Protein delivery from biodegradable microspheres

Controlling timing and location in vaccines

Vaccines are one of the most powerful technologies supporting public health. The adaptive immune response induced by immunization arises following appropriate activation and differentiation of T and B cells in lymph nodes. Among many parameters impacting the resulting immune response, the presence of antigen and inflammatory cues for an appropriate temporal duration within the lymph nodes, and further within appropriate subcompartments of the lymph nodes- the right timing and location- play a critical role in shaping cellular and humoral immunity. Here we review recent advances in our understanding of how vaccine kinetics and biodistribution impact adaptive immunity, and the underlying immunological mechanisms that govern these responses. We discuss emerging approaches to engineer these properties for future vaccines, with a focus on subunit vaccines.

IVIVC from Long Acting Olanzapine Microspheres

In this study, four PLGA microsphere formulations of Olanzapine were characterized on the basis of their in vitro behavior at 37°C, using a dialysis based method, with the goal of obtaining an IVIVC. In vivo profiles were determined by deconvolution (Nelson-Wagner method) and using fractional AUC. The in vitro and in vivo release profiles exhibited the same rank order of drug release. Further, in vivo profiles obtained with both approaches were nearly superimposable, suggesting that fractional AUC could be used as an alternative to the Nelson-Wagner method. A comparison of drug release profiles for the four formulations revealed that the in vitro profile lagged slightly behind in vivo release, but the results were not statistically significant (P < 0.0001). Using the four formulations that exhibited different release rates, a Level A IVIVC was established using the deconvolution and fractional AUC approaches. A nearly 1 : 1 correlation (R² > 0.96) between in vitro release and in vivo measurements confirmed the excellent relationship between in vitro drug release and the amount of drug absorbed in vivo. The results of this study suggest that proper selection of an in vitro method will greatly aid in establishing a Level A IVIVC for long acting injectables.

Fragmentation of poly(lactic acid) nanosheets and patchwork treatment for burn wounds

Freestanding poly(L-lactic acid) (PLLA) nanosheets are mass-produced by a simple combination of a spin-coating-assisted multi-layering process and a peeling technique. The resulting PLLA nanosheets are fragmented by homogenization and then reconstructed into a "patchwork" sheet on various surfaces without any adhesive reagents. The patchwork is shown to offer excellent protection against burn wound infection with Pseudomonas aeruginosa, and may therefore be an alternative to conventional burn therapy for prevention of infection.

Localized controlled release of stratifin reduces implantation-induced dermal fibrosis

Localized controlled release of anti-fibrogenic factors can potentially prevent tissue fibrosis surrounding biomedical prostheses, such as vascular stents and breast implants. We have previously demonstrated that therapeutic intervention with topically applied stratifin in a rabbit ear fibrotic model not only prevents dermal fibrosis but also promotes more normal tissue repair by regulating extracellular matrix deposition. In this work, the anti-fibrogenic effect of a controlled release form of stratifin was investigated in the prevention of fibrosis induced by dermal poly(lactic-co-glycolic acid) (PLGA) microsphere/poly(vinyl alcohol) (PVA) hydrogel implants. Pharmacodynamic effects were evaluated by histopathological examination of subcutaneous tissue surrounding implanted composites. Controlled release of stratifin from PLGA microsphere/PVA hydrogel implants significantly moderated dermal fibrosis and inflammation by reducing collagen deposition (30%), total tissue cellularity (48%) and infiltrated CD3(+) immune cells (81%) in the surrounding tissue compared with the stratifin-free implants. The controlled release of stratifin from implants markedly increased the level of matrix metalloproteinase-1 expression in the surrounding tissue, which resulted in less collagen deposition. These stratifin-eluting PLGA/PVA composites show promise as coatings to decrease the typical fibrosis exhibited around implanted biomedical prostheses, such as breast implants and vascular stents.

Synthesis and in vitro characterization of poly(ethylene glycol)-albumin hydrogel microparticles

High water content hydrogel microparticles based on the cross-linking of albumin with activated poly(ethylene glycol) were synthesized. The influence of different synthesis parameters on the physicochemical characteristics of the microparticles, such as the type of oil and of albumin, and the molecular weight of PEG, was evaluated. The water content of the microparticles ranged from 95 to 98%, increasing with an increase of the molecular weight of PEG. At optimal conditions, microparticles with sizes ranging from 3 to 50 μm were prepared. These microparticles showed a negatively charged surface. They were freely dispersed in PBS buffer and they were stable at 4°C for times varying from 0.5 to 10 months. Initial stirring speed and molecular weight of PEG were the 2 main factors that significantly affected microparticle size. High hydrophilicity, good stability and modulable size make this hydrogel an attractive matrix for protein or cell immobilization for biomedical applications.

Stromal-derived factor-1 alpha-loaded PLGA microspheres for stem cell recruitment

PURPOSE

Stromal-derived factor-1 alpha (SDF-1α) is a chemoattractant that has been investigated for treating various diseases, with the goal of recruiting endogenous stem cells to the site of injury. Biodegradable PLGA microspheres were investigated as a means to deliver SDF-1α in a sustained-release manner.

METHODS

We encapsulated SDF-1α into biodegradable poly(lactide-co-glycolide) (PLGA) microspheres using a double-emulsion solvent extraction/evaporation technique. We varied several formulation parameters, characterized the in vitro release profile of SDF-1α and the size and morphology of microspheres, and determined the bioactivity of the released SDF-1α of stimulating migration of mesenchymal stem cells (MSCs).

RESULTS

We found that microspheres fabricated using end-capped PLGA, BSA as an excipient, and low solvent volumes yielded a high encapsulation efficiency (>64%) and released SDF-1α over a >50-day timeframe. The released SDF-1α was bioactive and caused significant migration of MSCs throughout the duration of release from the microspheres.

CONCLUSIONS

We have identified several variables that led to successful encapsulation of SDF-1α into PLGA microspheres. We envision that SDF-lα-loaded microspheres may serve as injectable sources of sustained-release chemokine for promoting the recruitment of endogenous stem cells to the site of injury.

Bioactive electrospun scaffolds delivering growth factors and genes for tissue engineering applications

A biomaterial scaffold is one of the key factors for successful tissue engineering. In recent years, an increasing tendency has been observed toward the combination of scaffolds and biomolecules, e.g. growth factors and therapeutic genes, to achieve bioactive scaffolds, which not only provide physical support but also express biological signals to modulate tissue regeneration. Huge efforts have been made on the exploration of strategies to prepare bioactive scaffolds. Within the past five years, electrospun scaffolds have gained an exponentially increasing popularity in this area because of their ultrathin fiber diameter and large surface-volume ratio, which is favored for biomolecule delivery. This paper reviews current techniques that can be used to prepare bioactive electrospun scaffolds, including physical adsorption, blend electrospinning, coaxial electrospinning, and covalent immobilization. In addition, this paper also analyzes the existing challenges (i.e., protein instability, low gene transfection efficiency, and difficulties in accurate kinetics prediction) to achieve biomolecule release from electrospun scaffolds, which necessitate further research to fully exploit the biomedical applications of these bioactive scaffolds.

Biodegradable microspheres of ketorolac tromethamine for parenteral administration

Ketorolac tromethamine loaded microspheres were prepared using two different polyesters, namely poly (lactic acid) and poly (glycolic acid) by solvent evaporation technique. The morphology of microspheres was analysed by scanning electron microscopy. In vitro release profiles of these microspheres were studied in phosphate buffered saline pH 7.4. The release kinetics of ketorolac tromethamine from the microspheres was evaluated by fitting the release data to the zero-order, Higuchi and korsemeyer-peppas equations. All microspheres showed initial burst release, followed by fickian diffusion of drug through microspheres. These microspheres were formulated as parenterals to have controlled release system.

Mechanisms controlling protein release from lipidic implants: Effects of PEG addition

Different types of tristearin-based implants for controlled rh-interferon alpha-2a (IFN-alpha) release were prepared by compression and thoroughly characterised in vitro. Hydroxypropyl-beta-cyclodextrin (HP-beta-CD) was added as a co-lyophilisation agent for protein stabilisation and different amounts of polyethylene glycol (PEG) as efficient protein release modifier. To get deeper insight into the underlying mass transport mechanisms, the release of IFN-alpha, HP-beta-CD and PEG into phosphate buffer pH 7.4 was monitored simultaneously and appropriate analytical solutions of Fick's second law of diffusion were fitted to the experimental results. Importantly, the addition of only 5-20% PEG to the lipidic implants significantly altered the resulting protein release rates and the relative importance of the underlying mass transport mechanisms. The release of IFN-alpha from PEG-free implants was purely diffusion controlled. In contrast, in PEG-containing devices other phenomena were also involved in the control of protein release: the IFN-alpha release rate remained about constant over prolonged periods of time and the total amounts of mobile IFN-alpha increased. Interestingly, the release of PEG itself as well as of HP-beta-CD from the implants remained purely diffusion controlled, irrespective of the amount of added PEG. Thus, different mass transport mechanisms govern the release of the drug, co-lyophilisation agent and release modifier out of the lipidic implants.

Proprietary Rel-Ease drug delivery technology: opportunity for sustained delivery of peptides, proteins and small molecules

Proprietary Rel-Ease (Praecis Pharmaceuticals) drug delivery technology uses biocompatible polymers as carriers to incorporate a drug into a polymer matrix through opposite charge interaction or complexation. The resulting low solubility complexes can be used to prepare sustained release depot injections or potentially sustained release formulations for oral administration. As a regulatory approved and commercialised drug delivery technology, Rel-Ease is used in abarelix for injectable suspension, a monthly depot injection for the treatment of patients with advanced prostate cancer. The technology offers high drug loading and minimal-to-no initial burst effect in vivo. It uses aqueous processes and is compatible for complexation with many peptide and protein therapeutics; its mechanism can also be applied to many small-molecule therapeutics and offers conventional and alternative methods for sustained release delivery via an oral route.

NIR spectroscopy—a non-destructive analytical tool for protein quantification within lipid implants

Lipid implants have been proposed as promising sustained release devices for the parenteral application of pharmaceutical proteins. Near infrared spectroscopy (NIRS) has been reported in the literature to be a non-destructive tool for drug quantification within controlled release matrix systems based on poly-(lactic-co-glycolic) acid (PLGA). The objective of this study was to evaluate the potential application of NIRS for protein content determination within lipid matrices containing stabilizing and release modifying additives. Bovine serum albumin (BSA) and rh-interferon alpha-2a (IFN alpha-2a) were initially lyophilized with trehalose and then blended with tristearin (matrix material) and optionally with polyethylene glygol 6000 (PEG, release modifier). Implants were prepared by compression. NIR transmittance spectra were measured on a NIRTab spectrometer. Partial least squares regression (PLSR) calibration models were developed to predict protein content in implants from the NIRS results. Additional samples were measured after performing release studies. It could be shown that NIRS allowed protein quantification in complex matrix systems with good accuracy after implant manufacture and during release studies [e.g., standard error of prediction (SEP) between 57 microg-176 microg]. In addition, small protein amounts down to 70 microg of incorporated protein per implant could be determined, thus demonstrating the low detection limit of NIRS.

Effect of additives on encapsulation efficiency, stability and bioactivity of entrapped lysozyme from biodegradable polymer particles

Low encapsulation efficiency, incomplete and erratic release profiles are the most common features of controlled released protein delivery systems employing biodegradable polymers. In the present study, lysozyme as a model protein was encapsulated in biodegradable microspheres using solvent evaporation method and the effect of amphiphilic stabilizer, a basic salt and a lyoprotectant on microparticle formulation was evaluated. Incorporation rat serum albumin (RSA) in the internal aqueous phase during emulsion increased the encapsulation efficiency of lysozyme and maintained the bioactivity. Use of NaHCO3 improved the encapsulation efficiency of lysozyme from 15-94%, but at the cost of reduced in vitro release characteristics. Incorporation of both RSA and NaHCO3 improved the bioactivity of lysozyme and decreased burst release of the protein from the polymer particle, but reduced the encapsulation efficiency from 90-70%. Addition of sucrose in the internal aqueous phase lowered the encapsulation efficiency which was restored by its addition in the external aqueous phase. Maintenance of internal aqueous phase pH close to the iso-electric point of the protein and osmotic balance between the internal aqueous phase and the external aqueous phase during solvent evaporation method helped in better encapsulation of the protein drug. In vitro release of the lysozyme correlated with the effect of different excipients on entrapment in polymer matrix. Entrapment efficiency as high as 76%, low burst effect and high bioactivity of the entrapped lysozyme was observed from the polymer particles. Use of RSA, sucrose and NaHCO3 helped in a co-operative way towards the formulation of particles entrapping bioactive lysozyme.

Subcutaneous drug delivery and the role of the lymphatics

Subcutaneous injections are widely utilised as a delivery route for compounds with limited oral bioavailability or as a means to modify or extend the release profile. In this review, factors affecting absorption from the subcutaneous space are discussed with particular emphasis on differential drug absorption into either the underlying blood or lymphatic capillaries. Formulation and targeted delivery approaches, which utilise the subcutaneous administration route, are reviewed with reference to associated technologies and future challenges.:

Drug transport to brain with targeted nanoparticles

Nanoparticle drug carriers consist of solid biodegradable particles in size ranging from 10 to 1000 nm (50-300 nm generally). They cannot freely diffuse through the blood-brain barrier (BBB) and require receptor-mediated transport through brain capillary endothelium to deliver their content into the brain parenchyma. Polysorbate 80-coated polybutylcyanoacrylate nanoparticles can deliver drugs to the brain by a still debated mechanism. Despite interesting results these nanoparticles have limitations, discussed in this review, that may preclude, or at least limit, their potential clinical applications. Long-circulating nanoparticles made of methoxypoly(ethylene glycol)- polylactide or poly(lactide-co-glycolide) (mPEG-PLA/PLGA) have a good safety profiles and provide drug-sustained release. The availability of functionalized PEG-PLA permits to prepare target-specific nanoparticles by conjugation of cell surface ligand. Using peptidomimetic antibodies to BBB transcytosis receptor, brain-targeted pegylated immunonanoparticles can now be synthesized that should make possible the delivery of entrapped actives into the brain parenchyma without inducing BBB permeability alteration. This review presents their general properties (structure, loading capacity, pharmacokinetics) and currently available methods for immunonanoparticle preparation.

PLGA-PEG microspheres of teverelix: influence of polymer type on microsphere characteristics and on teverelix in vitro release

Teverelix microspheres were produced by coacervation using a new type of poly(ester-carbonates) made of block copolymers of poly(lactic-glycolic acid) (PLGA) and poly(ethylene glycol) (PEG). Five different PLGA-PEG copolymers and one PLGA were used. The 'stability window' has been determined for all polymers. It varied depending on the molecular weight and the weight percentage of PEG. With increasing core loading (from 9.4 to 34.2%), the microparticle size increased from 10-50 to 5-1000 micrometer. The core loading did not have any influence on encapsulation yield, which remained above 80%. The influence of polymer type on microsphere characteristics was studied at two different core loadings: 9.4 and 28%. At a low core loading, the nature of the polymer had no influence on microsphere characteristics whereas at 28%, only PLGA-PEG copolymers gave acceptable microparticles in term of particle size. At 28%, the glass transition temperature (T(g)) of loaded particles was 1-8 degrees C higher than the T(g) of the corresponding polymer. Increasing the core loading increased teverelix release whereas polymer degradation was decreased. All microparticles made of PLGA-PEG copolymers showed a faster release of teverelix than PLGA-based microspheres, whatever the core loading. One PLGA-PEG was selected on the basis of in vitro release rate for further in vivo investigations.

Peripheral nerve regeneration with sustained release of poly(phosphoester) microencapsulated nerve growth factor within nerve guide conduits

Prolonged delivery of neurotrophic proteins to the target tissue is valuable in the treatment of various disorders of the nervous system. We have tested in this study whether sustained release of nerve growth factor (NGF) within nerve guide conduits (NGCs), a device used to repair injured nerves, would augment peripheral nerve regeneration. NGF-containing polymeric microspheres fabricated from a biodegradable poly(phosphoester) (PPE) polymer were loaded into silicone or PPE conduits to provide for prolonged, site-specific delivery of NGF. The conduits were used to bridge a 10 mm gap in a rat sciatic nerve model. Three months after implantation, morphological analysis revealed higher values of fiber diameter, fiber population and fiber density and lower G-ratio at the distal end of regenerated nerve cables collected from NGF microsphere-loaded silicone conduits, as compared with those from control conduits loaded with either saline alone, BSA microspheres, or NGF protein without microencapsulation. Beneficial effects on fiber diameter, G-ratio and fiber density were also observed in the permeable PPE NGCs. Thus, the results confirm a long-term promoting effect of exogenous NGF on morphological regeneration of peripheral nerves. The tissue-engineering approach reported in this study of incorporation of a microsphere protein release system into NGCs holds potential for improved functional recovery in patients whose injured nerves are reconstructed by entubulation.

Release mechanism of insulin encapsulated in trehalose ester derivative microparticles delivered via inhalation

The aim of this study was to evaluate properties of amorphous oligosaccharide ester derivative (OED) microparticles in order to determine drug release mechanisms in the lung. Trehalose OEDs with a wide range of properties were synthesised using conventional methods. The interaction of spray dried amorphous microparticles (2-3 microm) with water was investigated using attenuated total reflectance Fourier transform infra-red spectroscopy (ATR-FTIR) and dynamic vapour sorption (DVS). The in vivo performance of insulin/OED microparticles was assessed using a modified Higuchi kinetic model. A modified Hansen solvent parameter approach was used to analyse the interactions with water and in vivo trends. In water or high humidity, OED powders absorb water, lose relaxation energy and crystallise. The delay of the onset of crystallisation depends on the OED and the amount of water present. Crystallisation follows first order Arrhenius kinetics and release of insulin from OED microparticles closely matches the degree of crystallisation. The induction period depends on dispersive interactions between the OED and water while crystallisation is governed by polarity and hydrogen bonding. Drug release from OED microparticles is, therefore, controlled by crystallisation of the matrix on contact with water. The pulmonary environment was found to resemble one of high humidity rather than a liquid medium.

Monitoring the in vivo delivery of proteins from carbomer hydrogels by X-ray fluorescence

PURPOSE

To measure the effect of protein size on their disappearance from subcutaneously implanted carbomer hydrogel matrices.

METHODS

A series of different molecular weight (MW) proteins were iodinated, incorporated into Carbopol hydrogels, injected subcutaneously into rats, and monitored using X-ray fluorescence (XRF).

RESULTS

A 10 mg/mL minimum concentration of Carbopol-940 was necessary before protein 150 mg/mL iodinated bovine serum albumin (I-BSA)] retention times increased with increasing hydrogel concentration. The decreasing protein signal was not caused by outward protein diffusion or iodoprotein hydrolysis. As the protein MW increased, protein retention times lengthened [e.g.. 6.2 h for insulin (5.7 kDa) to 13.3 h for thyroglobulin (669 kDa)]. Protein disappearance was monophasic first-order for some proteins and biphasic first-order for others. The disappearance rate constant ranged from 0.093 +/- 0.005 h(-1/2), to 0.187 +/- 0.057 h(-1/2), indicating gel erosion rather than protein diffusion as the rate-limiting mechanism. Entrapped I-BSA in Carbopol-1342 NF. pH 7.4, and Carbopol 2001-ETD, pH 7.4, gel matrices yielded different disappearance rates and profiles than Carbopol-940. The overall 50% disappearance rate of I-BSA was greatest for Carbopol-1342 NF (41 +/- 8 h), followed by Carbopol-2001 ETD (25 +/- 2 h) and Carbopol-940 (10.5 +/- 0.7 h).

CONCLUSION

XRF is a noninvasive technique that can be used to follow the status of macromolecules in vivo.

Growth factor delivery for bone tissue engineering

Bone is a dynamic tissue that undergoes significant turnover during the life cycle of an individual. Despite having a significant regenerative capability, trauma and other pathological scenarios commonly require therapeutic intervention to facilitate the healing process. Bone tissue engineering, where cellular and biological processes at a site are deliberately manipulated for a therapeutic outcome, offers a viable option for the treatment of skeletal diseases. In this review paper, we aim to provide a brief synopsis of cellular and molecular basis of bone formation that are pertinent to current efforts of bone healing. Different approaches for engineering bone tissue were presented with special emphasis on the use of soluble (diffusible) therapeutic agents to accelerate bone healing. The latter agents have been used for both local bone repair (i.e. introduction of agents directly to a site of repair) as well as systemic bone regeneration (i.e. delivery for regeneration throughout the skeletal system). Critical drug delivery and targeting issues pertinent for each mode of bone regeneration are provided. In addition, future challenges and opportunities in bone tissue engineering are proposed from the authors' perspective.

Therapeutic efficacy of IL-2-loaded hydrogels in a mouse tumor model

Interleukin-2 (IL-2) is a highly effective anticancer drug if it is applied locally for 5 consecutive days. In most cases this requires 5 invasive treatments, which is not usually acceptable for either the patient or the clinician. For this reason we have developed dextran-based hydrogels from which the required amount of encapsulated IL-2 (1-4 x 10(6) IU of IL-2) is gradually released during 5-10 days. Initially IL-2-containing macroscopic cylinder-shaped gels (implants), and later IL-2-containing injectable microspheres, were developed. These preparations were characterized in vitro, and the therapeutic activity was tested in DBA/2 mice with SL2 lymphosarcoma. The therapy was given to mice with a large and extensively metastasized tumor load (at least 5% of the body weight). If 1-4 x 10(6) IU of IL-2 was slowly released from the hydrogels over a period of 5-10 days, the therapeutic effects were very good and comparable to the effects of free IL-2 injections for 5 consecutive days. In conclusion, dextran-based hydrogels are promising systems for the controlled release of IL-2.

HPMA-hydrogels result in prolonged delivery of anticancer drugs and are a promising tool for the treatment of sensitive and multidrug resistant leukaemia

Treatment of an established BCL1 leukaemia in mice showed that the use of hydrogels is advantageous in comparison with free doxorubicin (DOX), partially due to the different pharmacokinetic profile of the drug release. Pharmacologically active concentrations ranging from 100 to 800 ng/ml were detectable in the bloodstream for more than 4 days when DOX-loaded hydrogels were implanted into mice. Animals treated with free DOX survived for 35 days, survival of hydrogel-DOX treated animals increased up to 60 days and long-term survivors were achieved, when the second hydrogel was implanted 2 weeks after the first one. Hydrogels containing vinblastine (VLB) were ineffective. N-(2-hydroxypropyl)methacrylamide (HPMA) hydrogels were also used in combined therapy against multidrug resistant leukaemia P388-MDR to achieve a synergistic effect of both the cytostatic drug and chemosensitising agent. It was shown that when 4 times the maximal tolerated dose (MTD) of free DOX was incorporated into HPMA-hydrogels, tumour volume was reduced by approximately 50% after implantation of the hydrogel containing DOX and cyclosporine A (CsA) and survival was slightly prolonged.

Lipid-based microtubular drug delivery vehicles

Lipid microtubules that self-assemble from a diacetylenic lipid are suitable structures for the sustained release of bioactive agents. Microtubules were loaded with agents under aqueous conditions and embedded in an agarose hydrogel for localization at areas of interest. Protein release from our microtubule-hydrogel delivery system was characterized in vitro, and in vivo biocompatibility was examined. The influences of protein molecular weight and initial loading concentration on release profile were evaluated by releasing test proteins myoglobin, albumin, and thyroglobulin. Protein molecular weight inversely affected the release rate, and loading with a higher protein concentration increased the mass but not the percent of initially loaded protein released daily. Preservation of protein activity was demonstrated by the ability of a neurotrophic factor released from the delivery system to induce neurite extension in PC12 cells. Bovine aortic smooth muscle cells co-cultured with the microtubule-hydrogel system showed no evidence of cytotoxicity and proliferated in the presence of the microtubules. Subcutaneous implantation of microtubules in rodents revealed no significant inflammatory response after 10 days. Our microtubule-hydrogel system is useful for applications where sustained release without contact between agent and organic solvents is desired.

Stabilizing insulin-like growth factor-I in poly(D,L-lactide-co-glycolide) microspheres

This study aimed at developing a controlled drug delivery system for recombinant human insulin-like growth factor-I (IGF-I) for localized delivery in bone healing. IGF-I was microencapsulated into an end-group uncapped 14 kDa poly(D,L-lactide-co-glycolide) 50:50 (PLGA 50:50) by solvent extraction from a W(1)/O/W(2) dispersion. Prior to encapsulation, IGF-I was exposed to ultrasonication in a water/dichloromethane dispersion, and its stability tested in the presence and absence of various excipients in the W(1) phase. HPLC and RIA were used for the assessment of IGF-I stability. Microencapsulated IGF-I was tested again for its structural intactness and also for in vitro release from various formulations containing appropriate co-encapsulated excipients. A specific fat cell assay was used to determine the biological activity of released IGF-I. Moderate ultrasonic treatment of aqueous IGF-I/dichloromethane mixtures caused approx. 50% IGF-I degradation. However, IGF-I was fully protected when bovine serum albumin, succinylated gelatin or poly(ethyleneglycol) were added to the aqueous IGF-I. Co-encapsulation of these excipients protected efficiently the protein upon microencapsulation. IGF-I release from microsphere formulations was sustained for up to 13 days featuring a moderately pulsatile pattern, depending on the microsphere composition. Typically, the amounts of IGF-I released within the first 24 h (burst) and during the second release pulse were in the order of 20 and 40%, respectively, of the total dose. The biological activity of released IGF-I was confirmed at selected time-points by the fat cell assay. In conclusion, the developed microspheres proved to be suitable to release biologically intact IGF-I over up to 13 days, a time-period considered to be relevant to promote bone fracture healing.

Lectin-mediated bioadhesion: preparation, stability and caco-2 binding of wheat germ agglutinin-functionalized Poly(D,L-lactic-co-glycolic acid)-microspheres

To take advantage of the cytoadhesive characteristics of Wheat germ agglutinin (WGA) for improved particulate drug delivery, the interaction between WGA-grafted poly(D,L-lactic-co-glycolic acid)-microspheres and Caco-2 monolayers was investigated using bovine serum albumin (BSA) or glycine coated microspheres as a control. Covalent immobilization of WGA by the carbodiimide/N-hydroxysuccinimide-method on 4 microm microspheres yielded a surface density of 9.67+/-1.21x10(6) molecules/particle, whereas 0.22+/-0.04x10(6) WGA-molecules were bound by physical adsorption. After storage for 21 days in HEPES-buffer and treatment of the particles with 5 M urea, 86% of covalently linked lectin was still attached to the particles. At 4 degrees C the Caco-2 binding rate of both, WGA- and BSA-modified particles increased with addition of increasing numbers of particles until saturation was reached at 38150+/-1740 (WGA) or 12066+/-1195 (BSA) microspheres bound/mm(2) Caco-2 monolayer. Inhibition of Caco-2 binding of WGA-functionalized microspheres by chitotriose indicated for specificity of the interaction. As observed by confocal laser scanning microscopy, the fluorescein-loading of the particles was accumulated intracellularly after incubation of Caco-2 monolayers with WGA-modified microspheres contrary to glycine-grafted microspheres. Additionally, in case of WGA-functionalized microspheres the amount of cell associated fluorescein was 200-fold higher than that of the free solution. In conclusion, WGA-modified microspheres are expected to enhance intestinal transport of incorporated drugs due to cytoadhesion provided by the lectin coating.

Inactivation of lysozyme by sonication under conditions relevant to microencapsulation

Controlled release dosage forms of proteins and other biomolecules can be prepared by microencapsulating them in polymeric microspheres. Proteins are subjected to potentially damaging effects of sonication and exposure to organic solvents during the microencapsulation process. The relatively stable enzyme lysozyme was dissolved in aqueous buffer and sonicated in the presence of methylene chloride to mimic the initial step of the microencapsulation process. The stability of lysozyme was evaluated by determining the enzyme activity before and after sonication, size-exclusion chromatography, native polyacrylamide gel electrophoresis, and by measuring the amount of precipitates formed. Following sonication, the total protein introduced was distributed between a soluble and an insoluble fraction. Sonication of lysozyme solutions in the presence of methylene chloride led to an increase in precipitates. The precipitates were enzymatically inactive, did not dissolve easily, and were held by non-covalent interactions. No fragments or aggregates of lysozyme were detectable in the soluble fraction. Sonicating aqueous lysozyme solutions with and without methylene chloride decreased the specific activity of the enzyme in the soluble fraction. Excipients such as dimethyl sulfoxide (DMSO), mannitol, sucrose, and tween 80 were included in the sonication mixtures containing lysozyme. With the exception of tween 80, the addition of the excipients to aqueous solutions of lysozyme led to a greater decrease in the specific activity of lysozyme when sonicated in the presence of methylene chloride. DMSO caused the greatest loss of enzyme activity following sonication. Sonication of lysozyme with water, methylene chloride, and DMSO yielded methyl radicals, which were trapped with alpha-phenyl N-tert-butylnitrone and detected by ESR spectroscopy.

5-Fluorouracil-loaded chitosan coated polylactic acid microspheres as biodegradable drug carriers for cerebral tumours

The development of injectable microspheres for anticancer drug delivery into the brain is a major challenge. The possibility of entrapping 5-fluorouracil (5-FU) in chitosan coated monodisperse biodegradable microspheres with a mean diameter of 10-25 um was demonstrated. An emulsion of 5-FU (in water) and polylactic acid (PLA) dissolved in acetone-dichloromethane mixture was poured into an aqueous solution of chitosan (or poly-vinyl alcohol) with stirring using a high-speed homogenizer, for the formation of microspheres. 5-FU recovery in microspheres ranged from 44-66% depending on the polymer and emulsification systems used for the preparation. Scanning electron microscopy revealed that the chitosan coated microspheres had less surface micropores compared to PVA based preparations. The drug release behaviour from microspheres suspended in phosphate buffered saline exhibited a biphasic pattern. The amount of drug release was much higher initially (approximately 25%), followed by a constant slow release profile for a 30 days period of study. This chitosan coated PLA/PLGA microsphere formulation may have potential for the targeted delivery of 5-FU to treat cerebral tumours.

Colchicine encapsulation within poly(ethylene glycol)-coated poly(lactic acid)/poly(epsilon-caprolactone) microspheres-controlled release studies

Smooth muscle cell proliferation plays a major role in the genesis of restenosis after angioplasty or vascular injury. Local delivery of agents capable of modulating vascular responses have the potential to prevent restenosis. However, the development of injectable microspheres for maintaining high tissue levels of drugs at the site of vascular injury is a major challenge. We demonstrated the possibility of entrapping an antiproliferative agent, colchicine, in polyethylene glycol (PEG)-coated biodegradable microspheres composed of poly(lactic acid)/poly(epsilon-caprolactone) blends, with a mean diameter of 3-6 microm. A solution of colchicine and blends of polylactic acid (PLA)/polycaprolactone (PCL) dissolved in acetone-dichloromethane mixture was poured into an aqueous solution of PEG (or polyvinyl alcohol) with stirring by a high-speed homogenizer to form microspheres. Colchicine recovery in microspheres ranged from 30-50% depending on the emulsification system and the ratio of polymer blends used for the preparations. Scanning electron microscopy revealed that the PLA/PCL microspheres were spherical in shape and had a smooth surface texture. Results of in vitro release studies showed that it is possible to control the colchicine release by choosing the appropriate particle size, loading, and PLA/PCL composition. Water permeability through the PLA membrane was greater, when compared with PCL blends. The amount of drug release also was much higher (58.3%) in PLA compared with PCL (39.3%) microspheres, for 30 days. Therefore, we concluded that the drug release from the microspheres followed a diffusion mechanism where bulk erosion and surface deposition were negligible. These PEG-coated PLA/PCL microspheres may have potential for targeting antiproliferative agents for prolonged periods to treat restenosis.

Bioabsorbable polymer scaffolds for tissue engineering capable of sustained growth factor delivery

Engineering new tissues utilizing cell transplantation on biodegradable polymer matrices is an attractive approach to treat patients suffering from the loss or dysfunction of a number of tissues and organs. The matrices must maintain structural integrity during the process of tissue formation, and promote the vascularization of the developing tissue. A number of molecules (angiogenic factors) have been identified that promote the formation of new vascular beds from endothelial cells present within tissues, and the localized, controlled delivery of these factors from a matrix may allow an enhanced vascularization of engineered tissues. We have developed a gas foaming polymer processing approach that allows the fabrication of three-dimensional porous matrices from bioabsorbable materials (e.g., copolymers of lactide and glycolide [PLG]) without the use of organic solvents or high temperatures. The effects of several processing parameters (e.g., gas type, polymer composition and molecular weight) on the process were studied. Several gases (CO(2), N(2), He) were utilized in the fabrication process, but only CO(2) resulted in the formation of highly porous, structurally intact matrices. Crystalline polymers (polylactide and polyglycolide) did not form porous matrices, while amorphous copolymers (50:50, 75:25, and 85:15 ratio of lactide:glycolide) foamed to yield matrices with porosity up to 95%. The mechanical properties of matrices were also regulated by the choice of PLG composition and molecular weight. Angiogenic factors (e.g., vascular endothelial growth factor) were subsequently incorporated into matrices during the fabrication process, and released in a controlled manner. Importantly, the released growth factor retains over 90% of its bioactivity. In summary, a promising system for the incorporation and delivery of angiogenic factors from three-dimensional, biodegradable polymer matrices has been developed, and the fabrication process allows incorporation under mild conditions.

Pulsatile release from subcutaneous implants

Pulsatile delivery of antigens and hormones from subcutaneous implants could have uses in the animal production and veterinary medicine. Development of single-shot vaccines which release both initial and booster antigen from a single administration and hormonal preparations that release in a similar manner to the natural secretion patterns are two areas with potential. Formulation approaches employed to produce subcutaneous implants with pulsatile release profiles are reviewed.

Controlled release of a model protein from enzymatically degrading dextran microspheres

Protein-loaded dextran microspheres were prepared by a water-in-water emulsion technique. With this technique, an aqueous solution of methacrylated dextran (dex-MA) is emulsified in an aqueous solution of poly(ethylene glycol) (PEG). Subsequently, the dispersed dex-MA phase is crosslinked by radical polymerization of the dextran-bound methacryloyl groups. This method renders microspheres with a hydrogel character of which the crosslink density can be controlled by the water content and the degree of substitution of the dex-MA (DS, the number of methacrylates per 100 glucopyranose residues). If an IgG solution was added to the dex-MA/PEG aqueous system prior to the polymerization reaction, the protein could be encapsulated in the dextran microspheres with a high yield (88-98%). The release of IgG was studied as a function of the water content, the DS and the degradation rate of the microspheres. The microspheres were rendered degradable by co-encapsulation of an endo-dextranase. Non-degrading microspheres mainly showed a burst release, which decreased with increasing crosslink density. By either a low water content (50%, w/w, or lower) or a high DS (DS 13), it was possible to reduce the burst release to about 10%, meaning that almost complete entrapment of the protein could be achieved. The release of IgG from degrading microspheres was predominantly dependent on the DS and the amount of encapsulated dextranase. No differences in release of IgG from microspheres with and without dextranase were observed at high DS (DS 13). This was ascribed to the inability of the enzyme to degrade these microspheres. On the other hand, the entrapped protein was completely released from enzymatically degrading microspheres with a DS 4. Moreover, the release rate of IgG was proportional to the degradation rate of these microspheres (depending on the amount of co-encapsulated dextranase). Interestingly, an almost zero-order release was observed from these microspheres for periods up to 30 days.

FTIR characterization of the secondary structure of proteins encapsulated within PLGA microspheres

A commonly used technique for protein encapsulation in microspheres is the double-emulsion method wherein an initial water-in-oil (w/o) emulsion of protein and polymer is formed via sonication, and then a second emulsion (w/o)/w is formed by dispersion in an aqueous phase via homogenization. This approach is often used to produce microspheres of biodegradable poly(lactic-co-glycolic acid) (PLGA). The harsh processing associated with this method can cause denaturation of the encapsulated protein. Herein, we have used Fourier transform infrared (FTIR) spectroscopy to determine the secondary structures of two model proteins, bovine serum albumin (BSA) and chicken egg-white lysozyme, within PLGA microspheres. The alpha-helix content of both proteins in the microspheres was about a third lower than in the lyophilized state, indicating conformational changes upon protein entrapment within the microspheres. BSA microspheres containing the stabilizing excipient trehalose have a higher alpha-helix content than those without excipient, suggesting that trehalose partially prevents the denaturing effects incurred during processing. In addition, BSA released from microspheres is improved by incorporation of trehalose: analysis of the protein released from the microspheres indicates that there is less BSA dimer formation in the trehalose-containing microspheres than in those without trehalose.

New advances in microsphere-based single-dose vaccines

Polymer microspheres have shown great potential as a next generation adjuvant to replace or complement existing aluminum salts for vaccine potentiation. Microsphere-based systems can now be made to deliver subunit protein and peptide antigens in their native form in a continuous or pulsatile fashion for periods of weeks to months with reliable and reproducible kinetics, often obviating the need for booster immunizations in animal models. Microspheres have also shown potential as carriers for oral vaccine delivery due to their protective effects on encapsulated antigens and their ability to be taken up by the Peyer's patches in the intestine. The potency of these optimal depot formulations for antigen may be enhanced by the co-delivery of vaccine adjuvants, including cytokines, that are either entrapped in the polymer matrix or, alternatively, incorporated into the backbone of the polymer itself and released concomitantly with antigen as the polymer degrades. In this article we review the use of polymer microspheres for single-step immunization and discuss future applications for the improvement of vaccines and immunotherapies by utilizing encapsulation technology.