660

Polymers in drug deliveryStanley S Davis*t+, Lisbeth lllum"! and Snjezana Stolnik*

Polymeric systems have wide applications in drug delivery,both as soluble and as particulate systems. Polymers canalso be used as coating agents to protect or deliver drugsto specific sites in the body. Particulate systems based onbiodegradable polymers have recently become of specialinterest. Many of these systems are based on microparticlesmade from polylactide or polylactide-co-glycolide copolymers,although some new materials such as poly(organo}phosphazenes have been introduced. Recent research onthe use of microparticles for drug targeting has seen theintroduction of new poly(ethylene oxide} copolymers, suchas polylactide-poly(ethylene oxide}. In aqueous media thesematerials spontaneously form micelle-like particles where thecore, made from the hydrophobic moiety of the copolymer,may encapsulate a drug.

Addresses·Department of Pharmaceutical Sciences, University of Nottingham,University Park, Nottingham NG7 2RD, UKtDanBioSyst (UK) Limited, Albert Einstein Centre, Highfields SciencePark, Nottingham NG7 2TN, UKfe-mail: stanley.davis@nottingham.ac.ukCorrespondence: Stanley S Davis

Current Opinion in Colloid & Interface Science 1996, 1:660-666

©Current Chemistry Ltd ISSN 1359-0294

IntroductionPolymeric materials have a large number of uses indrug delivery in both their soluble and particulate forms.The polymers may be synthetic or of natural origin(biopolymers). This review will focus on the use ofpolymers to achieve defined biological effects and will notconsider in detail the uses of polymers as simple coatingor binding agents.

In theory, the pharmaceutical scientist has available awide .range of materials for use in the development ofnovel delivery systems. The number of materials thatare approved for human use, however, is relatively small.The ability to achieve early registration of a new productmay well dictate the nature of the chosen polymer. Inthe emerging field of gene therapy, however, the choicemay be wider. Many of the polymeric materials used fordrug delivery have been selected from other biomedicalareas or food science (e.g, polylactide-eoglycolide (suturematerial), polycyanoacrylarc (tissue adhesive), alginate(wound healing), starch (food stuffs), and so on.

The broad interest in the use of polymeric microparticlcsin the formulation of drug delivery systems over thepast decade may be best illustrated by a number ofliterature references, reviews and published books [1-3).

The various systems have been investigated for possibleapplications over a range of therapeutic applications, forexample, to provide an increase in the selectivity of anti­cancer drugs, sustained release of contraceptive agents andas vehicles for protection and delivery of peptide drugs.A variety of polymeric materials, preparation methods andcharacterization techniques has been applied.

The size of the polymeric delivery system can varyfrom a few nanornetres to a few microns depending onthe intended use and route of administration. The areaof vaccine delivery represents a special area of interestwhere microspheres can be used with success to provideenhanced immune response following injection into themuscle or administration into body cavities (e.g. nose,gastrointestinal tract).

The novel approaches that comprise micro particle engi­neering for drug delivery will be discussed in detail in thepresent review.

Soluble polymers as drug carriers and ascoating agentsSoluble polymeric materials can be employed as drugcarriers that are injected into a body cavity or moreparticularly into the bloodstream. The polymer may beused to provide a slow release of the drug, through thecleavage of bonds that attach the drug to the polymerbackbone or via pendant side chains. Such release canbe controlled by environmental conditions (pH, redoxpotential) or the presence of enzymes. Within the vascularcompartment, the polymer may have attached groupingsor ligands that can provide site specific delivery to organs,specific cells or even to structures within cells [4). Thebest known of such ligands are monoclonal antibodies (orfragment thereof) or sugar residues.

Soluble polymers may also be used as coating agentsthat can determine the fate of a particulate system. Forexample, the coating of polymeric particles with blockcopolymers that are adsorbed strongly to the surface ofthe particle through hydrophobic or ionic interactions,can be used to control the fate of particles injectedinto the vascular and lymphatic compartments. Attemptshave also been made to modify the surface of particlesadministered into the gastrointestinal tract [5-). Furtherdetails of appropriate systems arc given below.

Some soluble polymers are able to self-associate intomicellar-like structures and in doing so arc then able tocarry a drug pay load. The block copolymers of polylactidcand polyethylene glycol have been studied in detail inthis regard [6-). Nonself-dispersible equivalents can alsobe useful as solid particular carriers as discussed below.

An exciting area where soluble polymers could play aviral role in delivery is in the field of gene therapy. Herethe therapeutic agent, plasmid DNA, can be modifiedthrough its electrostatic interaction with polymers ofopposite charge. This interaction leads to compaction ofthe DNA into a nanoparticle and preferential propertiesfor cell uptake, and hopefully subsequent expression ofthe gene product. The cationic materials polylysine andpolyamidoarnines have been used for . this purpose. Ofspecial interest are the more complex structures known asdendrimers (7,8].

BiopolymersBiopolyrners represent a fascinating group of materialsthat are of increasing interest in the pharmaceutical arena.Thcy can be used as soluble materials or formed intoparticulates by processing methods such as emulsification,spray drying, and so on, sometimes with a cross-linkingstep. Thus starch, dextran, chitosan and albumin are allbeing investigated as carriers for different routes andas different systems (soluble and particulate). Polymerinteractions (e.g, between proteins and carbohydratesystems) and polymer syncrgy are concepts of interest.These are familiar to the colloid scientist who has studiedthe phenomena of complex coacervation [9].

The material chitosan, a cationic polymer derived fromthe partial deacetylation of chitin, is a material ofspecial interest. Besides bcing bioadhesivc, through theinteraction of the positively charged amino group inchitosan with negatively charged sialic acid groups ofmucin, it is apparently nontoxic when administered to

epithelial surfaces. Chitosan has special properties inmodifying the intercellular (tight junction) properties ofcells. By using chitosan it is possible to enhance thetransrnucosal uptake of challenging polar molecules suchas peptides and proteins [10°].

Polymeric carrier particles (microspheres,nanoparticles)Polymers in solid form can bc used for drug delivery in tWOways: as controlled release materials and as drug targetingsystems. Included in this rubric of polymeric particles arevaccine systems.

The best known synthetic polymer material used inadvanced drug delivery is polylactide and its copolymerswith polyglycolide. These materials can be fabricated intorods and microparticles [11]. Drugs and antigens can beloaded in, as well as on, the surface of such systems. Thematerials carried on the surface can be attached by physicalas well as chemical bonds (grafted systems). The rate ofrelease of the drug or antigen can be controlled by thenature of .the polymer (i.c. ratio of lactide to glycolidesegments) and the molecular weight [12].

In theory, a large number of polymers are available for usein the formulation of solid drug deliverysystems, but not

Polymers in drug delivery Davis, ilium and Stolnik 661

many have been approved for human usc. A list of in­vestigated materials includes polyphosphazenes, polylac­tides, polylactide-co-glycolides, polycaprolactones, polyan­hydrides, polymalic acid and its derivatives, polyalkyl­cyanoacrylates, polyhydroxybutyrates, polycarbonates,polyaminoacids, polyorthoesters, albumin, gelatin, chi­tosan, collagen, alginate, and so on. Of the syntheticpolymers listed, however, only polylactic acid, polylactide­co-glycol ide, polycaprolactone and polyanhydrides havebeen approved for human use.

The material of first choice for the development ofmicroparticlcs is still the group comprising polylactide orthe polylactide-co-glycolide (co)polymcrs. These materialsare now commercially available in various molecularcompositions and molecular weights, and properties suchas degradation rare can be tailored by choosing theoptimal copolymer composition and molecular weight.Improvements in the production methods of polylactideand polylactide-co-glycolidc microspheres are being pub­lished on a regular basis, both as patents and as scientificpapers [13]. The modifications of the emulsificationsolvent evaporation method have been utilized mostly.The double water in oil in water (w/o/w) emulsificationprotocols, where an aqueous solution of a drug is dispersedin organic solvent containing the polymer in the first stepand in the second step this emulsion is then emulsifiedin an aqueous phase containing surfactant, were advocatedfor the more efficient incorporation of water soluble, highmolecular weight agents, such as proteins, and also forbetter protection of the drug from damages during theencapsulation process [14]. A reduction in particle size ofpolylactide and polylacride-co-glycolide microspheres to

a subinicron size range can be achieved by introductionof water-rniscible organic solvent to the organic phaseof the emulsification solvent evaporation protocol. Themodification was named 'spontaneous emulsification sol­vent evaporation method' and the reduction achieved inparticle size was explained by the occurrence of the socalled Marangoni effect [15]. Further reduction in theparticle size [0 a sub-200 nm range can be reached byapplying a preparation method based on precipitation ofthe polymer from its solution by the addition of misciblenonsolvent [16]. This method has attracted the attention ofgroups working on drug targeting aspects, where paniclesin die size range 50 to 250 nrn have been prepared.

Despite various attempts, an efficient incorporation of po­lar, water-soluble agents in the polylactide and polylactide­co-glycolidc microparticlcs, especially of small size, stillremains an elusive goal. The work on using polymericblends to achieve high loadings of proteins and zero orderrelease is illustrative 'of the type of research now beingundertaken in this area [17].

It should be noted that a drug associated with themicrosphcres can be located at the particle surface, closeto the surface or inside the polymeric material. The rate

662 Biological aspects

of release of the encapsulated drug molecules will dependon their location, whereby an initial burst release can occurif a high portion of the drug is located on the surface.Also, if encapsulated proteins are actually associated withthe surface rather then being 'buried' inside the polymericmaterial, then they can be damaged during passagethorough the stomach after oral administration. This wouldbe, for instance, relevant in oral delivery of vaccines. Asubsequent coating of the poly(L-lactide) microparticleswith polymers that 'would form an additional diffusionlayer to protect the surface-present drug, and reduce theburst effect, has recently been described [18] and may bea possible solution.:

Studies on the degradation of polylactide and polylactide­co-glycolide microspheres [19] have demonstrated anaccumulation of low molecular weight oligomers inside themicrospheres during degradation and the existence of pHvalues as low as two [19]. Such an acid environment hasto be carefully considered when encapsulating substancessensitive to low pH. High molecular weight drugs suchas peptides, encapsulated into the polymeric matrix, arecharacterized by a relatively slow release, and, dependingon the degradation rate of the polymer, the decrease in pHmay be detrimental to the peptide still remaining in themicrospheres.

Previously, 'polymers based upon alkylcyanoacrylates re­ceived considerable attention from the standpoint of theirability to produce small particles that could be usefulfor drug targeting [20]. The interest in these systems,however, has declined because of concerns over toxicity.Formaldehyde is produced during degradation of thispolymer in vivo. Reports on the use of such systems totarget to the brain, however, are intriguing [21]. Similarly,the polycaprolactones are also very slowly degradingpolymers and interest in their use has decreased. Poly­hydroxybutyrate represents an interesting material, butits degradation rates in vivo appear to be extremely slowand initial enthusiasm for this material seems to havewaned [22].

Other .polyrners such as polyanhydrides, have the advan­tage of degrading by surface erosion which normally leadsto a highly defined release profile. Formulations, mostlyin the form of small slabs aimed at implantations, havebeen studied for controlled drug release [23]. Recently,polyanhydride microspheres, produced from copolymersof fumaric and sebacic acid, were used to attemptthe bioadhesion to intestinal tissue through hydrogenbonding between hydrophilic functional groups and mucusglycoproteins [24·]. A detailed rationale of why suchinteractions would be relevant to clinical situations was notprovided.

Among the new polymeric materials under development,the polymalic acid derivatives have attracted attention.The benzyl ester derivatives of polymalic acid with

varied levels of pendent carboxyl groups in the polymerchain have been produced [25]. From these materials,150 nm sized nanoparticles were prepared by a precipita­tion/solvent evaporation method aimed at drug targetingpurposes. After injection these nanoparticles showedprolonged systemic circulation [26], relative to the levelof pendent carboxyl groups. At the present time thesepolymers are, however, not commercially available and noteasy to produce synthetically in large quantities.

A novel poly(benzyl glutamate) polymer was appliedin the production of microparticles for liver imaging[27]. Similarly, to polymalic acid derivatives, the benzylglutamate polymer provides functional groups to whichactive agents can be bound. An introduction of suchmaterials into colloidal particles, that at the same time havefree functional groups on the polymer chain, provides theopportunity of covalent binding of drugs that may increasethe incorporation level. Also binding a of targeting moietyon the particle surface is possible. It appears, however, thatsuch possibilities have not yet been fully explored.

The polyphosphazenes represent an interesting class ofnovel polymers: the interest arises from the fact that bythe use of appropriate nucleophiles, or a combinationof nucleophiles, the composition can be adjusted tosynthesize polymers with tailored physical and chemicalproperties. As' for all novel materials, the question oftoxicity and regulatory approval has to be clarified. It hasbeen demonstrated that poly(organo)phosphazenes, thatis, amino acid ester-substituted derivatives, are suitablefor a production of microparticles that range in sizefrom 5 to ZOO urn [28] or small 100-200 nm particles[29]. Furthermore, amino-methoxy poly(ethylene oxide)substituted derivatives were shown to be able to affectthe biodistribution after intravenous injection to the ratand rabbit [30]. One may speculate that materials withhydrophobic amino acids (such as phenylalanine) andarnino-mcthoxy poly(ethylene oxide), c'bsubstituted onthe same polyphosphazene chain, may show interestingproperties with self-forming nanoparticlcs,

Recently, products of novel polymer chemistry in theform of star-like structure's, the so called dendrimers, haveintrigued drug delivery researchers. These well defined,highly branched organic macromolecules in the nanometersize range have specific symmetrical three-dimensionalstructures with internal cavities that can be used forencapsulation of a guest molecule (drugs) [30,31]. Thestructure of the cavity, which is similar to the cyclodextrinsystem, will define the incorporation of a guest molecule.A prospect of dendrimeric delivery depends on thedevelopment of synthetic conditions mild enough not todamage a guest molecule, and of finding ways of producingdendrirncrs with cavities suitable for incorporation of largermolecules. Dendrimcr polymers have also been proposedfor the compaction of plasmid DNA in gene therapy [32].

A variety of natural polymers and macromolecules hasbeen applied in microsphere production. Some of thesesystems, such as dextran-, albumin- and hyaluronic acidester-based microspheres are mentioned later in the text.

Application of colloidal polymericparticles - controlled release and drugtargetingOne of the first therapeutic applications considered formicroparticle drug delivery was the delivery of antiturnouragents, with the aim of improving the site-specificity ofthe drug. Incorporation of drugs such as 5-fluorouracil ormitomycin into the polylactide or polylactide-co-glycolide,albumin or gelatin microparticles for sustained drug releaseis a recent example of this approach [33,34-). The needfor improved anticancer therapy may be best describedby the following summary by Jain "Soon we will be ableto predict an individual's life-time chances of getting atumour on the basis of his genetic profile, and we willbe able to systematically dissect a tumour to determinewhich genes are mutated. We will nevertheless be forcedto tell patients that although we have a set of wonderfulmolecular agents, we cannot deliver them to target cells inthe solid tumours in effective quantities" (35).

An emerging field is the use of microparticles as vehiclesfor delivery of new biotechnology and bioengineeringproducts such as peptides and proteins. Strategies toovercome difficulties in the delivery of peptides acrossbiological membranes, including low permeability, pro­teolysis and first pass metabolism, have included theuse of permeation enhancers, bioadhesive formulations orreceptor-mediated transcytosis. One of the approaches isthe use of bioadhesive systems for 'alternative' routes ofadministration, for example, dextran [36-) or hyaluronicacid ester microspheres (37) for the nasal delivery ofinsulin.

'Oral immunization' via targeting to Peyers patches bymicroencapsulated antigen, has been extensively studied.Namely, there is now considerable evidence that smallquantities of 'micropa~ticulate material are translocatedacross the gastrointestinal tract (the extent of the processis still a controversy), and thus induce immune responsesto microencapsulated or surface-adsorbed antigen. Variousacademic and industrial groups are undertaking researchwith various antigenic materials. Target diseases includeinfluenza, polio, measles, pertussis, cholera, tetanus,and so on. The systems mostly investigated are basedon polylactide-co-glycolide microparticles produced bya double w/o/w emulsion solvent evaporation method,and the work has been concentrated on improving "theloading and finding the optimal microparticle size andsurface properties (charge, hydrophobicity) to enhancethe efficiency of the system [38-). The mcchanisrnts) ofenhanced response to encapsulated antigen is not yet clear,although the current view is that both incorporated and

Polymers in drug delivery Davis. ilium and Stolnik 663

surface-adsorbed antigen, demonstrating slow release fromthe microparticles (depot effect) is essential.

A new area of possible application of microencapsulationtechnology is in nonviral gene delivery. Although colloidalparticles can be a suitable size for cellular penetrationand are a logical choice for delivery of such agentsto endocytic cells, there is still little work done onexploring microparticlc gene delivery (the research inthe field of liposomes has demonstrated feasibility ofthe use of gene complexes wiro cationic liposomesas delivery vehicles) (39). Systems based on gelatinmicrospheres have been investigated by various groups.A complicated system consisting of gelatin-chondroitinsulphate microspheres, with a surface-bound monoclonalantibody for targeting, appeared able to achieve cellspecific delivery of genes (40).

Drug targeting-poly(ethylene oxide)copolymersPolymeric colloidal systems administered parenterally arerecognized rapidly by the reticuloendothelial system, andare consequently sequestered by the phagocytic cellsof the liver and (to a lesser extent) the spleen. Thispassive type of targeting could be an advantage fordiseases involving the reticuloendothelial system, butnormally it represents a major barrier to targeting ofother sites within the body. The pioneering work" ofIlIum and Davis in the eighties, using model polystyrenecolloids, has shown that it is possible to reduce retic­uloendothelial uptake and thereby keep particles inthe general circulation by formation of a poly(ethyleneoxide) layer on the colloid surface (41). This concepthas now been applied successfully to biodegradablepolylactide and polylactide-co-glycolide nanoparticles (inthe liposomal field this idea has been used in the socalled 'stealth' liposome technology) (42). Attention hasrecently been focused on the use of poly(ethylene oxide)copolymers, such as polylactide-polytethylcne oxide),poly(organo)phosphazene-poly(ethylene oxide), dextran­poly(ethylene oxide) and albumin-poly(ethylene oxide).Water-soluble polylacride-polytethylene oxideis have beenemployed for the surface modification of polylactide­co-glycol ide nanoparticles, either by subsequent adsorp­tion onto the nanoparticles or by preparation of thenanoparticles from mixtures of polytlactide-co-glvcolide)and polylactidc-polytethylene oxide) copolymers [43--45).The polylacride-polyterhylene oxide) copolymers withhigher molecular weight polylactide portions are beinginvestigated as 'self-forming' particulate carriers. Particlesize and surface properties are critical issues in targetingand, novel production and drug incorporation methods arein development. Also, the effective incorporation of drugneeds to be resolved,

Recent work by Torchilin et al. (46) has demonstratedthat some other synthetic polymers such as polytacrylamide) and polytvinyl pyrrolidone) can exhibit similar

664 Biological aspects

long-circulating effects to polytethylenc oxide). A highflexibility of the polymer chain appears to be essential.This field has recently been reviewed in a special issueof Advanced Drug Delivery Reviews on Long-CirculatingDrug Delivery Systems (\'01. 16, Iss 2, 1995) as well as insubsequent review articles [47].

More often than not, current work in the field of drugtargeting has concentrated on the vascular compartmentas the most appropriate means of achieving site specificdelivery. The lymphatic system, however, can havean important role to play in pathological conditions,particularly the spread of metastases from cancer. It hasbeen found recently that nanoparticles can be preparedthat have preferential deposition characteristics so far aslymph node targeting is concerned (48). Such systemscould be used not only as drug carriers, but also for thedelivery of diagnostic agents, particularly in X-ray andmagnetic resonance imagery (l\IRI).

Physicochemical characterization ­correlation with biological performances andparticle engineeringA knowledge on the physicochemical properties of themicro particulate systems is essential to assist in our under­standing of their biological behaviour, and in engineeringthe systems with tailored properties. In the past fewyears, a classical characterization protocol for rnicroparticles(analysis of the morphology, size, surface charge and invitro release) has been extended by novel approaches andcharacterization methods, Some of these are already usedin other scientific fields. Surface analysis techniques suchas electron spectroscopy for chemical analysis (ESCA) orsecondary ion mass spectrometry .(SIi\IS) are applicable forthe analysis of micro particle surfaces, particularly systemsfor drug targeting. For example, ·these techniques havebeen applied in the analysis of a range of nanoparticles to

determine the level of the poly(ethylene oxide) presenton the surface. The data collected were correlated within vivo performances (49). ESCA has also been appliedto verify the change in the surface chemical structureafter microparticlcs had been coated with another polymerlayer [18]. The disadvantage of the techniques, that theyrequire a dry sample in a high vacuum environment, maybe overcome by new cryo-techniques. The rationale forthe use of surface analysis techniques, and also whichanalysis can be performed by contemporary tools, havebeen summarized by Ratner [SO··].

Recently, an innovative application of atomic force mi­croscopy allowed in situ visualization of surface erosionof polyanhydride films [51]. The same techniques arebeing considered for probing the forces of molecularinteractions. These may be interactions' between bio­logical macromolecules, between colloidal particles andmacromolecules and so on. Thermal methods, such asdifferential scanning calorimetry, have been applied to

assess the interaction of a drug with a polymeric materialand the state of the incorporated drug. Isothermal titrationmicrocalorimetry has been used to assess adsorption ofpolytethylene oxide) copolymers onto microparricles [52).

An example of the application of novel characterizationtechniques is the use of nuclear magnetic resonanceimaging of polytlactide-co-glvcolide) cylindrical monolithsto probe, qualitatively and quantitatively, the effect ofthe presence of drug on the polymer morphology, drugdistribution in the polymer and drug-polymer interaction.Nuclear magnetic resonance has been shown able to

analyze the presence of poly(ethylene oxide) on thecolloidal surfaces [53). Small angle neutron scatteringis still a relatively inaccessible method, although ithas been applied in the analysis of polymers adsorbedonto deuterated polystyrene colloids and perfluorocarbonemulsions [54).

ConclusionsIn the past few years, work on drug delivery usingpolymers has been focused on various key areas ofmedical science but in particular to solve the problemsof delivering challenging drugs such as peprides andproteins and the nonparenteral delivery of vaccines. Theprospect of microparticulate delivery of such molecules,however, depends on their effective incorporation intothe microparricles, which still needs to be resolved.Research on targeted delivery has been dominated byan introduction of poly(ethylene oxide)-based copolymers,(such as polylactide-polylethylene oxide» in nanoparricleproduction. These materials form micelle-like structures,whereby a drug may be incorporated into the core. Novelphysicochemical characterization techniques have beenintroduced to assist in extending the knowledge of theproperties of drug-polymer interaction, or the chemicalnature of particles surfaces.

In the future, further work will be focused' on polymericrnicroparticlcs for the more effective delivery of vaccinesand drugs. Hopefully, the range of polymer materialapproved by regulatory authorities will increase to allow awider choice of starring materials. Polymers will also findan increasing role in the development of 'smart' systemsfor the improved delivery of plasmid DNA to specifictissue sites as part of the development of novel strategiesfor gene therapy.

References and recommended readingPapers of particular interest, published within the annual period of review,have been highlighted as:

• of special interest•• of outstanding interest

1. Davis 55,IlIum l, McVie JG, Tomlinson E (Eds): Microspheresand Drug Therapy: Pharmaceutical, Immunological and MedicalAspects. Amsterdam: Elsevier; 1984.

2. Gregoriadis G, Senior J, Poste G (Eds): Targeting of Drugs withI Synthetic Systems. New York: Plenum Press; 1986.

'3. Tomlinson E, Davis SS (Eds): Site-Specific Drug Delivery: Cell! Biology, Medical and Pharmaceutical Aspects. Chichester: John

Wiley & Sons; 1986.

4. Morgan SM, Subr V, Ulbrich K, Woodley JF, Duncan R: Evaluationof N-(2·hydroxypropyl)methacrylamide copolymer-peptideconjugates as potential oral vaccines. Studies on theirdegradation by isolated rat small intestinal peptidases andtheir uptake by adult rat small intestinal tissue in vitro. Int JPharm 1996, 128:99-111.

5. Leroux JC, Cozens R, Roesel JL, Galli B, Kubel F, Doelker E,Gurny R: Pharmacokinetics of a novel HIV·1 protease inhibitorincorporated into biodegradable or enteric nanoparticlesfollowing intravenous and oral administration to mice. J PharmSci 1995, 84:1387-1391.

Additional evidence is provided that absorption of intact nanopartidesacross gastrointestinal mucosa is lower than was supposed in the past.

,6. Hyde TM, Galden LF, Payne R: A nuclear magnetic resonancestudy of the effect of incorporation of a macromolecular drugin poly(glycolic acld-co-Dk-lactlc acid). J Control ReI 1995,

I 36:261-275..The peptide molecule was incorporated into the poly(glycolic acid-co-pt-lac­tic acid) cylindrical monoliths, and diffusion of the buffer in the system wasfollowed. From obtained NMR images concentration profiles and kinetic datacan be extracted.

7. Wolfert MA, Seymour LW: Atomic force microscopic analysisof the influence of the molecular weight of poly(L)lysine onthe size of polyelectrolyte complexes formed with DNA. GeneTherapy 1996, 3:269-273.

8. Ledley FD: Nonviral gene therapy: the promise of genesas pharmaceutical products. Human Gene Therapy 1995,6:1129-1144.

9. Taravel MN, Domard A: Collagen and its interaction withchitosan. II. Influence of the physicochemical characteristicsof collagen. Biomater 1995, 16:865-871.

10. Aspden TJ, Adler J, Davis SS, Skaugrad 13, ilium L: Chitosan asa nasal delivery system - evaluation of the effect of chitosanon mucocilliary clearance rate in the i,og palate model. Int JPharm 1995, 122:69-78.

A description of the bioadhesive properties of chitosan.

11. Belbella A, Vauthier C, Fessi H, Devissaguet J-P, Puisieux F: Invitro degradation of nanospheres from poly(D,L-lactides) ofdifferent molecular weight and polydispersities. Int J Pharm1996, 129:95-102.

12. Grandfils C, Flandroy P,Jerome R: Control of the biodegradationrate of poly(DL-lactide) microparticles intended aschemoembolization materials. J Control ReI 1996, 38:109-122.

13. Tsai T, Mehta RC, DeLuca PP Adsorption of peptides topoly(D,L-lactide-co-glycolide). 1. Effect of physical factors onthe adsorption. Int J Pharm 1996, 127:31-42.

14. Prieto MJB, Delie F, Fallal E, Tartar A, Puisieux F,Gulik A,Couvreur P: Characterization of V3 BRU peptide loaded smallPLGA microspheres prepared by a W/OIW emulsion solventevaporation !"ethod. lnt J Pharm 1994, 111:137-145.

15. Niwa T, Takeuchi H, Hino T, Kunou N, Kawashima Y: In vitrodrug release behaviour of D,L-Iactide/glycolide copolymer(PLGA) nanospheres with nafarelin acetate prepared by anovel spontaneous emulsification solvent evaporation method.J Pharm Sci 1994, 83:727-732.

16. Fawaz F, Bonini F, Guyot M, Lagueny AM, Fessi H, Devissaguet JP:Disposition and protective effect against irritation afterintravenous and rectal administration of indomethacin loadednanocapsules to rabbits.lnt J Pharm 1996, 133:107-115.

17. Yeh MK, Davis SS, Coombes AGA: Improving the deliverycapacity of microparticle systems using blends of poly(DLlactide-co-glycolide) and 'Pluronic' PEO-PPO copolymers.In Proceedings of the International Symposium on Control andRelease of Bioactive Materials 1995, 22:412-413.

18. Gopferich A, Alonso MJ, Langer R: Development andcharacterization of microencapsulated microspheres. PharmRes 1994,11:1568-1574.

19. Carrio A, Schwach G, Coudane J, Vert M: Preparation anddegradation of surfactant-free PLAGA microspheres. J ControlReI 1995, 37:113-121.

Polymers in drug delivery Davis, ilium and Stolnik 665

20. Nakada Y, Fattal E, Foulquier M, Couvreur P: Pharmacokineticsand biodistribution of oligonucleotide adsorbed ontopoly(jsobutylcyanoacrylate) nanoparticles after intravenousadministration in mice. Pharm Res 1996, 13:38-43.

21. Schroder U, Sabel BA: Nanoparticles, a drug carrier system topass the blood-brain barrier, permit central analgesic effects ofi.v. dalargin injections. Brain Res 1996, 710:121-124.

22. Pouton CW, Akhtar S: Biosynthetic polyhydroxyalkanoatesand their potential in drug delivery. Adv Drug Del Rev 1996,18:133-162.

23. Mylonas CC, Tabata Y, Langer R, Zohar Y: Preparation andevaluation of polyanhydride microspheres containinggonadotropin-releasing hormone (GnRH), for inducingovulation and spermination in fish. J Control ReI 1995,35:23-34.

24. Chickering DE, Jacob JS, Mathiowitz E: Bioadhesivemicrospheres. 2. Characterisation and evaluation ofbioadhesion involving hard, bioerodible polymers and soft­tissue. Bioreactive Polym 1995, 25:189-206.

A microbalance-based method was used to measure interactions betweenpolyanhydride microspheres and intestinal tissue. The adhesion is believedto occur due to hydrogen bonding.

25. Stolnik S, Garnett MC, Davies MC, ilium L, Bousta M, Vert M,Davis SS: The colloidal properties of surfactant freebiodegradable nanospheres from poly(p-malic acid-co-benzylmalate)s and poly(lactic acld-co-glycolide). Coffoid Surf 1995,97:235-245.

26. Stolnik S, ilium L, Davis SS: Long circulating microparticulatedrug carriers. Adv Drug Del Rev 1995, 16:195-214.

27. Yang OJ,Li CL, Nikiforow S, Gretzer MB, Kuang LR, Lopez MS,Vargas K, Wallance S: Diagnostic and therapeutic potential ofpoly(benzyl L-glutamate). J Pharm Sci 1994, 83:328-331.

28. Goedemoed JH, Mense EHG, De Groot K, Claessen AME,Scheper RJ: Development of injectable antitumor rnlcrospheresbased on polyphosphazenes. J Control ReI 1991, 17:245-258.

29. Vandorpe J, Schacht E, Stolnik S, Garnett MC, Davies MC, ilium L,Davis SS: Poly(organo)phosphazene nanoparticles surfacemodified by poly(ethylene oxide). Biotechnol Bioeng 1996, inpress.

30. Zimmerman SC, Zeng F, Reichert DE, Kolotuchin SV: Self­assembling dendrimers. Science 1996,271:1095-1098.

31. Jansen JFG, De Brabander-van den Berg EMM, Meijer EW:Encapsulation of guest molecules into a dendric box. Science1994, 266:1226-1228.

32. Tomlinson E, Rolland AP: Controllable gene therapyPharmaceutics of non-viral gene delivery systems. J ControlReI 1996,39:357-372.

33. Cummings J, Allan L, Smyth JF: Encapsulation of mitomycinin albumin microspheres markedly alters pharmacokinetics,drug quinone reduction in tumor tissue and antitumor activity­implications for the drugs in vivo mechanism of action.Biochem Pharmaco/1994, 47:1345-1356.

34. Boisdroncelle M, Meneli P, Benoit JP: Preparation andcharacterisation of 5-fluorouraciHoaded microparticles asbiodegradable anticancer drug carriers. J Pharm Pharmacal1995,47:108-114.

The encapsulation of 5-f1uorouracil into polylactide-co-glycolide rnicroparti­des, prepared by emulsion-extraction method, achieved sustained releaseof the drug over 18 days.

35. Jain RK: Delivery of molecular medicine to solid tumours.Science 1996, 271:1079-1080.

36. Pereswetoffmorath L, Edman P: Dextran microspheres as apotential nasal drug-delivery system for insulin - in vitro andin vivo properties. Int J Pharm 1995, 124:37-44.

The influence of the particle size on nasal absorption and localization of in­sulin in the spheres was studied. The spheres with the insulin on the surfacewere more effective in promoting insulin absorption.

37. ilium L, Farraj NF, Fisher AN, Gill I, Miglietta M, Benedetti L:Hyaluronic-acid ester microspheres as a nasal delivery systemfor insulin. J Control ReI 1994, 29:133-141.

38. Lavelle EC, Sharif S, Thomas NW, Holland J, Davis SS: The• importance of gastrointestinal uptake of particles in the design

of oral delivery systems. Adv Drug Del Rev 1995, 18:5-22.A comprehensive review of the work on oral immunization using polymericmicroparticles published in an 'Advanced Drug Delivery Reviews' issue onnonparenteral vaccines.

666 Biological aspects

39. Schreier H: Sawyer SM: Liposomal DNA vectors for cysticfibrosis gene therapy. Current applications, limitations andfuture directions. Adv Drug Del Rev 1996, 19 :73-87.

40. Truong Vl, Guarniery FG, Hildreth JE, Williams JR, August JT,Leong KW: Immuno-microspheres as gene delivery vehicle:Targeting of LAMp·l to lysosomal membrane. Proceedingsof the fnternational Symposium on the Control and Release ofBioactive Materials 1994, 21 :142-.143.

41. Davis S8, ilium L: Particulate systems for site specific drugdelivery In Targeting of Drugs 4: Advances in System Constructs.Edited by Gregoriadis G, McCormack B, Poste G. New York:Plenum Press; 1995:183-194.

42 . Bedu·Addo FK, Huang L: Interaction of PEG-phospholipidconjugates with phospholipid : implications in liposomal drug

.delivery. Adv Drug Del Rev 1995, 16 :235-247.

43 . Stolnik 5, Dunn 5E, Garnett MC, Davies MC, Coombes AGA,ilium L, Davis S5, Taylor DC, Irving MP, Purkiss 5G et al.: Surfacemodification of poly(lactide'co-glycolide) nanoparticles bynovel biodegradable poly(lactide)-poly(ethylene glycol)ccpolymers. Pharm Res 1994, 11 :1800-1808.

44. Verrecchia T, Spenlehauer G, Bazile DV, Murry-Brelier A,Archimbaud Y, Veillard M: Non-stealth (poly(lactic acid/albumin»and stealth (poly(lactic acid-polyethylene glycol)) nanoparticlesas injectable drug carriers. J Control ReI 1995, 36:49-61.

45 . Gref R, Domb A, Quellec P, Blunk T, Muller RH, Verbavatz JM,Langer R: The controlled intravenous delivery of drugs usingPEG-coated sterically stabilized nanospheres. Adv Drug DefRev 1995, 16:215-233.

46. Torchilin Vp, Shtilman ML, Trubetskoy VS, Whiteman K,Milstein AM: Amphiphilic vinyl polymers effectively prolongIiposome circulation time in vivo. Biochim Biophys Acta 1994,1195:181-184.

47. Storm G, BeUiotSO, Daemen T, Lasic DD: Surface modificationof nanoparticles to oppose uptake by the mononuclearphagocyte system. Adv Drug Del Rev 1995, 17 :31-48.

48. Moghimi SM, Hawley AE, Christy NM, Gray T, ilium L, Davis S5:Surface engineered nanospheres with enhanced dr<:inage intolymphatics and uptake by macrophages of the regional lymphnodes. FEBS Lett 1994, 344:25-30.

49 . Brindley A, Davis S5, Davies MC, Watts JF: Polystyrene colloidswith surface grafted poly(ethylene oxide) as model systemsfor site-specific drug delivery. Part 1: Preparation and surfacecharacterisation, J Colloid Interface Sci 1995, 171 :150-161.

50. Ratner BD: Surface analysis and controlled release systems.Proceedings of the InternationalSymposium on the Control andRelease of Bioactive Materials 1995, 22:133-134.

A summary on the possible applications of surface techniques in the con'trolled delivery research. Also gives references for further reading.

51 . Shakesheff KM, Davies MC, Roberts CJ, Tendler SJB, Shard AG,Domb A: In situ atomic force microscopy imaging of polymerdegradation in an aqueous environment. Langmuir 1994,10:4417-4419.

52 . Stolnik S, Heald CR, Garnett MG, Ilium L, Davis S5:Thermodynamics of adsorption of poly(ethylene oxide)copolymers onto model colloidal drug carrier. The use ofisothermal titration microcalorimetry. In 90th Colloid andSurface Science Symposium: 1996 June 16-19; Potsdam.Potsdam, New York: publishers; 1996.

53 . Cosgrove T:'Volume-fraction profiles of adsorbed polymers.J Chem Soc Faraday Trans 1990, 86 :1323-1332.

54 . Washington C, King SM, Heenan RK: Structure of blockcopolymers adsorbed to perfluorocarbon emulsions. J PhysChem 1996, 100:7603-7609.