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Pharmaceutical Journal Vol 263 No 7060 p309-318
August 28, 1999 Special Feature

Science in pharmacy

Parenteral drug delivery: 1

By Ijeoma F. Uchegbu, BPharm, PhD

This article, the first in an occasional series, provides an update on developments in the area of parenteral drug delivery

The earliest documented record of parenteral drug administration using hypodermic needles dates back to the mid 19th century when Alexander Wood reported the injection of morphine in the treatment of neuralgia.1 A number of technological advances have since been made in the area of parenteral drug delivery leading to the development of sophisticated systems that allow drug targeting and the sustained or controlled release of parenteral medicines. The main drug delivery technologies currently being tested and their potential applications are summarised in Table 1 and are discussed in further detail below.

Table 1: Main applications of modern drug delivery technology
Drug delivery technology Main applications
Liposomes Passive tumour targeting
  Vaccine adjuvants
  Passive targeting to lung endothelium in gene delivery
  Targeting to regional lymph nodes
  Targeting to cell surface ligands in various organs/areas of pathology
  Sustained release depot at point of injection
Niosomes Passive tumour targeting
  Vaccine adjuvants
  Sustained release depot at point of injection
Nanoparticles Passive tumour targeting
  Vaccine adjuvants
Microparticles Sustained release depot at point of injection
  Vaccine adjuvants
Cyclodextrins Lipophilic drug solubilisation for parenteral use
Emulsions Lipophilic drug administration vehicles
  Targeting to cell surface antigens
ADEPT Active tumour targeting
Polymer drug conjugates Passive tumour targeting
Implant system Localised depot systems for the treatment of infections and cancers
  Sustained drug release systemic therapies
Needle free injections Decreased pain on injection
  Increased bioavailability of intradermal vaccines

Liposomes

Liposomes were discovered in the mid 1960s2 and originally studied as cell membrane models. They have since gained recognition in the field of drug delivery. Liposomes are formed by the self-assembly of phospholipid molecules in an aqueous environment. Shown schematically in Figure 1a, the amphiphilic phospholipid molecules form a closed bilayer sphere in an attempt to shield their hydrophobic groups from the aqueous environment while still maintaining contact with the aqueous phase via the hydrophilic head group. The resulting closed sphere may encapsulate aqueous soluble drugs within the central aqueous compartment (Figure 1, left) or lipid soluble drugs within the bilayer membrane (Figure 1, centre). Alternatively, lipid soluble drugs may be complexed with cyclodextrins and subsequently encapsulated within the liposome aqueous compartment.3 The encapsulation within/association of drugs with liposomes alters drug pharmacokinetics, and this may be exploited to achieve targeted therapies. Alteration of the liposome surface is necessary in order to optimise liposomal drug targeting.
Figure 1a Figure 1b Figure 1c
Figure 1: Liposomes - (left) A = aqueous soluble drug encapsulated in aqueous compartment; (centre) B = a hydrophobic drug in the liposome bilayer; (right) C = hydrophilic polyoxyethylene lipids incorporated into liposome

Liposomal anticancer drugs The toxic side effects associated with the administration of anticancer drugs makes these drugs ideal candidates for drug targeting. Anticancer agents have been encapsulated within liposomes in an effort to target such agents to tumours.
The use of liposomes as anticancer drug delivery systems was originally hampered by the realisation that liposomes are rapidly cleared from the circulation and largely taken up by the liver macrophages.4 Liposome macrophage uptake by the liver and spleen (the reticuloendothelial system) hampered the development of liposomes as drug delivery systems for over 20 years. However, two discoveries helped liposomes transfer from the bench to the clinic. The first was the finding in the late 1980s and early 1990s that the presence of liposome surface ligands, such as monosialoganglioside5 or polyoxyethylene,6,7 decreased liposome clearance5,8 by partially preventing liver and spleen uptake of intravenously injected liposomes.5,8 The realisation that liposomal biodistribution could be altered in this way was facilitated by the results of similar studies with polystyrene nanoparticles coated with a polyoxyethylene polymer.9 The reduced liver and spleen uptake of stealth liposomes, as polyoxyethylene liposomes came to be known (Figure 1, right), is believed to be due to a reduced coating (opsonisation) of these liposomes by plasma proteins, thus enabling them to escape recognition by the liver and spleen10,11 and enjoy long circulation times. It is likely that the reduced aggregation of stealth liposomes in the blood is also responsible for the increased circulation time.12,13
Other methods of extending liposome blood circulation times include the incorporation into liposomes of cholesterol,14 polyvinyl-pyrrolidone polyacrylamide lipids,15 glucoronic acid lipids16 or the high phase transition temperature phospholipid distearoyl phosphatidylcholine.17 Liposome size also affects biodistribution and a size of between 70 and 200nm is necessary to achieve prolonged circulation times with stealth liposomes.18,19

Stealth liposomes of about 100nm in size passively target20-22 solid tumours by extravasation23 into their extracellular space on intravenous administration. Extravasation is achieved due to the disorganised tumour vasculature24 (Figure 2). Non-stealth liposomes prepared from high phase transition temperature phospholipids also enjoy prolonged circulation times and accumulate within tumour tissue despite high levels of liver uptake.17
The second factor that enabled liposomes to be developed into anticancer drug delivery systems was the elucidation of an efficient doxorubicin loading procedure.25 This produced liposomes with a high drug payload, thus reducing the level of phospholipid that had to be administered in order to achieve a desired dose.

Figure 2 - full size
Figure 2: Accumulation of liposomes within solid tumours — (right) liposome extravasation from the disorganised tumour vasculature and (left) liposomes in normal tissue

Doxorubicin loaded stealth liposomes circulate for prolonged periods,21, 26 accumulate21 and extravasate27,28 within tumours and also improve tumoricidal activity29-31 in mice. In patients, liposomal doxorubicin accumulates within Kaposi’s sarcoma lesions32 and produces a good therapeutic response.32-34 Liposomal doxorubicin is now licensed, as Caelyx,35 for the treatment of Kaposi’s sarcoma. This formulation is currently in clinical trials for ovarian cancer and could be approved shortly for use in ovarian cancer patients who have failed to respond to paclitaxel and cisplatin.36 Sales of Caelyx (known as Doxil in the US) are predicted to exceed US$75m (£50m) in 1999.37
Non-stealth liposomes prepared from high phase transition temperature lipids, such as distearoyl phosphatidylcholine, also accumulate within tumours17,38 and have been developed into the product Daunoxome39 — liposomal daunorubicin. Daunoxome is also licensed for use in the treatment of Kaposi’s sarcoma.
There are a variety of other methods employed to target liposomal anticancer drugs. These include the use of antibody targeted (immuno-) liposomes,40,41 discussed further below, and the use of local hyperthermia (42C)42,43 in combination with thermosensitive liposomes. Thermosensitive liposomes release their contents at elevated temperature.43 Local hyperthermia in itself improves tumoricidal activity with daunorubicin thermostable liposomes42 as does its use in combination with thermosensitive liposomes.43 Hyperthermia appears to aid drug extravasation.42
Technology exists to target liposomes to the spleen44,45 and lung.46,47 Liposomes designed to combat cancer chemotherapy associated multidrug resistance48 are also being developed.
The success achieved with the anthracycline anticancer agents led to a more intense search for similar formulations with other anticancer drugs and a number of liposomal formulations are in preclinical development. These include liposomal formulations of a 5-fluorouracil lipid analogue,49 vincristine,50-52 a porphyrin derivative for use in combination with laser light irradiation,53 bleomycin,54 mitozantrone55 and paclitaxel.56 A few liposomal combination chemotherapy regimens, such as liposomal valinomycin in combination with cisplatin, have been tested preclinically and found to show favourable results.57
Liposomal cancer chemotherapy has thus resulted in product licensing within the past decade although skin toxicity in the clinic is still a problem. Efforts to actively target liposomes are outlined below.
Liposomes as vaccine adjuvants Liposomes have been firmly established as immunoadjuvants (enhancers of the immunological response), potentiating both cell mediated and humoral immunity.58 Liposomal immunoadjuvants act by slowly releasing encapsulated antigen on intramuscular injection and also by passively accumulating within regional lymph nodes.59 Strategies that enhance the targeting of liposomes to regional lymph nodes, for example the use of phosphatidylserine liposomes,60 may thus improve adjuvanticity. Liposomal vaccines can be made by associating microbes, soluble antigens, cytokines58 or deoxyribonucleic acid (DNA)61 with liposomes, the latter stimulating an immune response on expression of the antigenic protein.62 Liposomes encapsulating antigens which are subsequently encapsulated within alginate lysine microcapsules,63 to control antigen release, improve the antibody response. Liposomal vaccines may also be stored dried at refrigeration temperatures for up to 12 months and still retain their adjuvanticity.64
Liposomal anti-infective agents The use of amphotericin B, a polyene antibiotic, in the treatment of systemic fungal infections is associated with extensive renal toxicity.65 Amphotericin B acts mainly by binding to sterols such as ergosterol in membranes of sensitive fungi, thus increasing membrane permeability.65 The toxicity associated with this compound is probably the result of its interaction with cholesterol in mammalian cells. Liposomal amphotericin B (Ambisome),66 the first liposomal preparation to be licensed for clinical use,67 is used for the treatment of systemic fungal infections. Liposomal amphotericin B, by passively targeting the liver and spleen, reduces the renal67 and general68 toxicity of the drug at normal doses, although renal toxicity appears to remain unaffected when the formulation is administered at elevated doses.69 The latter is presumably due to a saturation of the macrophage uptake mechanisms of the liver and spleen. Ambisome may also be used to treat drug-resistant leishmaniasis, a parasitic infection of the reticuloendothelial system.70 The ability of liposomes to be taken up by macrophages and to concentrate in the liver and spleen undoubtedly makes them ideal for the treatment of diseases of the liver and spleen, such as leishmaniasis. Another polyene antibiotic with a similar mechanism of action to amphotericin B, hamycin, improves the survival of mice infected with systemic Candida albicans when encapsulated in liposomes.71
Liposomes may be targeted to the lung by coating vesicles with 0-stearoyl amylopectin and polyoxyethylene or monosialoganglioside.72 The encapsulation of the anti-tuberculosis agents rifampicin or isoniazid in lung targeted liposomes modulates the toxicity72 and improves the efficacy73 of these drugs.
To summarise, liposomes by passively targeting the liver and spleen and also by actively targeting the lung have been shown to improve the efficacy and modulate the toxicity of certain anti-infectives.

Active targeting of liposomes

Immunoliposomes The production of stealth liposomes, which are not rapidly cleared by the liver and spleen, made the active targeting of liposomes a real possibility. Various antibodies have thus been conjugated to the surface of stealth liposomes to produce immunoliposomes (Figure 3, left and centre) for active targeting, as opposed to the passively targeted species discussed above.
Figure 3 (left) Figure 3 (centre) Figure 3c (right)
Figure 3: Immunoliposomes - (left) antibodies (A) attached to the surface of stealth liposomes; (centre) antibodies attached to the distal ends of polyoxyethylene chains in stealth liposomes; (right) antibodies attached to the surface of non-stealth liposomes

Stealth liposomes bearing an antibody that specifically binds intracellular myosin in ischaemic or necrotic cardiomyocytes targets infarcted areas in rabbits.74 The antibody does not bind to cardiomyocytes with intact plasma membranes and hence avoids healthy tissue. The infarcted heart antibodies were attached to the liposome surface as shown in Figure 3 (left) and the effectiveness of the antibody in homing the liposome found to be directly related to the level74,75 and molecular weight76 of polyoxyethylene on the surface. A high polyoxyethylene density74 and polyoxyethylene with a molecular weight in excess of 200076 hampers the directing ability of the antibody by shielding the antibody from its target. To overcome this masking effect, surface antibodies such as the OX26 transferrin antibody41 and a lung endothelial cell antibody77,78 may be attached to the distal ends of polyoxyethylene chains (Figure 3, centre). This attachment of antibodies improves the targeting ability when compared with the attachment of antibodies to the surface of non-stealth liposomes78 (Figure 3, right). This indicates that the extended circulation time achieved by stealth liposomes improves targeting simply because the prolonged circulation allows an extended period of time for liposomes to make contact with their target. Additionally, the exposure of antibodies to the target is enhanced by their attachment to the distal ends of the polyoxyethylene chains. Antibody Fab fragments attached to the distal ends of polyoxyethylene chains in stealth78,79 or the liposome surface of non-stealth80,81 liposomes may also be used to target liposomes to solid tumours.
Although the stealth immunoliposomes with antibodies conjugated to the distal ends of polyoxyethylene chains (Figure 3, centre) are effective targeting agents, the conjugation accentuates the immune response to the antibody.82 This could ultimately result in the liposomes being cleared from the circulation prematurely,59 a feature that could limit the clinical development of these immunoliposomes. Unfortunately, although elegant science, anticancer drug targeting with immuno-stealth liposomes83 and immuno-non-stealth liposomes84 does not improve the tumoricidal activity when compared with ordinary stealth and non-stealth liposomes not bearing immunotargeting moieties. It is possible that a high binding of immunoliposomes to the periphery of solid tumours prevents their penetration into deeper layers.85 Alternatively, the lack of an incremental improvement in tumoricidal activity with immuno-targeted species could indicate that the extravasation of stealth liposomes is the rate limiting step to their accumulation at tumour sites.83
A further innovative utilisation of immunoliposomes involves a form of antibody directed enzyme prodrug therapy (ADEPT). The use of antibodies to direct prodrug activating enzymes to tumour tissue is the principle behind ADEPT,86 as discussed below. ADEPT-type liposomes bearing antibodies and enzymes on their surface87 may be used to localise enzymes at the tumour site before administration of a prodrug. In vitro, prodrugs of epirubicin88 and daunorubicin87 may be activated by pre-treating cells with liposomes bearing cell specific antibodies and prodrug specific enzymes. The density of surface enzyme directly influences the tumoricidal activity of the drug87 and hence these rather complex systems must be carefully titrated to produce an effective formulation.
While most immunoliposomes have been developed to target potentially toxic cancer chemotherapeutic agents, they may also prove beneficial in the treatment of other pathological conditions. The conjugation of interleukin 2 (IL-2) to the surface of non-stealth liposomes allows the targeting of toxic immunosuppressants to areas of the immune system participating in graft rejection, such as T-cells expressing the IL-2 receptor,89 without affecting other parts of the immune system.
Limitations to the clinical use of immunoliposomes include a failure to demonstrate an advantage in experimental cancer chemotherapy plus the fact that these systems are rather complex. Each antibody- disease state combination requires a specially tailored carrier90 and specialised manufacture to retain the activity of the antibody.85
Ligand bearing liposomes An alternative targeting strategy that may ultimately have a wider application than the antibodies described above utilises surface moieties that are upregulated in certain diseases as targets. These include the folate receptor, overexpressed in ovarian carcinoma, and the cell adhesion molecules (eg, selectins and integrins), which are implicated in metastatic events.85 Liposomes bearing specific ligands such as folate may be used to target ovarian carcinomas, while specific peptides or carbohydrates may be used to target integrins and selectins.85 Targeting with small ligands appears more likely to succeed than the use of antibodies since these ligands are easier to handle and manufacture. Specifically, hamycin liposomes bearing mannosyl human serum albumin, which directs them to macrophages in vivo, are superior at suppressing leishmanial parasites than non-targeted liposomes.91 Hence, although liposomes are rapidly cleared by macrophages in vivo, an enhancement of this macrophage uptake by the use of targeting ligands results in an enhancement of the antileishmanial activity. Liposomes may also be targeted to hepatocytes by conjugating with asialofetuin.92 Such liposomes may be useful in the delivery of genes to hepatocytes for the treatment of specific diseases.

Liposomes in gene delivery

Human genome characterisation and recombinant DNA technology have created opportunities for gene therapy that previously never existed. Target diseases for such technology include certain cancers,93 arteriosclerosis,94 cystic fibrosis,95 haemophilia, sickle cell anaemia and other genetic diseases. In addition, genes encoding for various antigens may be administered as vaccines.

The administration of the gene of interest should theoretically result in the expression of the therapeutic protein. However, the delivery of the large anionic bioactive DNA across cell membranes is by no means a simple feat. DNA is easily degraded by circulating and intracellular deoxyribonucleases and also must be delivered intact across the cell and nucleolar membranes to the nucleus. Liposomes have been used to achieve efficient intracellular delivery of DNA.96,97 Such liposomes are prepared from phospholipids with an amine hydrophilic head group.98 The amines may be either quaternary ammonium, tertiary, secondary or primary and the liposomes prepared in this way are commonly referred to as cationic liposomes, because they possess a positive surface charge at physiological pH. The use of cationic liposomes as gene delivery systems was pioneered in the late 1980s when in vitro studies demonstrated that the complexation of genes with liposomes (Figure 4) promoted gene uptake by cells in vitro.99 Since this time, cationic liposomes of varying description have been used to promote the cellular uptake of DNA with resultant therapeutic protein expression by various organs in vivo.

Figure 4
Figure 4: DNA-liposome complex

On intravenous injection, cationic liposomes are rapidly cleared from the blood and most of the protein expression is found in the lung, the first capillary bed encountered after IV administration.98 Lung expression could be the result of the formation of liposomal aggregates on interaction with blood proteins, which would then become trapped in the tiny lung capillaries. Alternatively, the cationic liposomes could become opsonised with lung specific opsonins on IV administration and hence be directed to the lung. It is not clear which of these, if either, is responsible for the high protein expression observed in the lung endothelium.
The knowledge gained in the development of anticancer liposomes, such as the prolongation of liposome circulation using polyoxyethylene lipids, may be applied in the development of gene delivery liposomes in order to produce efficient carriers.100 This strategy should aid liposome targeting in gene delivery. Thus far, targeting efforts with cationic liposomal gene formulations have been few and far between. A rare example is the targeting to mouse hepatocytes by the portal vein administration of cationic liposome DNA complexes coated with chylomicron remnants.101 Such liposomes are specifically taken up by the apolipoprotein receptor. However, these chylomicron remnant coated liposomes were not targeted to the hepatocytes on tail vein administration.
Although clinical trials with liposomal gene delivery in cystic fibrosis patients have concentrated on administration via non-parenteral routes,102,103 for the obvious reason that the disease manifests itself in the alveolar epithelium, it may be possible to treat this condition by the intravenous administration of the replacement gene-liposome complex. This is because such a strategy results in gene expression in distal regions of the alveolar epithelium.95
Liposomal DNA is superior to free DNA in the treatment of experimental tumours. A cationic liposome formulation of the human interleukin 2 (IL-2) gene improves the tumoricidal activity of IL-2 in experimental xenografts.93 Cell kill in this case is mediated by expression of the IL-2 cytokine, which stimulates natural killer (NK) cell activity. Efficient arterial gene transfer will be beneficial in the treatment of arteriosclerosis and gene transfer into porcine arteries, via catheter, is enhanced by complexing the gene with liposomes.104 Additionally, IV injection into tumour-bearing mice of the gene for diphtheria toxin A-chain complexed with cationic liposomes significantly reduces tumour volume compared with untreated controls.105
DNA vaccines administered as liposomal complexes also improve the antibody response over that seen with free DNA.61 Cell and humoral mediated immunity after the intramuscular administration of plasmids encoding for the S region of hepatitis B is enhanced by liposomal complexation. Cationic liposomes, by virtue of being able to transfer DNA more efficiently into cells than can be achieved with naked DNA alone, should lead to an increase in the level of antigen expression and hence an increase in the immune response to the antigen.

Although the experimental data indicate that cationic liposomes are able to facilitate the transfer of DNA into live mammalian cells, there are still major problems that need to be overcome in order to approach the ideal (Figure 5). These include a reduction in the rapid clearance of cationic liposomes and the production of efficiently targeted liposomes. At the cellular level, the problems may be overcome by improving receptor mediated uptake using appropriate ligands, the endowment of liposomes with endosomal escape mechanisms, a more efficient translocation of DNA to the nucleus and the efficient dissociation of the liposome complex just before the entry of free DNA into the nucleus.97

Figure 5 - full size
Figure 5: The optimisation of liposomal gene delivery

Other liposome formulations

Another interesting liposome-based multivesicular formulation has recently been reported. This sustained release formulation, being developed by Depotech, has a particle size of 1-5mm and is formed from phospholipids, cholesterol and the oil tripalmitolein.106-108 The system effectively controls the release of insulin-like growth factor I and is morphologically similar to the multivesicular niosome formulation described by Sternberg et al.109 A similar formulation developed by Depotech for the intrathecal sustained release of cytarabine, Depocyt,106,107,110 has been recommended by the United States Food and Drug Administration’s oncologic drugs advisory committee for accelerated approval in the treatment of lymphomatous meningitis.111
It can safely be said that liposomes are the most widely studied modern drug delivery system. While effective anticancer formulations have been developed after years of painstaking research, developments in the field of gene delivery are predicted to be the next growth area.

Non-ionic surfactant vesicles (niosomes) and other synthetic surfactant vesicles

Niosomes The success achieved with liposomal systems stimulated the search for other vesicle forming amphiphiles. Non-ionic surfactants were among the first alternative materials studied112 and a large number of surfactants have since been found to self assemble into closed bilayer vesicles113 (Figure 6) which may be used for drug delivery.

Anticancer niosomes Anticancer niosomes, if suitably designed, will be expected to accumulate within tumours in a similar manner to that depicted in Figure 2 for liposomes. Indeed, the niosomal encapsulation of methotrexate114 and doxorubicin115,116 increases drug delivery to the tumour and tumoricidal activity. Unlike non-stealth liposomes, 800nm doxorubicin niosomes, possessing a triglycerol115,117 or 200nm doxorubicin niosomes possessing a muramic acid118 surface are not taken up significantly by the liver. As such, these triglycerol niosomes accumulate in the tumour.115 However, muramic acid vesicles do accumulate in the spleen.118 On the other hand, 200nm doxorubicin niosomes with a polyoxyethylene (molecular weight 1,000) surface are rapidly taken up by the liver116 and accumulate to a lesser extent in tumour. A polyoxyethylene coating with a molecular weight of 2,000 was found to be optimal for stealth liposomes.7 It is obvious that, like liposomes and presumably any parenterally administered colloidal particulates, body disposition may be controlled by alterations in particulate surface chemistry.

Figure 6
Figure 6: Cryo-transmission electron micrograph of doxorubicin Span 20 based niosomes, bar = 500nm (Uchegbu IF, Moody M, Florence AT, unpublished data)

The accumulation of certain niosomes within the liver may be exploited, however. Doxorubicin polymer conjugates119 are prepared by conjugating the anticancer drug to a polymeric backbone via an enzymatically cleavable spacer. The encapsulation of these polymeric prodrugs within 200nm niosomes120 provides a depot system within the liver from which free drug is gradually cleaved off and liver levels of the drug are seen to rise over a 24-hour period.120 This technology may prove advantageous for the treatment of hepatic neoplasms.
The activity of other anticancer drugs, such as vincristine,121 bleomycin122 and plumbagin, a plant derived anticancer agent,123 are also improved on niosomal encapsulation.
Niosomes and other drugs Uptake by the liver and spleen113 make niosomes ideal for targeting diseases manifesting in these organs. One such condition is leishmaniasis and a number of studies124-126 have shown that niosomal formulations of sodium stibogluconate improve parasite suppression in the liver,124,125 spleen and bone marrow.127 Niosomes may also be used as depot systems for short acting peptide drugs on intramuscular administration.128
Niosomes as vaccine adjuvants Niosomal antigens are potent stimulators of the cellular and humoral immune response.129,130 The formulation of antigens as a niosome in water-in-oil emulsion further increases the activity of antigens.131 The controlled release property of the emulsion formulation is responsible for enhancing the immunological response.

Other vesicles Vesicles for use in drug delivery have also been prepared from amphiphilic carbohydrate and amino acid based polymers132,133 (Figure 7) and also from aged red blood cells.134 The latter increase intracellular drug delivery to virus infected cells.
Non-ionic surfactants and other amphiphiles may be used to prepare a number of different drug carriers with varied surfaces. This should provide the basis for the development of drug delivery vesicles with different biodistribution profiles.

Figure 7
Figure 7: Freeze fracture electron micrograph of polymeric chitosan based vesicles

Solid nanoparticles and microparticles

Solid nanoparticles and microparticles differ from liposomes and niosomes in that they are prepared from polymers and do not have an aqueous core but a solid polymer matrix. Microparticles and nanoparticles are usually prepared by the controlled precipitation of polymers solubilised in one of the phases of an emulsion.135-137 Precipitation of the polymer out of the solvent takes place on solvent evaporation, leaving particles of the polymer suspended in the residual solvent. Drug loading in both instances occurs simultaneously with particle formation and the presence of surfactants in the aqueous phase may be used to ensure a small particle size. Microparticles may also be prepared by chemical cross-linking of soluble polymers138 and nanoparticles by the polymerisation of a monomer in a good solvent for the monomer but a poor solvent for the polymer139 or by high-pressure homogenis-ation.140,141 The loading of water soluble drugs into the hydrophobic polymer particle matrix is still difficult but may be enhanced by the salting out of the active ingredient with soluble inorganic salts,142 the formation of lipophilic ion pairs of soluble amine drugs with monalkyl phosphate esters,143 or by exploiting the ionic attraction between basic amine drugs and the carboxylic groups of polylactic acid co-glycolic acid.144
Most of the early investigations into the science of solid particulate drug delivery used non-biodegradable polymers such as polystyrene.9,145 However, recent studies have quite rightly focused on the development of biodegradable146 or at least bioerodible particles. Polylactic acid, polyglycolic acid,146 poly ß-hydroxybutyrate139 and fibrin147 are biodegradable polymers used to make drug delivery particles, while the alkyl cyanoacrylates139 are bioerodible polymers. These solid nanoparticles and microparticles may be used to prepare sustained release parenteral formulations or to achieve drug targeting, some examples of which are discussed below. Since the particle size of an injected dispersion has a significant impact on its biodistribution and pharmacodynamics, the drug delivery applications of nanoparticles and microparticles will be treated separately. For the purposes of this review, nanoparticles are defined as particulate dispersions having a particle size of between 30 and 500nm while microparticles are defined as particulate dispersions having a particle size in excess of 0.5microm.

Solid nanoparticles Tumour targeting — Because of their small size, nanoparticles may be injected intravenously and used to target drugs to particular organs. The particles, as with all intravenously injected and non-polymer coated colloidal particulates, are cleared from the circulation by the liver and spleen.145,148 To facilitate drug targeting, in tumour tissue for example, a reticuloendothelial system avoidance (stealth) facility may be incorporated using polyoxyethylene.
This may be achieved using block polyoxyethylene copolymers9,149-151 in which the hydrophobic portion of the molecule (eg, polylactic acid) forms the nanoparticle matrix while the water soluble polyoxyethylene block forms a hydrophilic coating on the particle (Figure 8). Stealth nanoparticles may also be prepared by coating them with soluble polyoxyethylene152 or by using dialkyl polyoxyethylenes and phospholipids153 to make nanoparticles. The hydrophobic portions of these amphiphilic molecules, in the latter case, comprise the particle matrix. Stealth nanoparticles increase the tumour accumulation152,153 and tumoricidal activity153 of anticancer drugs in mice.

Figure 8
Figure 8: Stealth nanoparticlesv - A = polymer matrix containing drug, B = polyoxyethylene chains

The accumulation of non-stealth doxorubicin nanoparticles within the Kupffer cells of the liver may be used to target hepatic neoplasms indirectly.154 This is achieved by providing a depot of drug for killing nearby neoplastic tissue, as the nanoparticles are not actually taken up by neoplastic tissue.
Vaccine adjuvants — Nanoparticles have also been used as vaccine adjuvants. Antigens adsorbed onto the surface or entrapped in the matrix of polymethylmethacrylate nanoparticles induce an enhanced immunological response.155 Polymethylmethacrylate nano- particles containing the influenza antigen may protect against challenge with the disease to a greater extent than the antigen alone or an alum preparation of the antigen.155 Decreasing particle size and increasing hydrophobicity improves the adjuvancy of these particles.155 The particles offer a prolonged and controlled presentation of the antigen to the immune system.
ISCOMS (immunostimulating complexes) are cage like 40nm supramolecular assemblies comprising quillaja saponins, cholesterol and phospholipids, and hydrophobic antigens.156 ISCOMs are used as parenteral and non-parenteral immunoadjuvants and are currently in clinical trials.156,157
Other applications Restenosis, defined as the re-obstruction of an artery following procedures such as angioplasty or artherectomy may be treated by the local application of dexamethasone-loaded polylactic acid co-glycolic acid nanoparticles.158 Cyclosporin A, an immunosuppressant drug used to prevent graft rejection after transplantation by the inhibition of T-lymphocytes, may be targeted to regional lymph nodes by the intramuscular administration of cyclosporin A polylactic acid nanoparticles.159
In summary, by virtue of their small size, solid nanoparticles provide opportunities for targeted parenteral therapies and may also be used as immunoadjuvants.
Solid microparticles Microparticles are generally injected either intraperitoneally, intramuscularly, subcutaneously or directly to the target organ and, because of their large size (10-160mm), are only really used to provide a sustained release depot of the drug. Drug is gradually released on erosion or by diffusion from the particles.
The rate of release may be increased by decreasing polymer molecular weight160,161 and particle size161, 162 and also by controls on the nature of the polymer/copolymer.160,163
Antitumour formulations — Microparticulate technology has been evaluated in experimental tumours but their large particle size means that these formulations may only be administered by certain routes, thus ultimately limiting their potential. Hence, anti-tumour microparticles are either administered intra-arterially, and hence straight into the organ,164 or into body cavities such as the peritoneum.165 Doxorubicin ion exchange resin microparticles164 are superior to the free drug when administered via the hepatic artery. Improved tumoricidal activity is seen in animals after intraperitoneal administration of mitozantrone chitosan166 or doxorubicin polylactic acid165 microparticles when compared with the free drug and 100mm cisplatin polylactic acid microparticles may be used to provide an intraperitoneal depot of the drug in ovarian cancer patients.167
Apart from the intra-arterial and intraperitoneal routes, microparticles may also be injected directly into tumours. The direct injection of microparticles into solid tumours increases the tumoricidal activity of the drugs 5-fluorouracil168 and doxorubicin.169
Vaccine adjuvants — Biodegradable polylactic acid and polylactic acid co-glycolic acid microspheres also act as immune adjuvants by providing a depot formulation of the antigen at the site of administration.170 The antigen is thus continually released to the antigen presenting cells. Ovalbumin polylactic acid171 and polylactic acid/polyglycolic acid172 microparticles enhance the immune response to ovalbumin when compared with free ovalbumin on parenteral administration. Additionally, intramuscular injection of spermine-chondroitin sulphate microparticles enhances the virus specific immune response to rotavirus.173
Controlled release peptide formulations Depot formulations of short acting peptides have been successfully developed using microparticle technology. Such peptides include leuprorelin acetate174 and triptoreline,163 both luteinising hormone releasing hormone agonists. Leuprorelin polylactic acid co-glycolic acid microspheres may be used as monthly and three monthly dosage forms in the treatment of advanced prostate cancer, endometriosis and other hormone responsive conditions.174 These microspheres effectively halt the progression of prostate cancer or endometriosis in patients174 and are currently marketed as Prostap SR.
Other peptides formulated as sustained release microparticles include the angiotensin receptor antagonist L-158809 for the treatment of hypertension,175 thyrotropin releasing hormone for central nervous system stimulation,144 salmon calcitonin161 for the treatment of hypercalcemia and postmenopausal osteoporosis and the immunosuppressant drug cyclosporin A.162

Dr Uchegbu is lecturer in pharmaceutics in the department of pharmaceutical sciences, University of Strathclyde

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