Lesson 2: Physico-chemical and

analytical properties of drugs

PharmDr. Pavol Ježko, PhD.Department of Pharmaceutical ChemistryFaculty of Pharmacy, Comenius UniversityOdbojarov 10, 832 32 Bratislava, Slovakia

Tel: + 421 250 117 221e-mail: jezko@fpharm.uniba.sk; p.jezko@gmail.com

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Why are physico-chemical

properties of drugs important?

• Approximately 30-40% of potential drugs are

later rejected for their inappropriate

pharmacokinetic (ADME) parameters

• ADME

– Absorption

– Distribution

– Metabolism

– Elimination2

Acidity and basicity of drugs• Drugs are weak organic acids (acetylsalicylic acid ) or weak organic bases (procaine), or their salts

(ephedrine hydrochloride).

• The degree of ionized drugs are in solution is highly dependent on the pH.

• Ionization of a drug has an important effect on its absorption, distribution and elimination.

• There are many examples of the alteration of pH to change these properties.

• The pH of urine may be adjusted (for example by administration of ammonium chloride or

sodium bicarbonate) in cases of overdosing with amfetamines, barbiturates, narcotics and

salicylates, to ensure that these drugs are completely ionized and ready for excretion.

• The pH of the urine may be altered to prevent ionization of a drug in cases where reabsorption is

required for therapeutic reasons.

• Sulphonamide crystalluria may also be avoided by making the urine alkaline.

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Table shows the nominal pH values of

some body fluids and sites, which are

useful in the prediction of the

percentage ionisation of drugs in vivo.

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Acid drugs (HA)

• HA + H2O H3O+ + A-

non-ionized drug ionized drug

• for diluted solutions, the dissociation constant Ka is:

[H3O +]. [A-] [molar concentrations of the reactants]

Ka = ------------------

[HA]

• Ka values can be expressed in the form of their nega@ve logarithm (pKa ):

pKa = - log Ka

% dissociated acid = 100 / 1 + 10 pKa – pH

(depending of the pH value) 5

Basic drugs (B)B + H2O BH + + OH-

unionized drug ionized drug

• for diluted solutions, the dissociation constant is:

[BH +]. [OH-] [molar concentrations of the reactants]

Kb = ------------------

[B]

• In this expression, Kb is the dissociation constant of the base and reflects the force of

the base. The equilibrium of the proton transfer, which includes the base form, can be

expressed with form of conjugate acid BH +:

BH + + H2O B + H3O +

[H3O +]. [B]

Ka = ------------------

[BH +]

• % dissociated base = 100 / 1 + 10 pH – pKa

(depending of the pH value) 6

• The pKa and pKb values provide a convenient means of comparing the strengths of

weak acids and bases.

• The lower the pKa , the stronger the acid.

• The lower the pKb , the stronger is the base.

• Both acidic and basic drugs are exactly 50% ionized at the pH of their pKa values.

• Ionic form of the drug has a positive or negative electrostatic charge and preferentially

dissolves in water (blood).

• Non-ionized drug molecule is electroneutral and is better dissolved in lipids.

• The drug passes through the hydrophobic lipid membrane in its non-dissociated form.

• The degree of ionization of the drug in cell membrane is therefore an important

parameter that determines what amount of the undissociated drug (soluble in lipids

(fats) is available for transport across lipid membrane.

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pKa can be determined by several methods:

• potentiometry

• spectrofotometric methods (ultraviolet (UV) spectrophotometry)

• using the partition coefficient (in the case of ionizable drugs)

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Theoretical methods for pKa prediction

• SPARC

– use computational algorithms based on fundamental chemical structure

theory to estimate a variety of chemical reactivity parameters (such as

ionization pKa , kinetics, heat of vaporization, diffusion coefficient, etc.).

• Jaguar -The pKa Prediction Module

– The module uses a combination of correlated ab initio quantum

chemistry, a self-consistent reaction field (SCRF) continuum treatment

of solvation, and empirical corrections to repair deficiencies in both

the ab initio and continuum solvation models.

– This combination leads to high accuracy for a wide range of organic

compounds, in conjunction with tractable computational

requirements.

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Drug lipophilicity• Is an important physico-chemical parameter that influences the distribution and destiny of drugs in the body.

• Lipophilicity is the measure of the partitioning of a compound between a lipidic and an aqueous phase.

• Lipophilicity is one of the most informative physicochemical properties in medicinal chemistry and since long

successfully used in quantitative structure – activity relationship (QSAR) studies.

• Its important role in governing pharmacokinetic and pharmacodynamic events has been extensively documented.

• Increased lipophilicity was shown to correlate with:

– poorer aqueous solubility,

– increased plasma protein binding,

– increased storage in tissues

– more rapid metabolism and elimination.

– increased rate of penetration through the skin,

– sometimes with a shorter duration of action

• Lipophilicity is also a highly important descriptor of blood – brain barrier (BBB) permeability.

• Last, but not least, lipophilicity plays a dominant role in toxicity prediction .

• partition coefficient P , is defined as the ratio of the concentrations of a neutral compound in organic and aqueous

phases of a two - compartment system under equilibrium conditions. It is commonly used in its logarithmic form,

log P . Whereas 1 - octanol serves as the standard organic phase for experimental determination, other solvents are

applied to better mimic special permeation conditions such as the cyclohexane – water system for BBB permeation. 17

• during the transport of biological systems drug

exceeds the interphase interface between polar

and lipid bi-phase

• most drugs are transported to the site of action

by blood;

• water solubility makes dissociation of the drug

and the disposition for the reactions

• drug solubility in lipids allows drug to pass

throught the lipid membranes, also the increased

concentration in the lipophilic nerve tissue18

• drug solubility in water and lipids are applied side by side,

we evaluate them in relation to each other

• lipophilicity is defined by the distribution of substances between water and

non-aqueous phase:

P = corg / caq

• P

- is the partition coefficient of the system

n-octanol - water

- informs about the ratio of drug solubility

in an aprotic environment (lipids)

and protic environment (in water)

- it informs about the amphiphilic nature of the substance in vitro

- parameter P (log P) is considered the most important lipophilic

parameter, which is fairly good correlation to the overall biological effects in

many groups of drugs 19

• In the case of ionizable compounds in the distribution of non-ionized form in the

octanol phase depends on the pH value at which was experiment maked

• Effective lipophilicity of the compound is given by the distribution coefficient D.

• Distibution coefficient (at a given pH) is a function of lipophilicity non-ionized

compounds and the degree of ionization.

• The ionization of acids and bases in the first degree of distribution coefficient

can be calculated from the equation:

• for acid HA: HA = H + + A- D = [HA]org / [HA]aq + [A-]aq

• for the base B: BH + = H + + B D = [B]org / [B]aq + [BH+]aq

• According to this hypothesis, only non-ionized form of the compound is able to

distribution in non-aqueous phase. If this condition is performed, then

compound shows a maximum value of the lipophilicity under the pKa value of the

acid and above the pKa value of the base.

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Experimental determination

partition coefficientShake - Flask Method (in combination with UV spectroscopy)

• (The method usually involves the following: solubilization of the compound in a mixture of mutually presaturated

buffered water and octanol, separation of octanol and aqueous phases, and direct measurement of the solute

concentration in both phases. )

Potentiometric Method

• (The potentiometric method for log P determination has been correlated with the shake - flask method .

• The potentiometric log P is characterized by comparing an aqueous pKa to an apparent pKa measured in the two

– phase system (generally octanol – water) using difference curve analysis .

• Therefore, the method is appropriate only for ionizable compounds with accurately determined aqueous pKa

values. )

Chromatographic Methods

• (lipophilicity can be determinated by liquid chromatography, including reversed phase, thin - layer, micellar,

reversed - phase (RP) – HPLC, RP - ion - pair and countercurrent chromatography.

• from TLC RF values , respec@vely. RM of HPLC and GC P calculated from the reten@on @mes tM)

Electrophoretic Methods

• (Microemulsion electrokinetic chromatography (MEEKC), a variation of the capillary electrophoresis method

• The method only suitable for estimating log P of neutral compounds. )21

Theoretical calculations of lipophilicity• most used methods are:

• Substructure - based Methods

• Fragmental Methods (1.)

• Atom - based Methods (2.)

• Property - based Methods (3.)

• Methods based on 3D structure representation

• Methods based on topological descriptors

• Substructure - based approaches cut molecules into fragments or down to the single -

atom level; summing the substructure contributions gives the final log P .

• Property - based approaches utilize descriptions of the entire molecule including

molecular lipophilicity potentials (MLP), topological indices or molecular properties like

charge densities, volume and electrostatic potential to quantify log P .22

Fragmental Methods• Fragmental methods cut molecules down into fragments and apply correction

factors in order to compensate for intramolecular interactions.

• Defining fragments larger than single atoms guarantees, that significant

electronic interactions are comprised within one fragment; this is a prime

advantage of using fragments. On the other hand, fragmentation can be

arbitrary and missing fragments may prevent calculation. These are the main

disadvantages.

• reductionistic approaches:

– KLOGP and KOWWIN

• CLOGP and ACD/LogP

– are based on the principle of constructionism.

• AB/LogP

– combines the advantages of both reductionistic and constructionistic approaches by using

hierarchical cluster analysis.

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Atom - based Methods• Atom - based methods cut molecules down to single atoms and commonly do not apply

correction rules.

• Since the partition coefficient is not a simple additive property, the constitutive feature

is covered by classifying huge numbers of atom types according to structural

environment. An advantage of atom - based methods is that ambiguities are avoided.

• software packages such as MOLCAD, TSAR, PROLOGP, ALOGP98 and XLOGP.

• MOLCAD, TSAR and PROLOGP are based on the original Ghose – Crippen approach .

• ALOGP98 is based on a refined version .

• XLOGP is the only atom - additive method applying corrections .

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Methods Based on 3D Structure Representation

• For two relatively immiscible solvents log P can be considered proportional to the molar Gibbs free energy

of transfer between octanol and water: Eq. (1)

• Empirical Approaches

– SPARC

• Methods Based on Quantum Chemical Semiempirical Calculations

– QLOGP

• Approaches Based on Continuum Solvation Models

– A) COSMO - RS (Full) Approach B)COSMOfrag (Fragment - based) Approach

– Ab Initio Methods

• Models Based on MD Calculations

– QikProp is based on a study which used Monte Carlo simulations to calculate 11 parameters

– QikProp does not perform MD simulations but calculates required parameters from supplied 3D

structures of molecules

• MLP Methods

– Hydrophobic Interactions (HINT)

– Calculated Lipophilicity Potential (CLIP)27

Methods Based on Topological Descriptors

• One of their main advantages is speed

• Speed renders these methods an important tool for predicting large datasets, in

particular for screening virtual combinatorial libraries.

• MLOGP

– MLOGP uses the sum of lipophilic (carbons and halogens) and hydrophilic atoms (nitrogens and

oxygens) as two basic descriptors.

• Graph Molecular Connectivity– TLOGP

• Methods Based on Electrotopological State (E - state) Descriptors– VLOGP

– ALOGPS

– CSlogP

– A_S+logP

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Drug solubility

• The solubility of drugs in water is important for oral drug absorption.

• To simplify, a drug must be soluble in the aqueous contents of the gastrointestinal lumen to

be orally absorbed.

• Drug solubility in DMSO is important in the biology testing of a compound formatted as a

DMSO stock solution.

• Solubility is the amount of a solute that can be dissolved in a specific solvent under given

conditions.

• depends on the nature of the solvent and temperature

• the degrees of drug solubility is defined by European Pharmacopoeia 8th Edition

• An important feature of the ADME properties of drug 29

• increased water solubility:

- Chemical modification of the molecule by substitution with

– Neutral hydrophilic group

– Ionizable organic acids or basis

– Preparation of salts

– Using appropriate solubilizers (benzoate, Sodium salicylate

as solubilizers for purine alkaloids)

• reduction of water solubility:

- Modification of the structure

- Design the less soluble salt

• increase the solubility lipids:

- Blocking of hydrophilic groups (eg esterification)

- The introduction of lipophilic substituents (eg alkyl, halogens)

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Drug salts

• The selection of an appropriate salt form for

a potential drug candidate is an opportunity

to modulate its characteristics to improve

bioavailability, stability, manufacturability,

and patient compliance.

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Drug salts

• Most used drug salts

– Anions

• Hydrochloride

• Citrate

• Chloride

• Bromide

• Acetate

• Maleate

• Mesylate

• Phosphate

• Sulphate

• Tartrate

• Nitrate

• Most used drug salts

– Cations

• Sodium

• Potassium

• Calcium

– Aminoacids

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Prediction of the solubility

• Drug solubility is one of the important factors, which affect the movement

of a drug from a site of administration into the blood.

• Knowing of drug solubility is important. It is well known that insufficient

solubility of drugs can lead to poor absorption [1]. Investigation of the

rate-limited steps of human oral absorption of 238 drugs (including

warfarin) has been shown [1] that the absorption of a drug is usually very

low if the calculated solubility is <0.0001 mg/L.

• 1. Y.H. Zhao, M.H. Abraham, J. Lee, A. Hersey, Ch.N. Luscombe, G. Beck, B. Sherborne, I.

Cooper, Pharm. Res. 19 (2002) 1446.

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Prediction of the solubility

• Log S – an intrinsic solubility in neutral state is indicative of a compound’s

solubility (S).

• When experimental solubilities of compound is not known, the log S

values can be calculated using ALOGPS a predictor.

• This method uses E-state indices as descriptors and a neural network [2]

as the modeling ‘‘engine”.

• Another methods for prediction of solubility:

– AC logS

– AB/logS

• [2] I.V. Tetko, V.Y. Tanchuk, T.N. Kasheva, A.E.P. Villa, J. Chem. Inf. Comput. Sci. 41 (2001) 1488.

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