest and the smallest diameter of pellet (Baert et al., 1992a,b, 1993a,b; Ligarski et al., 1991;

Baert and Remon, 1993; Baert et al., 1991); shape factor calculated by means of the projected

area of the pellet and its perimeter measured with computer –aided image analysis (Woodruff

and Nuessle, 1972; Robinson and Hallenbeck, 1991; Fielden et al.,1992a).An indirect

indication of the sphericity of a pellets from the determination of the repose angle Φ (Woodruff

and Nuessle, 1972). Tangent Φ is the ratio of the pile height and the pile radius measured after a

certain amount of pellets are allowed to fall from a given height onto a hard surface through a

specific orifice.

10. Water content

The residual water content present in the pellets after drying was determined by thermgravimetric

or IR-LOD apparatus (Loss on drying) connected to a sample analyzer. Moisture content was

determined using IR-LOD apparatus (Mettler PC 440 equipment). A specified amount (3 g) of

pellets was kept so as to cover the full surface of the pan. The equipment was operated at 105o C

for 15 min. After 15 min, the percentage moisture content was recorded from the digital recorder.

11. Mechanical crushing force

At least 20 pellets in the size range 1-1.4 mm of each formulation were evaluated for their

diametral crushing force using a tablet strength tester (CT 40 Engineering Systems), at a

crosshead speed of 1 mm/min (Sausa et al., 2002).

12. Friability

10 g of pellets together with 25 steel/glass spheres (6.35mm diameter) were rotated in a Roche

friabilator for 10 min, the resulting material were placed on a 250 m sieve and shaken for 5 min,

the amount of material passed through the sieve was weighted and expressed as a percentage

(Steckel and Nogly, 2004). In an alternative method 5 gram accurately weighed of pellets were

taken from the modal class fraction of the pellets and placed in a Roche friabilator and tumbled

for 200 revolutions at 25 rpm. Twelve steel balls (diameter 6.3 mm, weighing 1.028 gm each)

were used as attrition agents. After friability testing, the pellets were sieved through a series of

sieves. The weight loss (%F) after friability testing was calculated by formula given below :



51

Where the initial weight of the pellets before friability testing and the final weight of pellets

retained above the sieve with 0.355 mm aperture size after friability testing were determined.

13. Disintegration time

The disintegration time of pellets in size fraction mode value was studied in deionised water at

370C using a disintegration test apparatus (Model ZT3, Erweka). Six pellets from each

formulation were evaluated. The end point was taken as the time for disintegration of the

pellets.The mesh size used is 35 (500 µm) in place of 10 (2000 µm) mesh.

14. Dissolution studies

Dissolution and release of self-emulsifying pellet or tablets is determined using USP XXIV

rotating basket apparatus (VanKel, mod. VK7000) at 370

C. The rotating speed is 50 rpm and

some time 100 rpm is used and the dissolution medium is either 900 ml of distilled water,

simulated gastric fluid (pH 1.2) or simulated intestinal fluid ( pH 7.4) for time period of 1 hour.

The dissolution and characterization of self-emulsifying tablet formulation is previously

addressed by (Nazzal et al., 2002a, 2002b; Franceschinis et al., 2005). Dissolution studies can

also be carried out for 12 h to evaluate the sustained release properties of the preparations.

15. Scanning electron microscopy

The pellets morphology is evaluated by scanning electron microscopy (SEM). Samples are

sputter-coated with Au/Pd using a vacuum evaporator (Edwards, Milano, Italy) and examined

using a scanning electron microscope (Model 500, Eindhoven; The Netherlands) at 10 kV

accelerating voltage using the secondary electron technique.

16. Stability studies

The optimized self-emulsifying pellet or tablet dosage forms containing the lipid formulation are

subjected to accelerated stability studies. Two ounce amber colored glass containers, each

containing 5 gm pellets are stored at 40C; in temperature controlled ovens at 25

0 and 30

0C; in a

% Friability = Initial weight – Final weight x 100

Final weight



52

light stability chamber at 250C UV irradiation (350 nm) using a 60W black light; and in humidity

chambers at 250C/60% RH, 30

0C /60% RH and 40

0C/75% RH. These conditions are selected to

facilitate comparison of stability data without strictly adhering to the ICH guidelines which

recommends 300C/65% RH as the intermediate storage condition. Saturated salt solutions of

sodium bromide (for 60% RH) and sodium chloride (for 75% RH) are used to maintain the

humidity conditions. Equivalent amount of pellets are removed at each time point (15 days, 1

month, 1.5 months and 4 months) and evaluated for their hardness and dissolution profiles

(Nazzal et al., 2002a, 2002b).

17. General guidelines for stability studies

In general case stability condition as per the ICH Guideline are defined in three types (Table

1.3). Accelerated stability study is done at 400C ± 2

0C /75% RH ± 5% RH, to know the result of

the study in short duration of time. The result of accelerated stability study is then extrapolated to

know the stability at ordinary conditions. Long-term study is mainly done at 250C ± 2

0C /60%

RH ± 5% RH, and the results are collected after 12 months. Some times intermediate stability

studies at 30 0C ± 2

0C /65% RH ± 5% RH, is done and the data is collected after 6 month period.

Table 1.3: Stability study of the self-nanoemulsifying pellets

Study Study conditions specification Time period

Long term* 250C ± 2

0C /60% RH ± 5% RH or

30 0C ± 2

0C /65% RH ± 5% RH

12 months

Intermediate 30 0C ± 2

0C /65% RH ± 5% RH 6 months

Accelerated 400C ± 2

0C /75% RH ± 5% RH 6 months

If 30 0C ± 2

0C /65% RH ± 5% RH is used for the long-term condition, there is no

intermediate conditions.

1.18. Mechanism of bioavailability enhancement for the Nanoemulmsion based

formulations



53

As we know that hydrophobic drugs with low aqueous solubility, poor bioavailability can often

be attributed to slow dissolution within the lumen of the gastrointestinal (GI) tract. Dissolution

becomes the rate limiting process, and the time available for absorption, during transit of the

crystalline drug through the GI tract, is insufficient to allow complete absorption. It has been

widely reported that the oral bioavailability of hydrophobic drugs, whose absorption is normally

dissolution rate limited, can be enhanced via reformulation in a oily vehicle (Humberstone and

Charman., 1996). The mechanism of enhancement of bioavailability has been attributed to a

variety of factors including; the effect of oils on gut motility (Yamahira et al., 1978) and the

stimulation of secretion of digestive juices, particularly bile, which enhances drug dissolution

(Welling, P., 1989; Gibaldi and Feldman, 1970; O’Driscoll., 1996). Many hypothetical

assumptions are given below for the bioavailability enhancement of nanoemulsion-based

formulationsas.

1.18.1. Absorption of hydrophobic drug from the GI tract

The rate of absorption of non-electrolytes from the crystalline state depends on drug dissolution

rate and the intrinsic rate of absorption across the epithelia of the intestine. However many drugs

are weak electrolytes and, though their unionized species may be hydrophobic, weak electrolytes

are often absorbed to an adequate extent because their pKa(s) allow them to exist predominantly

as ionized species in the lumen of the gut. In such cases the drug is absorbed as the lipid-soluble

unionized species but the ionized species represents a reservoir of drug from which the

crystalline drug unionized species is immediately available. One commonly used technique is to

solubilize the drug in a water-miscible cosolvent system, such as polyethylene glycol but it is

likely that the drug will crystallize on dilution of the cosolvent in the lumen of the gut. However,

the cosolvent approach could be advantageous if the drug precipitates as a microfine suspension

(i.e. a dispersion that is finer than could be achieved by micronisation. A better strategy for the

formulator is to maintain solubility of the drug throughout its passage through the gut, and this

can be achieved using oily systems. Furthermore, in cases where the drug is so lipophillic that

solution can be achieved in liquid paraffin, there is a possibility that the log P of the drug is so

high that it could be effectively sequestered within the oil droplets, defeating the purpose of the

formulation. These formulations will be expected to be digested during their passage through the

GI tract, thus the mechanism by which the drug is maintained in solution will depend on the

solubilization of drug by mixed micelles of bile and the products of lipolysis. There is sufficient

evidence in the literature to support the view that bile salt-lecithin mixed micelles (Carey and



54

Small, 1972; Tso, P., 1985; O’Connor and Wallace, 1985) are often good solvents for

lipophilic drugs (Bakatselou et al., 1991; Naylor et al., 1995; Mithani et al., 1996; Li et al.,

1996; Solomon et al., 1996). However the process of solubilization is not entirely driven by

hydrophobicity, molecular shape must also play a role.

1.18.2. Absorption of drugs from digestible oil formulations.

Following passage into the small intestine, the formulation will become exposed to local

secretions of lipase and bile salts from the pancreas and gall bladder respectively. These

secretions will together facilitate lipolysis, wherein dietary fat is ultimately broken down to form

fatty acids and monoglycerides. Microscopy studies (Patton et al., 1985) have provided evidence

to support the existence of a hydrophobic continuum, linking the dispersed and degrading oil

droplets to the interior of these intermediate product phases. Moreover, the liquid crystalline

phases are themselves perpetually broken down in the presence of (unsaturated) bile salt

micelles, leading to the formation of lipid rich, swollen, mixed micelles. This provides the

reservoir of drug from which partitioning will occur, allowing absorption of free drug from the

lumen of the gut. Therefore, if it is necessary to maximise the rate of drug partitioning into

aqueous intestinal fluids, and hence the absorption rate (Figure 1.14), the formulation should be

highly dispersable. One mechanism by which this can be achieved is by formulation of a self-

emulsifying drug delivery system (SEDDS) using surfactants.

Figure 1.14: Mechanism of uptake of the oil globules,micelles and water soluble solutes through

biological membrane



55

The extent to which the consistency of absorption for a hydrophobic drug can be improved by the

use of a SEDDS can be appreciated by comparing the absorption of cyclosporin A from

Sandimmune with that from a new formulation, Sandimmune Neoral. In contrast to

Sandimmune, the latter undergoes rapid dispersion in GI fluids to form a uniform, stable,

microemulsion. As a result this formulation becomes homogeneously distributed in the lumen of

the gut in vivo and provides a dispersion from which the drug can partition consistently into the

aqueous phase of the lumen (Kovarik et al., 1994) its cofactor from the oil/water interface.

Having made the case for SEDDS it is important to recognize that if rapid absorption of the drug

is not necessary, for many drugs the bioavailability may be high enough from a digestible system

formulated without surfactants, due to the dispersion caused by lipolysis. A recent study

comparing the bioavailability of a MK-386, a testosterone 5Lu-reductase inhibitor, from oil

solution and a SEDDS formulation illustrates this point (Loper et al., 1996). A delivery system

based on inert lipids would be the formulation of choice from a safety viewpoint to avoid

potential toxicological issues associated with chronic use of surfactants.

1.18.3. Solubilization of drugs by bile salt-lecithin mixed micelles

There is sufficient evidence in the literature to support the view that bile salt-lecithin mixed

micelles (Carey and Small., 1972; Tso., 1985; O’Connor and Wallace., 1985) are often good

solvents for lipophillic (Bakatselou et al., 1991; Naylor et al., 1995; Mithani et al., 1995; Li et

al., 1996; Solomon et al., 1996). As would be expected the total solubility of the drug is

proportional to bile salt concentration and there is an increase in solubilization with increase in

log P of the solute. Mithani et al., studied the solubilization of a range of drugs by taurocholate

solutions and calculated the solubilization ratio (SR) given by:

SCbs = The solubilization capacity of the bile salt micelles measured as moles drug/mole bile

salt.

Scaq = The equivalent parameter for water.

SCbs

SR = -------

SCaq



56

These authors reported that the value of SCbs was similar for a range of drugs where as the

SCaq was variable, suggesting that the driving force for bile salt solubilization was determined

by the hydrophobicity of the drug rather than by affinity for bile salt micelles. It was concluded

that there was a linear relationship between solubilization ratio SR and log P This relationship

could be used predictively to give good estimates of saturation solubility for most drugs.

However the process of solubilization is not entirely driven by hydrophobicity, molecular shape

must also play a role. Esters of hydro- cortisone, particularly long chain esters, were as well

solubilized as other steroids of comparable log P Total solubilities of steroids in the 15 mM bile

salt - 5 mM lecithin system were enhanced by up to two orders of magnitude for progesterone

and three orders of magnitude for testosterone acetate, indicating the potential enhancement to be

exploited.

The presence of bile may also be influential for drugs, which are present in the gut in

crystallinebility in the presence of bile and on wetting of solid particles. Figure 1.15 illustrates

that solubility and dissolution rate in the presence of mixed micelles are not necessarily directly

related, though both can be expected to be enhanced for hydrophobic drugs in the presence of

bile. In practice drugs with log P > 2 are likely to be solubilized by bile salt micelles too.

Figure. 1.15: Schematic diagram of the fate of drug and oil droplet after exposure to pancreatic lipase and

bile in the small intestine. Lipolytic products (fatty acids and monoglycerides) form at the oil-water

interface and are extruded from the droplet as a bilayer, forming a vesicular particle. Vesicles are

solubilised by bile salt micelles to form mixed micelles, which would include lipolytic products and drug.

Both vesicular particles and micellar structures exist in the dynamic environment of the GI tract (Patton

et al., 1985; Staggers et al., 1990; Hernell et al., 1990).



57

1.18.4. Impact of the GI tract on lipids and lipid based formulations

The GI tract impacts on the performance of lipid based formulations as triglyceride lipids are

quantitatively and efficiently hydrolysed prior to absorption. In the fed state, the presence of

hydrolytic lipid digestion products induces secretion of biliary and pancreatic fluids which

dramatically alter the luminal environment of the small intestine. The lipid digestion products,

which are more water soluble than the parent triglyceride, are solubilised within bile salt mixed

micelles for their subsequent delivery to the absorptive cells of the gastrointestinal tract

(Thomson et al., 1989; Tso, P., 1994).

1.18.5. Dispersion of the products of lipid digestion into an absorbable form

Lingual lipase secreted by the salivary gland, and gastric lipase secreted by the gastric mucosa,

are responsible for initiating triglyceride (TG) hydrolysis to the corresponding diglyceride (DG)

and fatty acid (FA) within the stomach. The liberation of these amphiphilic lipid digestion

products facilitates formation of a crude emulsion, which empties into the duodenum. The

optimal pH range for lingual and gastric lipase is 3-6, and medium chain triglycerides are

hydrolyzed at a faster rate than long chain triglycerides (Liao et al., 1984). The presence of lipid

in the duodenum stimulates secretion of bile salts. Biliary lipids adsorb to the surface of the TG

and DG emulsion entering from the stomach thereby stabilizing the system and reducing droplet

size (Carey et al., 1983). Pancreatic lipase is an interfacial enzyme, which preferentially acts at

the surface of emulsified TG droplets to quantitatively produce the corresponding 2-

monoglyceride (MG) and two fatty acids (FA) (Tso, P., 1994). As the FA and MG products of

lipid digestion are effective emulsifying agents, and because the presence of liberated FA further

promotes binding of the lipase/colipase complex to the emulsion surface (Bernback et al., 1989).

The enzyme is also known, as cholesterol esterase is required for maximal activity. A further

class of biliary-derived lipids is the phospholipids (e.g. phosphatidylcholine, PC), which have an

important role in solubilisation of lipid digestion products.

1.18.6. Solubilisation, physical chemistry and absorption of lipid digestion products

Bile plays a central role in the solubilisation of lipid digestion products and poorly water-soluble

drugs (Figure 1.16). Bile salts form polymolecular aggregates whose structural characteristics

are dependent upon solution pH, temperature, the compositional distribution of other bile acids,

and the presence of other biliary lipids and lipid digestion products (Carey and Small., 1970).



58

The inclusion of these lipidic components decreases the CMC, and increases the size and the

solubilization capacity of the micelles. More recently, Carey and co-workers systematically

studied the physical chemistry of fat and proposed that as lipolysis proceeds, digestion products

‗bud off‘ from the surface of the TG/DG emulsion to form large liquid crystalline structures,

which in the presence of sufficient bile salt concentrations, form multilamellar and unilamellar

structures (Tso, P., 1994; Staggers et al., 1990; Hernell et al., 1990).

Figure 1.16: A proposed model for the sequential steps involved in lipid digestion. Biliary lipids in

various physical states (simple micelles,mixed micelles and vesicles) and pancreatic lipase and colipase

adsorb to the surface of lipid emulsion droplets in the duodenum, thereby stabilising and reducing the size

of the droplets. The core lipids of the crude emulsion are triglycerides (TG), diglycerides (DG) and

cholesterol esters (CE). During lipolysis, the fatty acid (FA) and monoglyceride (MG) digestion products

further enhance emulsification as they concentrate at the surface. As the size of the emulsion droplet

decreases with continuing lipolysis, the build up of MG, FA and phospholipids (PL) at the surface

continues and portions of the surface lipids ‗bud off (probably as multilamellar bilayers) to form a variety

of structures which are dependent upon the concentration of bile salts (BS). It was postulated that

unilamellar vesicles originate from lamellar liquid crystals that form at the surface of the emulsion/water

interface.



59

1.18.7. Intracellular lipid processing and intestinal lymphatic transport of highly

lipophillic drugs

The majority of orally administered drugs gain access to the systemic circulation via absorption

into the portal blood. However, for some extremely lipophilic compounds, transport via the

intestinal lymphatics provides a route of access to the systemic circulation, which avoids hepatic

first-pass metabolism (Figure 1.17). The majority of long chain FA and MG lipid absorbed by

the enterocyte migrates to the endoplasmic reticulum where re-esterification and assembly into

intestinal lipoproteins occurs prior to secretion into mesenteric lymph. As a general guide, fatty

acids of chain length less than 12 carbons are absorbed primarily via the portal blood (Kiyasu et

al., 1952), whereas fatty acids with chain lengths greater than 12 carbons are re-esterified and

transported via intestinal lymph. However, the dependence of portal or lymphatic transport on the

chain length of the fatty acid is less clear-cut than this ‗rule of thumb‘ as there are reports of

lymphatic transport of medium chain fatty acids and portal blood absorption of long chain fatty

acids (McDonald and Weidman, 1987; McDonald et al., 1980). The major route for the re-

esterification of absorbed FA is the monoacylglycerol pathway which accounts for approximately

80% of chylomicron triglyceride.Following reesterification, TG is processed through a number of

intracellular organelles where it then becomes the core lipid of chylomicrons. The surface of

chylomicrons is stabilised by the addition of phospholipids and various apohipoproteins

(Zilversmit, D.B., 1965) prior to secretion into the lamina propria and the mesenteric lymphatics.

Figure 1.17: Inter-junction of blood vessels and lymph vessels showing the fat absorption site



60

Charman and Stella have suggested that drugs require a log octanol/ water partition coefficient in

excess of 5 and triglyceride solubility of at least 50 mg/ml before mesenteric lymphatic transport

is likely to become a major contributor to oral bioavailability. As previously noted, these simple

estimates of potential lymphatic transport ignore the issues of metabolic conversion, chemical

stability and bioavailability which can significantly decrease the lymphatic transport of otherwise

highly lipophilic drugs (Charman and Stella, 1986).

1.18.8. Lipids and intestinal permeability

The pre-epithelial unstirred water layer presents a barrier to the passive absorption of poorly

water-soluble drugs and lipid digestion products. In terms of drug absorption, the role of the

unstirred water layer is dependent on the physicochemical properties of the drug, with it being a

progressively greater barrier to the absorption of increasingly hydrophobic compounds.

Constituents of the mixed micellar phase may impact on the intestinal permeability of poorly

water-soluble compounds via three mechanisms.

Firstly, the presence of lipid digestion products and bile salts may alter the intrinsic

permeability of the intestinal membrane leading to increased absorption via paracellular or

transcellular routes.

Secondly, solubilization of lipophilic drugs within bile salt mixed micelles may facilitate

diffusion through the aqueous diffusion layer leading to increased absorption.

Thirdly, and conversely, drug solubilization may decrease the intermicellar ‗free‘ fraction

of drug, which could potentially lead to a decrease in absorption.

These issues are not new, and the points raised in a review published by Gibaldi and Feldman,

1970; are as relevant today as when the article first appeared. The inherent permeability

enhancing abilities of various bile salts, fatty acids and monoglycerides in the GI tract are well

known (Anderberg et al., 1993; Aungst., 1993; Swenson et al., 1994). Kvietys and co-workers

studied the role that lipolytic digestion products may play in causing mucosal injury (Kvietys et

al., 1991).

1.18.9. Influence of surfactants on lipolysis of oils



61

The formulation of SEDDS often involves combining hydrophilic surfactants with more

lipophilic counterparts. This is in part to produce efficient emulsification but is also used to raise

the solvent capacity of the complete formulation. Hydrophilic surfactants can inhibit significantly

the lipolysis of the triglyceride component. During screening exercise we identified that

Cremophor RH40, a commonly used hydrophilic ethoxylated triglyceride surfactant, completely

inhibited lipolysis of MCT oil over a 90 min period in vitro, when the oil and surfactant were

present in equal mass. The addition of a third component, a lipophilic surfactant, in some cases

caused recovery of lipolysis. The chemistry of the lipophilic component appeared to be a crucial

determinant of whether lipolysis was recovered , the most efficient being Imwitor 988 (a mixture

of medium chain length mono- and diglycerides). Though many other lipophilic materials had

similar HLB numbers, some such as Span 80 were not able to cause recovery of lipolysis. These

effects are presumably the result of the orientation of the surfactants at the oil-water interface but

it was not possible to suggest any explanation, which would explain the differences between

various lipophilic materials. Solomon et al, 1996; performed a study of the inhibition of MCT

lipolysis by nonyl phenol ethoxylates, which enabled a more systematic study of the influence of

HLB on inhibition. The rate of lipolysis was affected by nonyl phenol ethoxylates with HLB

greater than 12, the effect being most pronounced within the HLB range 13- 17. At very high

HLB the inhibitory effect appeared to decline, an effect which may be explained by the weak

surface activity of highly ethoxylated materials. When pure nonyl phenol or ethoxylates of very

low HLB were used, the initial rate of hydrolysis was unaffected but the total hydrolysis of fatty

acids from MCT oil was lower than expected. This effect could be explained by the phase

separation of the oil and alkyl phenol on mixing with water into two populations of droplets (one

rich in MCT oil and one in alkyl phenol), only one of which enables lipolysis to proceed. Within

the range of surface active materials which would be expected to be anchored at the oil-water

interface (HLB range 5-U), it is likely that lipolysis is inhibited once the oxyethylene mantle

generated by the surfactant exceeds a critical thickness, which prevents binding of the

colipaselipase complex to the surface of the oil droplet.study. The self-emulsifying lipolysing

formulations I were compared with a commercially available oily suspension formulation

containing 100 mg progesterg 70 one (Utrogestan). The Cmax and AUC were between 8-10 times

greater for the lipolysing SEDDS formulations. The initial data suggests that this approach has

considered a potential for optimising the absorption of hydrophobic drugs from the GI tract. It

has been found that the surfactants also increase the plasma drug profile by inhibiting the p-

glycoprotein efflux inhibition of drug ( Figure 1.18) from tissue (Dintaman and Silverman,

1999).



62

Figure 1.18: P-glycoprotein efflux inhibition of drug molecules by oily based formulations



63

1.18.10. Emulsification of the vehicle

Emulsification of the lipid vehicle has the obvious advantage of increasing the surface area for

release of drug from the vehicle. Undoubtedly, the facile in situ dispersion of digestible lipids

play a role in their superiority over non-digestible vehicles, and probably underpins some of the

differences observed between formulations based on XT, MCT or LCT lipids. Exogenous

surfactants are usually required for emulsification of the lipid vehicle and they primarily

influence drug absorption by altering the nature of the surface of the emulsified lipid. However,

they can also interact with aqueous colloidal phases and cell membranes and impact on drug

absorption by non-surface area dependent effects. Furthermore, emulsification will increase the

available surface area for binding of the lipase/colipase complex thereby facilitating the kinetics

of lipid digestion, while the presence of inter-facially active components could also affect lipase

binding and associated processes. Consequently, it is possible to over-interpret bioavailability

data when differences in emulsion droplet size is the only factor considered when comparing

between different lipid formulations. Notwithstanding these limitations, Table 1.4 lists the

selected examples where dispersion of the administered lipid vehicle has been addressed in the

context of the resultant bioavailability of the administered drug. Myers and Stella compared the

absorption of penclomedine after slow intra-duodenal administration to anaesthetized rats as

either an emulsified (10% emulsion comprising 50 µl of oil dispersed in 450 µl of water

stabilized with lecithin and glycerol), or non-emulsified (50 µl of oil followed by 450 µl of the

lecithin suspension) vehicle (Myers and Stella., 1992). The blood concentration-time profiles of

penclomedine after administration as emulsified and non-emulsified vehicles, employing either

trioctanoin or mineral oil as the lipids, are presented in Figure. 7. Administration of lipid as

emulsion increased drug absorption from both the trioctanoin and mineral oil lipids, compared

with administration as non-emulsified lipid. In the case of the non-digestible mineral oil,

sequently evaluated in humans where it afforded a 2-fold enhancement in bioavailability

compared with either an aqueous suspension or commercial tablet formulation (Bates and

Sequeira., 1975).



64

Table 1.4: Selected examples where the effect of lipid vehicle dispersion on the extent of drug absorption

has been investigated

Ref. Compound Study type Formulation

Observation and commnts References

49 Griseofulvin Oral BA in

rats

Corn oil

suspension

BA from emulsion > lipid

suspension > aqueous

suspension. BA from

emulsion less variable

Carrigan, and

Bates., 1973

63 Griseofulvin

Oral BA in

rats

Corn oil

emulsion

BA from emulsion >

aqueous suspension. BA

from emulsion less variable

Bates and Canigan.,

1975

64 Griseofulvin Oral BA in

human

Corn oil

emulsion

BA from emulsion 2-fold >

tablets or aqueous

suspension

Bates and Sequeira.,

1975

66 Cyclosporin

Oral BA m

rats

Lipid

microemulsions

BA greater from

microemulsion than from

standard lipid solution

Ritschel et al., 1990

68 Vitamin E

acetate

Oral BA in

rats

MCTllecithin,

soybean oil

MCT/lecithin > soybean oil Kimurd et al., 1989

69 Vitamin E

acetate

Intraduodenal

BA in rats

MCT/lecithin,

soybean oil

MCT/lecithin > soybean oil,

absorption a function of

particle size

Fukui et al., 1989

70 REV 5901

Oral BA in

rats

PEG 400,

polysorbate 80

and peanut oil

BA from PEG g polysorbate

80 > peanut oil > aqueous

suspension, formed a finely

dispersed emulsion in water

Serajuddin et al.,

1988

71 Cyclosporin

Rat intestinal

perfusion

Olive oil

emulsions

Increased absorption after

reduction in emulsion

droplet size

Tarr and

Yalkowsky., 1989

72 Cyclosporin

Oral BA in

human

Emulsion and

microemulsion

BA greater from

microemulsion, and dose

linearity with

microemulsion

Mueller et al., 1994

73 Cyclosporin

Oral BA in

human

Emulsion and

microemulsion

Reduced effect of food on

BA after administered

microemulsion

Mueller et al., 1994

74 Cyclosporin

Oral BA in

human

Emulsion and

microemulsion

Reduced inter and intra-

individual variability from

microemulsion

Kovarik et al., 1994

75 Cyclosporin

Oral BA in

patients

Emulsion and

microemulsion

BA greatest from

microemulsion

Sketria et al., 1994

76 Cyclosporin

Oral BA in

patients

Emulsion and

microemulsion

Least dependence of BA

microemulsion on bile flow

Trull et al., 1995

77 Danazol Fed/fasted

BA in

humans

MG emulsion

BA from emulsion >

conventionalcapsule(fasted),

but similar BA

whenemulsion administered

fed or fasted

Charman et al.,

1993

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1.1. Introduction - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/6436/6/06...It involves the modification of chemical substituents groups, the physiochemical properties or prodrug