ISSN:1748 0132 © Elsevier Ltd 2007JUNE 2007 | VOLUME 2 | NUMBER 322

Magnetic nanoparticles for drug deliveryControlled release of drugs from nanostructured functional materials, especially nanoparticles (NPs), is attracting increasing attention because of the opportunities in cancer therapy and the treatment of other ailments. The potential of magnetic NPs stems from the intrinsic properties of their magnetic cores combined with their drug loading capability and the biochemical properties that can be bestowed on them by means of a suitable coating. Here we review the problems and recent advances in the development of magnetic NPs for drug delivery, focusing particularly on the materials involved.

Manuel Arruebo, Rodrigo Fernández-Pacheco, M. Ricardo Ibarra, and Jesús Santamaría*

Nanoscience Institute of Aragon (INA), Pedro Cerbuna 12, University of Zaragoza, 50009 Zaragoza, Spain

*E-mail: iqcatal@unizar.es

NPs are submicron moieties (between 1 nm and 100 nm according

to the usual definition, although there are examples of NPs

several hundreds of nanometers in size) made of inorganic or

organic (e.g. polymeric) materials, which may or may not be

biodegradable. Their importance relates to the fact that the

characteristics of NPs are different from those of bulk materials of

the same composition, which is mainly because of size effects, the

magnetic and electronic properties, and the role played by surface

phenomena as the size is reduced.

Preparation methods for NPs generally fall into the category of so-

called ‘bottom-up’ methods, where nanomaterials are fabricated from

atoms or molecules in a controlled manner that is thermodynamically

regulated by means such as self-assembly1. Some biomedical

applications require core-shell magnetic NPs. They consist of a metal

or metallic oxide core, encapsulated in an inorganic or a polymeric

coating that renders the particles biocompatible, stable, and may serve

as a support for biomolecules. Their magnetic properties enable these

particles to be used in numerous applications, belonging to one or

more of the following groups:

(i) Magnetic contrast agents in magnetic resonance imaging (MRI)2;

(ii) Hyperthermia agents, where the magnetic particles are heated

selectively by application of an high frequency magnetic field.

(e.g. in thermal ablation/hyperthermia of tumors3); and

(iii) Magnetic vectors that can be directed by means of a magnetic

field gradient towards a certain location, such as in the case of the

targeted drug delivery4.

The scientific community is seeking to exploit the intrinsic properties

of magnetic NPs to obtain medical breakthroughs in diagnosis, and

drug delivery. Perhaps the most promising applications relate to the

diagnosis and treatment of cancer.

Magnetic nanoparticles for drug delivery REVIEW

JUNE 2007 | VOLUME 2 | NUMBER 3 23

Even though, according to the American Cancer Society, cancer

deaths in the US have dropped for a second straight year, which is

attributed to the decrease in smoking rates and to earlier detection and

more effective treatment of tumors5, cancer is still one of the leading

causes of death in developed countries. Conventional treatments,

including surgery, radiation, chemotherapy, and biologic therapies

(immunotherapy) are limited by the accessibility to the tumor, the risk

of operating on a vital organ, the spread of cancer cells throughout the

body, and the lack of selectivity toward tumor cells. Immunotherapy is

still relatively recent, and is most likely to be applied to small tumors,

since its effectiveness seems to decrease for more advanced stages

of cancer. Multimodal therapy that uses radiotherapy, chemotherapy,

immunotherapy, and other forms of treatment in combination with

surgery provides a better chance of survival6.

The potential of drug delivery systems based on the use of nano-

and microparticles stems from significant advantages such as: (i) the

ability to target specific locations in the body; (ii) the reduction of the

quantity of drug needed to attain a particular concentration in the

vicinity of the target; and (iii) the reduction of the concentration of

the drug at nontarget sites7 minimizing severe side effects. All these

benefits justify the exponential growth in the number of publications

dealing with NPs for drug delivery applications (Fig. 1).

NPs can act at the tissular or cellular level. The latter implies that

they can be endocytosed or phagocytosed (i.e. by dendritic cells,

macrophages) resulting in internalization of the NP. In this process, the

NP can reach beyond the cytoplasmatic membrane and, in some cases,

also beyond the nuclear membrane (i.e. transfection applications).

Tumor targeting with magnetic NPs may use passive or active

strategies. Passive targeting occurs as a result of extravasation of

the NPs at the diseased site (tumor) where the microvasculature is

hyperpermeable and leaky, a process aided by tumor-limited lymphatic

drainage. Combined, these factors lead to the selective accumulation

of NPs in tumor tissue, a phenomenon known as enhanced permeation

and retention (EPR)8. The majority of solid tumors exhibit a vascular

pore cut-off size between 380 nm and 780 nm, although vasculature

organization may differ depending on the tumor type, its growth rate,

and microenvironment9. Apart from tumors, size-dependent removal

of NPs is a common occurrence in healthy capillaries. Table 110 shows

the morphological pore sizes contributing to diffusive permeability

in the capillaries of the human vascular system. It can be seen that,

from the delivery point of view, there is practically no limitation as

the diameters of typical NPs are well below that of the narrowest

capillaries. Instead, the main limitation concerns the residence time

of NPs in the bloodstream. Thus, the use of conventional NPs for

drug delivery by passive targeting would be limited to tumors in

mononuclear phagocyte system (MPS) organs (liver, spleen, and bone

Fig. 1 Temporal evolution in the number of scientific papers published

involving drug delivery using NPs. (Source: ISI Web of Knowledge © The

Thomson Corporation. Search terms: ‘drug delivery’ and ‘nanoparticles’. Date

of search: December 2006.)

Table 1 Relevant sizes regarding particle distribution through and removal from the capillaries of the human vascular

system.

Particle removal* Tight-junction capillaries < 1 nm Central nervous system, blood-brain barrier

Continuous capillaries ~ 6 nm Tissues such as muscle, skin, and lung

Fenestrated capillaries ~ 50-60 nm Kidney, intestine, and some endocrine and exocrine glands

Sinusoidal capillaries ~ 100-1000 nm Liver, spleen, and bone marrow

Particle delivery Arteriole radius 0.005-0.07 mm Circulatory system. Particles supplied by intravenous

Artery radius 0.08-7.5 mm administration. Elimination involves opsonization and removal

Venule radius 8-100 µm by monocytes in blood

*It is noted that this table expresses only the morphological pores contributing to diffusive permeability. Actual transcapillary exchanges are

modified by vesicular transports, which are able to internalize particles with sizes up to 20-30 nm.

(Adapted with permission from10. © 1999 Elsevier.)

Size System/Organ

REVIEW Magnetic nanoparticles for drug delivery

JUNE 2007 | VOLUME 2 | NUMBER 324

marrow). Addressing other tumoral tissues does not seem feasible

without active targeting strategies because of the short circulation

times involved and the low concentration of NPs that is achieved in

the tumor area (despite the EPR effect), leading to drug concentrations

below the therapeutic level11.

Active targeting is based on the over or exclusive expression

of different epitopes or receptors in tumoral cells, and on specific

physical characteristics. Thus, vectors sensitive to physical stimuli (e.g.

temperature, pH, electric charge, light, sound, magnetism) have been

developed and conjugated to drugs. Alternatively, active targeting

may be based on over-expressed species such as low molecular weight

ligands (folic acid, thiamine, sugars), peptides (RGD, LHRD), proteins

(transferrin, antibodies, lectins), polysaccharides (hyaluronic acid),

polyunsaturated fatty acids, peptides, DNA, etc.

Different moieties including dendrimers, micelles, emulsions,

nanoparticulated drugs, and liposomes are used to target specific

areas in the body (Fig. 2)12,13. The NPs must be endowed with the

specific characteristics needed to reach a given target, which means

attaining a suitable combination of nature, size, way of conjugating

the drug to the NP (attached, adsorbed, encapsulated), surface

chemistry, hydrophilicity/hydrophobicity, surface functionalization,

biodegradability, and physical response properties (temperature, pH,

electric charge, light, sound, magnetism). Among these, size is the main

factor affecting the fate of NPs in passive targeting processes using the

permeability of the capillary vessels, as discussed above.

Here we shall concentrate on the therapeutic applications of

magnetic drug targeting using NPs. Hyperthermia/thermal ablation

will not be addressed, although the general implications between

both therapies based on magnetism can be inferred. Applications in

diagnosis, where magnetic NPs are widely used as contrast agents, will

not be addressed here either.

Magnetic drug deliveryThe development of magnetic drug deliveryAny overview on drug delivery should start with the deserved

recognition of Paul Ehrlich (1854-1915), who proposed that if an agent

could selectively target a disease-causing organism, then a toxin for

that organism could be delivered along with the agent of selectivity.

Hence, a ‘magic bullet’ would be created able to kill the targeted

organism exclusively. Ehrlich received the 1908 Nobel Prize in Medicine

for his work in the field of immunity, and the magic bullet idea was

even used as a script for the 1940 movie Dr. Ehrlich’s Magic Bullet.

Since then, various strategies have been proposed to deliver a drug

to the vicinity of a tumor including, as mentioned above, the use of

vectors sensitive to physical stimuli and tumor-recognition moieties

conjugated to a drug.

Fig. 2 NP systems for drug delivery applications. (Adapted with permission from119 © 2005 Elsevier; and from120 © 2005 PharmaVentures Ltd.)

Magnetic nanoparticles for drug delivery REVIEW

JUNE 2007 | VOLUME 2 | NUMBER 3 25

Prior to their use for drug delivery, magnetic microparticles were

proposed as contrast agents for localized radiation therapy14 and to

induce vascular occlusion of the tumors (antiangiogenic therapy)15,16.

Freeman et al.17 proposed in 1960 that magnetic NPs could be

transported through the vascular system and concentrated in a specific

part of the body with the aid of a magnetic field.

The use of magnetic micro- and NPs for the delivery of

chemotherapeutics has evolved since the 1970s. Zimmermann and

Pilwat18 in 1976 used magnetic erythrocytes for the delivery of

cytotoxic drugs. Widder et al.19 described the targeting of magnetic

albumin microspheres encapsulating an anticancer drug (doxorubicin)

in animal models. In the 1980s, several authors developed this

strategy to deliver different drugs using magnetic microcapsules and

microspheres20-23. In 1994, Häfeli et al.24 prepared biodegradable

poly(lactic acid) microspheres that incorporated magnetite and the

beta-emitter 90Y for targeted radiotherapy, and successfully applied

them to subcutaneous tumors25.

However, all these initial approaches were microsized. Magnetic

NPs were used for the first time in animal models by Lübbe et al.26. In

1996, the first Phase I clinical trial was carried out by the same group

in patients with advanced and unsuccessfully pretreated cancers using

magnetic NPs loaded with epirubicin27. However, in that first trial,

more than 50% of the NPs ended up in the liver.

Since then, a number of groups around the world have synthesized

magnetic vectors and shown potential applications. Different start-ups

now manufacture magnetic micro- and NPs, which are used in MRI,

magnetic fluid hyperthermia, cell sorting and targeting, bioseparation,

sensing, enzyme immobilization, immunoassays, and gene transfection

and detection systems.

FeRx, Inc. (founded in 1997) produced doxorubicin-loaded

magnetic NPs consisting of metallic Fe ground together with activated

carbon28,29. A Phase II clinical study in patients with primary liver

cancer was conducted using these NPs, although the trial was not

successful. Chemicell GmbH currently commercializes TargetMAG-

doxorubicin NPs involving a multidomain magnetite core and a

cross-linked starch matrix with terminal cations that can be reversibly

exchanged by the positively charged doxorubicin. The particles have

a hydrodynamic diameter of 50 nm and are coated with 3 mg/ml

doxorubicin30. These NPs loaded with mitoxantrone have already

been used in animal models with successful results31. Chemicell also

commercializes FluidMAG® for drug delivery applications. Magnetic

NP hydro-gel (MagNaGel®) from Alnis Biosciences, Inc. is a material

comprising chemotherapeutic agents, Fe oxide colloids, and targeting

ligands32.

In summary, for magnetic targeting, a drug or therapeutic

radionuclide is bound to a magnetic compound, introduced in the body,

and then concentrated in the target area by means of a magnetic field

(using an internally implanted permanent magnet or an externally

applied field). Depending on the application, the particles then release

the drug or give rise to a local effect (irradiation from radioactive

microspheres or hyperthermia with magnetic NPs)33. Drug release

can proceed by simple diffusion or take place through mechanisms

requiring enzymatic activity or changes in physiological conditions

such as pH, osmolality, or temperature34; drug release can also be

magnetically triggered from the drug-conjugated magnetic NPs.

Drug delivery with magnetic NPsDifferent organic materials (polymeric NPs, liposomes, micelles) have

been investigated as drug delivery nanovectors using passive targeting,

active targeting with a recognition moiety (e.g. antibody), or active

targeting by a physical stimulus (e.g. magnetism in magnetoliposomes).

However, these organic systems still present limited chemical and

mechanic stability, swelling, susceptibility to microbiological attack,

inadequate control over the drug release rate35, and high cost.

Polymer NPs also suffer from the problem of high polydispersity.

Synthesis produces particles with a broad size distribution and irregular

branching, which could lead to heterogeneous pharmacological

properties. One alternative is to use dendrimers, which have a

monodisperse character and globular architecture resulting from

their stepwise synthesis and can be purified at each step of growth36.

Visualization of dendrimers requires tagging with a specific moiety

(i.e. a fluorophore or metal). A major drawback of dendrimers and

dendritic polymers, however, is their high cost. The preparation

of dendritic polymers that circulate in the blood long enough to

accumulate at target sites but that can also be removed from the body

at a reasonable rate to avoid long-term accumulation also remains a

challenge.

Passive targeting using drug-conjugated dendrimers and dendritic

polymers has been widely studied, mainly using the EPR effect.

Therapies based on active targeting, such as antibody-conjugated

dendrimers, constitute a promising alternative in view of the potential

of antibodies for selective targeting37. Because of the disadvantages

of organic NPs for drug delivery, inorganic vectors constitute an

interesting option and are the subject of intense research. Some

examples of inorganic magnetic NPs will be given below.

The main advantages of magnetic (organic or inorganic) NPs are

that they can be: (i) visualized (superparamagnetic NPs are used in

MRI); (ii) guided or held in place by means of a magnetic field; and

(iii) heated in a magnetic field to trigger drug release or to produce

hyperthermia/ablation of tissue. It is important to point out that the

latter capability is not restricted to magnetic NPs, but also to other

particles capable of absorbing near-infrared, microwave, and ultrasound

radiation.

Depending on the synthesis procedure, magnetic NPs or

nanocapsules can be obtained. We refer to NPs when the drug is

covalently attached to the surface or entrapped or adsorbed within

the pores of the magnetic carrier (polymer, mesoporous silica, etc.).

Nanocapsules (‘reservoirs’) designate magnetic vesicular systems where

REVIEW Magnetic nanoparticles for drug delivery

JUNE 2007 | VOLUME 2 | NUMBER 326

the drug is confined to an aqueous or oily cavity, usually prepared by

the reverse micelle procedure, and surrounded by an organic membrane

(magnetoliposomes) or encapsulated within a hollow inorganic

capsule35.

The key parameters in the behavior of magnetic NPs are related to

surface chemistry, size (magnetic core, hydrodynamic volume, and size

distribution), and magnetic properties (magnetic moment, remanence,

coercivity). The surface chemistry is especially important to avoid the

action of the reticuloendothelial system (RES), which is part of the

immune system, and increase the half-life in the blood stream. Coating

the NPs with a neutral and hydrophilic compound (i.e. polyethylene

glycol (PEG), polysaccharides, dysopsonins (HSA), etc.) increases the

circulatory half-life from minutes to hours or days. Another possibility

is to reduce the particle size; however, despite all efforts, complete

evasion of the RES does not seem feasible38 and unwanted migration

to other areas in the body could cause toxicological problems.

In addition to cancer treatment, magnetic NPs can also be used

in anemic chronic kidney disease and disorders associated with

the musculoskeletal system (i.e. local inflammatory processes, side

effects) (Fig. 3). For those disorders, superparamagnetic Fe oxide NPs

(SPION), in conjunction with external magnetic fields, seem a suitable

alternative for drug delivery to inflammatory sites by maintaining

appropriate local concentrations while reducing overall dosage and side

effects39.

Limitations of magnetic drug deliverySince the magnetic gradient decreases with the distance to the target,

the main limitation of magnetic drug delivery relates to the strength

of the external field that can be applied to obtain the necessary

magnetic gradient to control the residence time of NPs in the desired

area or which triggers the drug desorption. Permanent Nd-Fe-B

magnets in combination with SPION, which have excellent magnetic

properties, can reach effective magnetic field depths up to 10-15 cm

in the body39. However, it must be noted that the magnetic carriers

accumulate not only at the desired site but also throughout the cross-

section from the external source to the depth marking the effective

field limit. Obviously, the geometry of the magnetic field is extremely

important and must be taken into account when designing a magnetic

targeting process.

As a means to elude the limitations of using external magnetic

fields, internal magnets can be located in the vicinity of the target by

using minimally invasive surgery40. Several studies have simulated the

interaction between a magnetic implant and magnetic NPs, enabling

drug delivery6,41,42. In addition, work in several laboratories40,43,44 is

addressing targeted drug delivery with magnetic implants.

Another limitation relates to the small size of NPs, a requisite

for superparamagnetism, which is in turn needed to avoid magnetic

agglomeration once the magnetic field is removed (see below). A small

size implies a magnetic response of reduced strength, making it difficult

to direct particles and keep them in the proximity of the target while

withstanding the drag of blood flow45. Targeting is likely to be more

effective in regions of slower blood velocity, and particularly when the

magnetic field source is close to the target site.

As for all biomedical applications, limitations also arise in

extrapolating from animal models to humans. There are many

physiological parameters to consider, ranging from differences in

weight, blood volume, cardiac output, and circulation time to tumor

volume/location/blood flow, complicating the extrapolation of data

obtained in animal models46,47. Related to this point is the fact

that studies on toxicity (not only direct toxicity, but also toxicity

Magnetic nanoparticles

Therapy Diagnosis

In vivo

MRI

In vitro

Sensing

Cell sorting

Bioseparation

Enzymeimmobilization

Immunoassays

Transfection

Purification

Drugdelivery

Hyperthermia/thermalablation

Radiotherapycombined with

MRI

Musculoskeletalsystem associated

diseases

Anemic chronic *kidney disease

Fig. 3 Biomedical applications of magnetic NPs.

*Ferumoxytol® (Advanced Magnetics, Inc.) is in Phase III multicenter clinical studies for use as an intravenous Fe replacement therapeutic for patients with anemic

chronic kidney disease.

Magnetic nanoparticles for drug delivery REVIEW

JUNE 2007 | VOLUME 2 | NUMBER 3 27

of the degradation products and induced responses48) and the fate

of magnetic carriers are insufficient and, in many cases, there is

insufficient characterization.

Finally, state-of-the-art magnetic drug delivery seems mainly

applicable to well-defined tumors, as treatment of metastatic

neoplasms and small tumors in the early stages of their growth

still remains a challenge. Treating emerging tumors will involve

the development of a new generation of seek-and-destroy NPs,

which specifically recognize small clusters of cancer cells and carry

the necessary elements (drugs or hyperthermia agents) for their

destruction. A strong interest continues in this field given the capability

of NPs to access tumors in regions where conventional surgery cannot

be applied.

Tailoring magnetic NPsEssential requisites

Magnetic NPs for biomedical applications must be endowed with

the specific characteristics required. As mentioned above, the first

requirement is often superparamagnetism.

Superparamagnetism occurs in magnetic materials composed of

very small crystallites (threshold size depends on the nature of the

material, for instance, Fe-based NPs become superparamagnetic at

sizes <25 nm49). In a paramagnetic material, the thermal energy

overcomes the coupling forces between neighboring atoms above the

Curie temperature, causing random fluctuations in the magnetization

direction that result in a null overall magnetic moment. However,

in superparamagnetic materials, the fluctuations affect the direction

of magnetization of entire crystallites. The magnetic moments of

individual crystallites compensate for each other and the overall

magnetic moment becomes null. When an external magnetic field is

applied, the behavior is similar to paramagnetism except that, instead

of each individual atom being independently influenced by an external

magnetic field, the magnetic moment of entire crystallites aligns with

the magnetic field (Fig. 4).

In large NPs, energetic considerations favor the formation of

domain walls. However, when the particle size decreases below a

certain value, the formation of domain walls becomes unfavorable

and each particle comprises a single domain. This is the case for

superparamagnetic NPs. Superparamagnetism in drug delivery is

necessary because once the external magnetic field is removed,

magnetization disappears (negligible remanence and coercivity, see

Fig. 4), and thus agglomeration (and the possible embolization of

capillary vessels) is avoided.

Another key requirement is the biodegradability or intact excretion

of the magnetic core. Thus, SPION are considered to be biodegradable

with Fe being reused/recycled by cells using normal biochemical

pathways for Fe metabolism50,51. For nonbiodegradable cores, a specific

coating is needed to avoid exposure (and possible leaching) of the

magnetic core and to facilitate intact excretion through the kidneys,

so that the half-life of the agent in the blood is determined by the

glomerular filtration rate (e.g. contrast agents based on gadolinium)51.

Coatings on magnetic NPs

The coatings on magnetic NPs often serve multiple purposes. Their

role in reducing leaching of the cores has already been mentioned. The

coating also often facilitates the stabilization of NPs in an environment

with a slightly alkaline pH or a significant salt concentration. For

instance, the isoelectrical point of SiO2 is reached at pH 2-3, meaning

that silica-coated NPs are negatively charged at the pH of blood,

inducing electrostatic repulsion that helps avoid aggregate formation.

Silica coatings also have additional advantages. On the one hand,

the external surface of silica coatings can be functionalized to allow

the binding of biomolecules. This is mainly related to the presence

of hydroxyl surface groups in significant concentrations that provide

intrinsic hydrophilicity and allow surface attachment by covalent

Fig. 4 Hysteresis loops (magnetization versus applied magnetic field) characteristic of ferromagnetic and superparamagnetic NPs. For comparison, para- and

diamagnetic behavior are also shown. The figure also indicates the values of the remanence, Mr, and coercive field, Hc.

REVIEW Magnetic nanoparticles for drug delivery

JUNE 2007 | VOLUME 2 | NUMBER 328

linkage of specific biomolecules. On the other hand, the internal

porosity of silica can be used to host a specific drug, a feat achievable

while avoiding the unwanted physical adsorption of larger molecules.

Thus, according to Ambrose and Fritz52, the smallest major protein in

serum is serum albumin, with a molecular mass of approximately

66 000 Da and an effective spherical radius of ~40 Å. This means that

serum albumin, and larger molecules, would be excluded from the

channels of microporous and mesoporous (with pore sizes <4 nm)

coatings (e.g. silica, carbon, zeolites) on magnetic cores. Finally, silica

and other microporous inorganic materials are heat resistant, with

high surface areas and good mechanical strength. Figs. 5 and 6 show

different types of inorganic coatings on metal cores developed in our

laboratory.

In addition, coatings play an essential role in retarding clearance

by the RES. Depending on their size, surface functionalization, and

hydrophilicity, a rapid uptake of uncoated NPs by the mononuclear

phagocyte system (MPS) is likely after systemic administration,

followed by clearance to the liver, spleen, and bone marrow. Different

proteins (antibodies) of the blood serum (opsonins) bind to the surface

of foreign bodies, accelerating phagocitation of the particles. To avoid

this, biodegradable (e.g. dextran) and nonbiodegradable organic and

inorganic coatings can be used as a means to retard detection and

uptake by the macrophages of the RES. Perhaps the most widely used

coating for this purpose is PEG, a linear neutral polyether, whose

attachment to NP surfaces provides a ‘stealth’ shielding effect, delaying

the action of the RES53. PEG shows little toxicity and immunogenicity,

and intact excretion is possible, either via the kidneys (for PEG <30

kDa) or in the feces (for PEG >20 kDa)54. Avoiding detection by the

RES stems from the protein-resistant character of PEGylated surfaces,

which has been ascribed to the combination of a low interfacial

energy in water and the steric stabilization effect55. Unfortunately, the

‘immunostealthing’ function provided by PEG is frequently concurrent

with the loss of biomolecular targeting capabilities; therefore, it is

necessary to optimize the coating.

The nature of the coating is also important in those cases where the

surface functionalization might cause hydrogen bonding and, therefore,

agglomeration of NPs. For such situations, surface modification can

reduce the aggregation and increase the stability of NPs in body fluids.

A wide variety of molecules has been loaded onto organic

and inorganic shells, e.g. by chemical functionalization or physical

absorption. The list includes tumor-recognition moieties such as

antibodies in ‘smart’ contrast agents56-58 and cell-penetrating

peptides for MRI applications59,60; and enzymes61, toxins62, genes63-68

(transfection), growth factors69,70, radionucleotides4,71-73, folic acid74,

and drugs (mitoxantrone4,75, tamoxifen76, cefradine77, doxorubicin78-83,

ammonium glycyrrhizinate84, fludarabine85, danorubicin86, cisplatin

and gemcitabine87, pingyangmycin88, nonsteroidal anti-inflammatory

drugs89, amethopterin90,91, mitomycin92, paclitaxel21,93, diclofenac

sodium94,95, and adriamycin96) for drug delivery applications.

FateThe distribution of NPs and their loads throughout the body depends

on numerous physicochemical factors: size of particles, toxicity, surface

charge, capacity for protein adsorption, surface hydrophobicity, drug

loading and release kinetics, stability, degeneration of carrier systems,

hydration behavior, electrophoretic mobility, porosity, specific surface

Fig. 6 HRTEM photographs of: (a) magnetite NPs encapsulated in a silica

matrix; and (b) EFTEM color map of Au NPs encapsulated in a silica matrix.

Fig. 5. High resolution transmission electron microscopy (HRTEM) images of

(a) magnetite NP encapsulated in silica; (b) magnetite NPs embedded in a

zeolitic (aluminosilicate) matrix; (c) Fe NPs encapsulated in silica (energy-

filtering TEM, EFTEM, color map); (d) magnetite NP encapsulated in graphite.

(b)(a)

(c) (d)

(b)

(a)

Magnetic nanoparticles for drug delivery REVIEW

JUNE 2007 | VOLUME 2 | NUMBER 3 29

characteristics, density, crystallinity, contact angle, and molecular

weight39. Nevertheless, the fate (and also the possible toxicity) of

magnetic NPs also depends strongly on the dose and administration

route (oral or parenteral: including delivery routes such as intravenous,

pulmonary, transdermal, and ocular, in addition to less conventional

routes, e.g. when used as scaffold coatings). Some of the implications

for the three most common ways of administration are now discussed.

Intravenous administrationThe general rule for magnetic NPs in parenteral applications it that

the carrier is nontoxic, nonimmunogenic, and of a size that avoids

embolization of capillary ducts. Once a NP enters the bloodstream,

opsonization processes activate the RES system response. Circulating

mononuclear phagocytes (monocytes) clear the NPs to the liver,

spleen, and bone marrow where resident cells (e.g. Kupffer cells in the

liver) capture the NPs prior to degradation, if possible. Depending on

biodegradability and size, some of the NPs present in the lysosomal

vesicles of Kupffer cells may be incorporated into the bile and be

removed in the feces. Other NPs will be filtered by the kidneys and

incorporated into the urine. In general, smaller NPs are subject to rapid

renal elimination, while larger ones show uptake by the liver, spleen,

and bone marrow (Fig. 7)97-103. Large particles will be removed by cells

capable of phagocytosis (i.e. by macrophages or dendritic cells), while

small particles can be removed by cells capable of endocytosis (i.e. by

B and T lymphocytes). Finally, if the magnetic NPs are biodegradable,

the decomposition products can be taken up by any cell by means of

pinocytosis.

According to Neuberger et al.39, magnetic particles smaller than

4 µm are removed by cells of the RES, mainly in the liver (60-90%) and

spleen (3-10%). Particles larger than 200 nm are usually filtered to the

spleen, whose cut-off point extends up to 250 nm104, while particles up

to 100 nm are mainly phagocytosed through liver cells. In general, the

larger the particles are, the shorter their plasma half-life-period101. This

clearance of particles by Kupffer cells can be, on the other hand, useful

for the treatment of liver diseases, such as cancer or leishmania105,

tuberculosis, listeria, leprosy, etc.; although it is important to consider

that it would simultaneously entail the depletion of a significant

number of the patient’s own defense cells.

Subcutaneous or intratumoral administrationUnlike water-soluble molecules, which are rapidly absorbed through

the blood capillary walls and pass into the circulatory system, small

particles injected locally infiltrate into the interstitial spaces around

the injection site and are gradually absorbed by the lymphatic capillary

system11. For this reason, subcutaneously or locally injected NPs

can be used for lymphatic targeting, i.e. as a tool for chemotherapy

for lymphatic tumors, although this route is rarely used in clinical

practice as it is not useful for targeting metastatic tumors. In this case,

magnetism is not required but may be a useful property to hold the

particles in place or to enable hyperthermia. As a general rule, colloidal

carriers aimed at regional lymph nodes through subcutaneous injection

need to be small (60 nm or less)106. Another limitation relates to

intratumoral pressure gradients caused by the fast cell proliferation in

solid tumors.

Oral administrationSeveral publications describe the use of magnetic NPs coated with an

organic shell as oral drug delivery vectors107, although most concern

Monocrystallineiron oxide nanoparticles< 10 nm

Ultrasmall superparamagneticiron oxide nanoparticles5-20 nm

Ferromagneticparticles200 nm Non-biodegradable

inorganic and polymericmicrometric spheres

Large superparamagneticiron oxide nanoparticles> 40 nm

Plasma half life = 2 hAccumulation in lymphnodes. Excreted by urineand feces

Removal by liver (80%)and spleen (15%)

Opsonization.Removal by liver, spleen,lungs, and bone marrowmacrophages

Blo

od r

esid

ence

tim

e

Particle size

101-103

100

98

97

99

Fig. 7 Qualitative diagram showing the evolution of blood residence time with particle size.

REVIEW Magnetic nanoparticles for drug delivery

JUNE 2007 | VOLUME 2 | NUMBER 330

magnetic NPs used as MRI contrast agents for the gastrointestinal

tract. The main problem here is that oral delivery of peptides and

proteins is hampered by their degradation in gastrointestinal acid,

low absorption, first-pass metabolism by the liver, and a significant

initial increase in drug concentration. Feng et al.108 describe the fate

of chemotherapeutical NPs in oral delivery: particles under 5 µm can

be removed via lymphatic drainage, particles up to 500 nm can cross

the membrane of epithelial cells through endocytosis, and particles

less than 50 nm can achieve paracellular passage between intestinal

epithelial cells.

Safety: influence of the magnetic fieldAlthough all the components of the body are either dia-, para-,

superpara-, ferri-, or ferromagnetic, the magnetic fields required to

produce obvious effect in the body are very large. Even red blood cells,

which each contain micrograms of the Fe protein hemoglobin, show a

relatively low response to large fields or steep field gradients, although

this low value is enough to be used in functional MRI (fMRI). The other

natural Fe-containing compounds in the body are hemosiderin, ferritin,

transferrin, and the cytochromes.

According to Schenck109, the US Food and Drug Administration

(FDA) initially considered applications for approval to market MRI

scanners on a case-by-case evaluation of the safety and efficacy

information provided. Based on positive clinical and safety experience,

the FDA classified magnets with field strength of less than 2 T as

nonsignificant risk devices in 1987. Further positive experiences led the

FDA to increase this threshold to 4 T in 1996 and again in 2003 (for

adults) to 8 T. Even though experiments with strong static magnetic

fields (8 T) have been shown to reduce the flow rate of human blood

by 30% in in vitro tests110 and it has been reported that magnetic fields

above 3 T might affect the normal behavior of erythrocytes, recent

studies evaluating human subjects for adverse effects in physiological

or neurocognitive functions resulting from exposure to static magnetic

fields (up to 8 T) from MRI systems have not shown any clinically

relevant effects111. According to Kangarlu and Robitaille112, within the

decade, human imaging at fields in excess of 10 T will most probably

be achieved and such projects are now being planned.

ToxicityWhen discussing the toxicity of NPs, generalization becomes difficult

because their toxicity depends on numerous factors including the dose,

chemical composition, method of administration, size, biodegradability,

solubility, pharmacokinetics, biodistribution, surface chemistry,

shape, and structure, to name but a few. With NPs, as with any new

biomedical discovery, the risk-benefit trade-off must be considered to

assess whether the risks can be justified. In general, the size, surface

area, shape, composition, and coating of an NP are the most important

characteristics regarding cytotoxicity113, and modifications of the NP

surface are a key tool to minimize toxicological effects114.

It is well documented that the large surface-to-volume ratio of

all nanosized particles can potentially lead to unfavorable biological

responses if they are inhaled and subsequently absorbed via the lung

or swallowed and then absorbed across the gastrointestinal tract115.

Interestingly, it has also been reported that, in 20-100 mg/ml

concentrations, large magnetic particles show higher cytotoxicity

than smaller ones even after normalizing for surface area116 despite

the lower surface-to-volume ratio, although it is difficult to perform

comparable experiments with differently sized particles. In any case,

toxicity studies should consider not only acute toxicity but also that of

degradation products, the possible stimulation of cells with subsequent

release of inflammatory mediators39, and long term toxicity.

For a magnetic carrier with potential as a drug delivery vector, it is

necessary, at the very least, to analyze its: (i) toxicity (acute, subacute,

and chronic toxicity, teratogenicity, and mutagenicity) in cellular

and animal models; (ii) hematocompatibility; (iii) biodegradation

(whenever possible); (iv) immunogenicity; and (v) pharmacokinetics

(body distribution, metabolism, bioavailability, elimination, organ-

specific toxicity) before the start of preclinical testing. The contrast

agents based on magnetic NPs currently on the market, either

involving superparamagnetic Fe oxides (i.e. Feridex®, Endorem™,

GastroMARK®, Lumirem®, Sinerem®, or Resovist®) or paramagnetic

metals encapsulated in a chelating agent (i.e. Magnevist®, Dotarem®,

Gadovist®, Teslacan®) have satisfied current regulations regarding use

in patients. The same applies to the magnetic drug delivery systems

that have already been commercialized, MagNaGel®32 and FluidMAG®

and TargetMAG® (Table 2).

Recently, the US Environmental Protection Agency has started

to regulate a large class of products made with Ag NPs using the

legislation developed for pesticides. This is the first federal restriction

to focus largely on nanotechnology117.

Perspectives and future challengesIn therapy, we are witnessing the early use of magnetic NPs as drug

delivery vectors and as tools for hyperthermia/thermal ablation.

Magnetic drug delivery constitutes a promising technology to treat

cancer, and several products are already on the market. The limitations

inherent in the use of external magnetic fields can, in some cases, be

circumvented by means of internal magnets located in the proximity

of the target by minimally invasive surgery40,43,44,98,118. Magnetic fluid

hyperthermia/thermal ablation is also promising and is currently being

applied (i.e. MagForce Nanotecnologies AG), but is limited by the fact

that the tumor needs to be localized. This route, therefore, cannot be

used in preventive medicine, or for treating early-stage tumors.

The greatest therapeutic potential is probably associated with

applications involving ‘intelligent’ particles with a magnetic core

(to direct the particles to the vicinity of the target and also for

hyperthermia or for temperature-enhanced release of the drug), a

recognition layer (to which suitable receptors are attached), and a

Magnetic nanoparticles for drug delivery REVIEW

JUNE 2007 | VOLUME 2 | NUMBER 3 31

therapeutic load (adsorbed inside the pores or hosted within internal

cavities of the particles). The challenges are formidable, especially

those related to the development of suitable recognition layers. Not

only must useful recognition moieties be identified and attached to the

particles, but they must be loaded to a high density while maintaining

their desired characteristics.

Finally, as already noted, the biomedical uses of magnetic NPs

are not restricted to drug delivery, and indeed new applications for

magnetic NPs are also likely in MRI, where contrast agents could be

tagged with a recognition moiety, cell sorting/targeting, bioseparation,

sensing, enzyme immobilization, immunoassays, and gene transfection/

detection systems.

AcknowledgmentsSupport from the Spanish Nanoscience Action NAN200409270-C3-1/2 and

from the Consolider CSD2006-00012 and Ciber Ingenio 2010 CB06/01/0026

programs, is gratefully acknowledged. MA acknowledges support of a contract

from the Juan de la Cierva program (project PPQ2003-04986). The authors

also gratefully acknowledge the Serveis Cientificotècnics of the University

of Barcelona for the use of the TEM, and Jordi Arbiol i Cobos for the TEM

photographs and helpful discussions.

Table 2 Some companies involved in the development and production of magnetic micro- and nanoparticles.

Company Application Website

Bangs Laboratories, Inc. Cell separation, DNA and RNA purification, immunoassays www.bangslabs.com

Polysciences, Inc. Magnetic separation, cell sorting, nucleic acid purification, flow cytometry,

calibration, immunoassay, fluorescent microscopy, diagnostic assays

www.polysciences.com

Micromod

Partikeltechnologie GmbH

Drug delivery, biomagnetic separation, nucleic acid purification www.micromod.de

Guerbet SA X-ray and MRI contrast agents www.guerbet.com

Ademtech SA Cell sorting, biomagnetic separation www.ademtech.com

Advanced Magnetics, Inc. Treatment of anemia, MRI contrast agents www.advancedmagnetics.com

Invitrogen Corp. Immunoassay and nucleic acid in vitro diagnostics, DNA and RNA

isolation, protein purification, cell separation and expansion, food and

environmental testing

www.invitrogen.com

Estapor (Merck & Co. Inc.) Chemiluminescent and radio-immunoassays, cell separation, protein

purification, immuno-precipitation, bacteria detection, immuno-

chromatographic assays

www.estapor.com

www.merck.com

MagForce

Nanotecnologies AG

Hyperthermia www.magforce.com

Polymicrospheres (division

of Vasmo, Inc.)

Drug delivery, diagnostic assays www.polymicrospheres.com

Spherotech, Inc. Cell separation, enzyme immunoassay www.spherotech.com

Alnis Biosciences, Inc. Drug delivery for anti-cancer and -infective treatments MagNaGel®105

and smart contrast agents

www.alnis.com

Triton Biosystems, Inc. Hyperthermia www.tritonbiosystems.com

Sirtex Medical Ltd. Radiation therapy www.sirtex.com

Biophan Technologies, Inc. Drug delivery www.biophan.com

Magnamedics GmbH Drug delivery, in vitro diagnostics www.magnamedics.com

Chemicell GmbH Drug delivery (FluidMAG®), bioseparation, gene transfection and detection www.chemicell.com

REFERENCES

1. Ferrari, M., Nat. Rev. Cancer (2005) 5, 161

2. Cunningham, C. H., et al., Magn. Reson. Med. (2005) 53, 999

3. Johannsen, M., et al., Int. J. Hyperthermia (2005) 21, 637

4. Jurgons, R., et al., J. Phys.: Condens. Matter (2006) 18, S2893

5. Jemal, A., CA: Cancer J. Clin. (2007) 57, 43

6. Cancer Reference Information, American Cancer Society, Inc. (2006)

7. Ritter, J. A., et al., J. Magn. Magn. Mater. (2004) 280, 184

8. Bogdanov, A., et al., J. Drug Targeting (1997) 4, 321

9. Hobbs, S. K., et al., Proc. Natl. Acad. Sci., USA (1998) 95, 4607

10. Okuhata, Y., Adv. Drug Delivery Rev. (1999) 37, 121

11. Brigger, I., et al., Adv. Drug Delivery Rev. (2002) 54, 631

12. Tomalia, D. A., Prog. Polym. Sci. (2005) 30, 294

13. Boyd, B. J., Drug delivery report Autumn/Winter, PharmaVentures Ltd., Oxford,

UK (2005)

REVIEW Magnetic nanoparticles for drug delivery

JUNE 2007 | VOLUME 2 | NUMBER 332

14. Meyers, P. H., et al., Am. J. Roentgenol. Radium Ther. Nucl. Med. (1963) 90,

1068

15. Alksne, J. F., and Fingerhut, A. G., Bull. LA Neurol. Soc. (1965) 30,153

16. Turner, R. D., et al., J. Urology (1975) 113, 455

17. Freeman, M. W., et al., J. Appl. Phys. (1960) 31, S404

18. Zimmermann, U., and Pilwat, G., J. Biosci. (1976) 31, 732

19. Widder, K. J., et al., Proc. Soc. Exp. Biol. Med.(1978) 158, 141

20. Kato, T., et al., Appl. Biochem. Biotechnol. (1984) 10, 199

21. Widder, K. J., et al., Proc. Natl. Acad. Sci., USA (1981) 78, 579

22. Gupta, P. K., et al., J. Pharmacol. Sci. (1989) 78, 290

23. Morimoto, Y., et al., J. Pharmacobio-Dynamics (1980) 3, 264

24. Häfeli, U. O., et al., J. Biomed. Mater. Res. (1994) 28, 901

25. Häfeli, U. O., et al., Nucl. Med. Biol. (1995) 22, 147

26. Lübbe, A. S., et al., Cancer Res. (1996) 56, 4694

27. Lübbe, A. S., et al., Cancer Res. (1996) 56, 4686

28. Goodwin, S., et al., J. Magn. Magn. Mater. (1999) 194, 132

29. Goodwin, S. C., et al., Toxicol. Sci. (2001) 60, 177

30. Steinfeld, U., et al., Int. J. Pharm. (2006) 311, 229

31. Wiekhorst, F., et al., J. Nanosci. Nanotechnol. (2006) 6, 3222

32. Sunderland, C. J., et al., Drug Develop. Res. (2006) 67, 70

33. Häfeli, U. O., Int. J. Pharm. (2004) 277, 19

34. Alexiou, C., et al., Cancer Res. (2000) 60, 6641

35. Arruebo, M., et al., Chem. Mater. (2006) 18, 1911

36. Fréchet, J. M. J., Prog. Polym. Sci. (2005) 30, 844

37. Duncan, R., Materials Today (2005) 8 (7-8, Suppl. 1), 16

38. Gaur, U., et al., Int. J. Pharmacol. (2000) 202, 1

39. Neuberger, T., et al., J. Magn. Magn. Mater. (2005) 293, 483

40. Ibarra, M. R., et al., Spanish Patent Application 200302803, 2006

41. Rotariu, O., and Strachan N. J. C., J. Magn. Magn. Mater. (2005) 293, 639

42. Iacob, G., et al., Biorheology (2004) 41, 599

43. Yellen, B. B., et al., J. Magn. Magn. Mater. (2005) 293, 647

44. Rosengart, A. J., et al., J. Magn. Magn. Mater. (2005) 293, 633

45. Pankhurst, Q. A., et al., J. Phys. D: Appl. Phys. (2003) 36, R167

46. Lübbe, A. S., et al., J. Surgical Res. (2001) 95, 200

47. Lübbe, A. S., et al., J. Magn. Magn. Mater. (1999) 194, 149

48. Maaßen, S., et al., STP Pharma Sci. (1993) 3, 11

49. Lee, J., et al., J. Colloid Interface Sci. (1996) 177, 490

50. Bulte, J. W. M., and Kraitchman, D. L., NMR Biomed. (2004) 17, 484.

51. Saebo, K. B., (ed.), Comprehensive summaries of Uppsala Dissertations form the

Faculty of Medicine, Uppsala University, Sweden, (2004)

52. Ambrose, D. L., and Fritz, J. S., J. Chromatogr., B (1998) 709, 89

53. Ferrari, M., Curr. Opin. Chem. Biol. (2005) 9, 343

54. Yamaoka, T., et al., J. Pharm. Sci. (1994) 83, 601

55. Meng, F., et al., J. Biomed. Mater. Res. (2004) A70, 49

56. Moats, R. A., et al., Angew. Chem. Int. Ed. (1997) 36, 726

57. Jacques, V., and Desreux, J. F., Top. Curr. Chem. (2002) 221, 123.

58. Arruebo, M., et al., Adv. Funct. Mater. (2007), in press

59. Bullok, K. E., et al., Mol. Imaging (2006) 5, 1

60. Nitin, N., et al., J. Biol. Inorg. Chem. (2004) 9, 706

61. Qasba, P. K., et al., AAPS J.(2006) 8, E190

62. James, J., and Kullberg, M., Ethnic. Disease (2005) 15, 113

63. Morishita, N., et al., Biochem. Biophys. Res. Commun. (2005) 334, 1121

64. Schillinger, U., et al., J. Magn. Magn. Mater. (2005) 293, 501

65. Yoon, T.-J., et al., Angew. Chem. Int. Ed. (2005) 44, 1068

66. Gersting, S. W., et al., J. Gene Med. (2004) 6, 913

67. Huth, S., et al., J. Gene Med. (2004) 6, 923

68. Scherer, F., et al., Gene Therapy (2002) 9, 102

69. Tanaka, H., et al., J. Biomed. Mater. Res. (2005) 73A, 255

70. Kullberg, M. et al., Med. Hyp. (2005) 64, 468

71. Cao, J., et al., J. Magn. Magn. Mater. (2004) 277, 165

72. Liang, S., et al., J. Radioanal. Nucl. Chem. (2006) 269, 3

73. Chen, J., et al., Cancer Lett. (2006) 231, 169

74. Kohler, N., et al., J. Am. Chem. Soc. (2004) 126, 7206

75. Alexiou, C., et al., J. Magn. Magn. Mater. (2005) 293, 389

76. Hu, F. X., et al., Biomaterials (2006) 27, 5725

77. Zhang, Y. Q., et al., J. Nanosci. Nanotechnol. (2006) 6, 3210

78. Arruebo, M., et al., Nanotechnology (2006) 17, 4057

79. Ma, Y., et al., J. Mater. Eng. Perform. (2006) 15, 376

80. Ma, Y., et al., J. Biomater. Sci.-Polym. Ed. (2004) 15, 1033

81. Nobuto, H., et al., Int. J. Cancer (2004) 109, 627

82. Leakakos, T., et al., Cancer Chemother. Pharmacol. (2003) 51, 445

83. Rudge, S., et al., J. Controlled Release (2001) 74, 335

84. Wu, Y., et al., Polymer (2006) 47, 5287

85. Port, R. E., et al., Cancer Chemother. Pharmacol. (2006) 58, 607

86. Zhang, R.,et al., Nanotechnology (2006) 17, 3622

87. Yang, J., et al., J. Microencapsulation (2006) 23, 203

88. Adriane, K., et al., J. Drug Targeting (2006) 14, 243

89. Taepaiboon, P., et al., Nanotechnology (2006) 17, 2317

90. Kohler, N., et al., Small (2006) 2, 785

91. Kohler, N., et al., Langmuir (2005) 21, 8858.

92. Cheung, R.Y., et al., Biomaterials (2005) 26, 5375

93. Zhang, J.Q., et al., Pharm. Res. (2005) 22, 573

94. Saravanan, M., et al., Int. J. Pharm. (2004) 283, 71

95. Jain, S., et al., Int. J. Pharm. (2003) 261, 43

96. Kubo, T., et al., Int. J. Oncology (2001) 18, 121

97. Vonarbourg, A., et al., Biomaterials (2006) 27, 4356

98. Fernández-Pacheco, R., et al., J. Magn. Magn. Mater. (2006) 311, 318

99. Mornet, S., et al., J. Mater. Chem. (2004) 14, 2161

100 Gould, P., Nano Today (2006) 1 (2), 34

101. Lu, J., et al., Nanotechnology (2006) 17, 5812

102. Bonnemain, B., J. Drug Targeting (1998) 6, 167

103. Weissleder, R., et al., Radiology (1990) 175, 489

104. Moghimi, S. M., et al., FASEB J. (2005) 19, 311

105. Chellat, F., et al., Biomaterials (2005) 26, 7260

106. Butterworth, M. D., et al., Colloids Surf., A (2001) 179, 93

107. Cheng, J., et al., Pharm. Res. (2006) 23, 557

108. Feng, S.-S., and Chien, S., Chem. Eng. Sci. (2003) 58, 4087

109. Schenck, J. F., Prog. Biophys. Mol. Biol. (2005) 87, 185

110. Haik, Y., et al., J. Magn. Magn. Mater. (2001) 225, 180

111. Chakeres, D. W., and de Vocht, F., Prog. Biophys. Molec. Biol. (2005) 87, 255

112. Kangarlu, A., and Robitaille P. M. L., Concepts Magn. Reson. (2000) 12, 321

113. Macaroff, P. P., et al., J. Appl. Phys. (2006) 99, S102

114. Park, S. I., et al., J. Magn. Magn. Mater. (2006) 304, e406

115. Duncan, R., and Izzo, L., Adv. Drug Delivery Rev. (2005) 57, 2215

116. Yin, H., et al., Biomaterials (2005) 26, 5818

117. Morris, J., Nanotechnology White Paper. US Environmental Protection Agency,

2007

118. Fernández-Pacheco, R., et al., Nanotech (2005) 1, 144

119. Tomalia, D. A., Prog. Polym. Sci. (2005) 30, 294

120. Boyd, B. J., Technology overviews, Colloidal drug delivery. Drug Delivery Report,

Autumn/Winter 2005, 63