Design and production of single-immunization vaccines using polylactide polyglycolide microsphere systems

Recent Progress in Drug Release Testing Methods of Biopolymeric Particulate System

Biopolymeric microparticles have been widely used for long-term release formulations of short half-life chemicals or synthetic peptides. Characterization of the drug release from microparticles is important to ensure product quality and desired pharmacological effect. However, there is no official method for long-term release parenteral dosage forms. Much work has been done to develop methods for in vitro drug release testing, generally grouped into three major categories: sample and separate, dialysis membrane, and continuous flow (flow-through cell) methods. In vitro drug release testing also plays an important role in providing insight into the in vivo performance of a product. In vitro release test with in vivo relevance can reduce the cost of conducting in vivo studies and accelerate drug product development. Therefore, investigation of the in vitro-in vivo correlation (IVIVC) is increasingly becoming an essential part of particulate formulation development. This review summarizes the principles of the in vitro release testing methods of biopolymeric particulate system with the recent research articles and discusses their characteristics including IVIVC, accelerated release testing methods, and stability of encapsulated drugs.

Injectable polymer microspheres enhance immunogenicity of a contraceptive peptide vaccine

Advanced contraceptive peptide vaccines suffer from the unavailability of adjuvants capable of enhancing the antibody response with acceptable safety. We sought to overcome this limitation by employing two novel poly(lactic-co-glycolic acid) (PLGA) microsphere formulations to deliver a synthetic human chorionic gonadotropin (hCG) peptide antigen co-synthesized with a T-cell epitope from tetanus toxoid (TT), C-TT2-CTP35: surface-conjugated immunogen to induce phagocytosis; and encapsulated peptide to provide a depot effect, with MgCO(3) co-encapsulated in the polymer to neutralize acidity from the biodegrading PLGA polyester. A single immunization of encapsulated peptide in rabbits elicited a stronger antibody response with equivalent duration relative to a positive control--three injections of the peptide administered in a squalene-based water-in-oil emulsion. Surface-conjugated peptide was less effective but enhanced antibody levels at 1/5 the dose, relative to soluble antigen. Most remarkable and unexpected was the finding that co-encapsulation of base was essential to attain the powerful adjuvant effect of the PLGA-MgCO(3) system, as the MgCO(3)-free microspheres were completely ineffective. A promising contraceptive hCG peptide vaccine with acceptable side effects (i.e., local tissue reactions) was achieved by minimizing PLGA and MgCO(3) doses, without significantly affecting antibody response.

Effect of the covalent modification of horseradish peroxidase with poly(ethylene glycol) on the activity and stability upon encapsulation in polyester microspheres

Encapsulation of proteins in polyester microspheres by coacervation methods frequently causes protein inactivation and aggregation. Furthermore, an often-substantial amount of the encapsulated proteins is released within the first 24 h from the microspheres. To overcome these problems poly(ethylene glycol) (PEG) was employed as excipient and protein-modifying agent. The model protein horseradish peroxidase (HRP) was chemically modified or co-lyophilized with PEG of differing molecular weights, namely PEG(5000), PEG(20000), and PEG(40000). The lyophilized preparations were encapsulated in poly(D,L-lactide-co-glycolic) acid (PLGA) microspheres by a coacervation method. Covalent modification of HRP with PEG increased the encapsulation efficiency (EE) from 83% to about 100% while PEG when used as an excipient reduced the EE. Encapsulation caused aggregation of ca. 5% of non-modified HRP and the residual specific activity was only 57%. Covalent modification with PEG reduced HRP aggregation to less than 1% and improved its residual activity to more than 95%. When PEG was used as excipient similar results were found with respect to a reduction in encapsulation-induced aggregation, but no more than 80% of residual activity was obtained even for the best formulation after encapsulation. It was also found that covalent modification of HRP with PEG substantially reduced the unwanted initial "burst" release observed during the initial 24 h of in vitro release from about 70% to 23%. Furthermore, HRP activity and stability were also improved during in vitro release for HRP-PEG conjugates. The data show that covalent modification of proteins with PEG might be useful to improve protein stability during coacervation encapsulation and subsequent release as well as to increase EE and reduce the burst release.

An Investigation of the Factors Controlling the Adsorption of Protein Antigens to Anionic PLG Microparticles

This work examines physico-chemical properties influencing protein adsorption to anionic PLG microparticles and demonstrates the ability to bind and release vaccine antigens over a range of loads, pH values, and ionic strengths. Poly(lactide-co-glycolide) microparticles were synthesized by a w/o/w emulsification method in the presence of the anionic surfactant DSS (dioctyl sodium sulfosuccinate). Ovalbumin (OVA), carbonic anhydrase (CAN), lysozyme (LYZ), lactic acid dehydrogenase, bovine serum albumin (BSA), an HIV envelope glyocoprotein, and a Neisseria meningitidis B protein were adsorbed to the PLG microparticles, with binding efficiency, initial release and zeta potentials measured. Protein (antigen) binding to PLG microparticles was influenced by both electrostatic interaction and other mechanisms such as van der Waals forces. The protein binding capacity was directly proportional to the available surface area and may have a practical upper limit imposed by the formation of a complete protein monolayer as suggested by AFM images. The protein affinity for the PLG surface depended strongly on the isoelectric point (pI) and electrostatic forces, but also showed contributions from nonCoulombic interactions. Protein antigens were adsorbed on anionic PLG microparticles with varying degrees of efficiency under different conditions such as pH and ionic strength. Observable changes in zeta potentials and morphology suggest the formation of a surface monolayer. Antigen binding and release occur through a combination of electrostatic and van der Waals interactions occurring at the polymer-solution interface.

Adsorption of a Novel Recombinant Glycoprotein from HIV (Env gp120dV2 SF162) to Anionic PLG Microparticles Retains the Structural Integrity of the Protein, Whereas Encapsulation in PLG Microparticles Does Not

PURPOSE

To evaluate the delivery of a novel HIV-1 antigen (gp120dV2 SF162) by surface adsorption or encapsulation within polylactide-co-glycolide microparticles and to compare both the formulations for their ability to preserve functional activity as measured by binding to soluble CD4.

METHODS

Poly(lactide-co-glycolide) microparticles were synthesized by a water-in-oil-in-water (w/o/w) emulsification method in the presence of the anionic surfactant dioctylsulfosuccinate (DSS) or polyvinyl alcohol. The HIV envelope glyocoprotein was adsorbed and encapsulated in the PLG particles. Binding efficiency and burst release measured to determine adsorption characteristics. The ability to bind CD4 was assayed to measure the functional integrity of gp120dV2 following different formulation processes.

RESULTS

Protein (antigen) binding to PLG microparticles was influenced by both electrostatic interaction and other mechanisms such as hydrophobic attraction and structural accommodation of the polymer and biomolecule. The functional activity as measured by the ability of gp120dV2 to bind CD4 was maintained by adsorption onto anionic microparticles but drastically reduced by encapsulation.

CONCLUSIONS

The antigen on the adsorbed PLG formulation maintained its binding ability to soluble CD4 in comparison to encapsulation, demonstrating the feasibility of using these novel anionic microparticles as a potential vaccine delivery system.

Water-based nanoparticulate polymeric system for protein delivery: permeability control and vaccine application

The idea of using polymeric nanoparticles as drug carriers is receiving an increasing amount of attention both in academia and industry, Nanoparticles have a number of potential applications in protein, drug and vaccine delivery, as well as gene therapy applications. In this article, we focus on this unique drug delivery technology as a method to control the release rate of substances, not only for protein delivery but also for delivering an experimental vaccine immunogen. Nanoparticles were assembled on the basis of ionic interaction between water-soluble polymers so that the resulting particles were stable in physiologic media. Among the typical polymers used to assemble nanoparticles, different polysaccharides, natural amines, and poly-amines were investigated. The entrapped substances tested included a protein and antigens. Polydextran aldehyde was incorporated into the particle core, to enable physiologic cross-linking as a method to control permeability. This resulted in long-term retention of substances that would otherwise rapidly leak out of the nanoparticles. Results of cross-linking experiments clearly demonstrated that the release rate could be substantially reduced, depending on the degree of cross-linking. For vaccine antigen delivery tests, we measured an antibody production after subcutaneous and oral administration. The data indicated that only the cross-linked antigen was immunogenic when the oral route of administration was used. The data presented in this article address primarily the utility of nanoparticulates for oral delivery of vaccine antigen.

Building drug delivery into tissue engineering

The creation of efficient methods for manufacturing biotechnology drugs--many of which influence fundamental but complex cell behaviours, such as proliferation, migration and differentiation--is creating new opportunities for tissue repair. Many agents are potent and multifunctional; that is, they produce different effects within different tissues. Therefore, control of tissue concentration and spatial localization of delivery is essential for safety and effectiveness. Synthetic systems that can control agent delivery are particularly promising as materials for enhancing tissue regeneration. This review discusses the state of the art in controlled-release and microfluidic drug delivery technologies, and outlines their potential applications for tissue engineering.

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.

Single-dose mucosal immunization with biodegradable microparticles containing a Schistosoma mansoni antigen

The purpose of this work was to assess the immunogenicity of a single nasal or oral administration of recombinant 28-kDa glutathione S-transferase of Schistosoma mansoni (rSm28GST) entrapped by poly(lactide-co-glycolide) (PLG)- or polycaprolactone (PCL)-biodegradable microparticles. Whatever the polymer and the route of administration used, the equivalent of 100 microg of entrapped rSm28GST induced a long-lasting and stable antigen-specific serum antibody response, with a peak at 9 to 10 weeks following immunization. Isotype profiles were comparable, with immunoglobulin G1 being the predominant isotype produced. The abilities of specific antisera to neutralize the rSm28GST enzymatic activity have been used as criteria of immune response quality. Pooled 10-week sera from mice receiving PLG microparticles by the nasal or oral route neutralized the rSm28GST enzymatic activity, whereas sera of mice receiving either PCL microparticles, free rSm28GST, or empty microparticles inefficiently neutralized this enzymatic activity. Finally, this study shows that a single administration of these microparticles could provide distinct and timely release pulses of microencapsulated antigen, which might greatly facilitate future vaccine development.

Stabilization of dichloromethane-induced protein denaturation during microencapsulation

This paper describes the denaturation of protein drugs by dichloromethane (DCM) during the primary emulsification step of the microencapsulation process using biodegradable polymer matrix for controlled-release application. It was found that interaction of proteins such as tetanus toxoid (TT), diphtheria toxoid (DT), ovine growth hormone (oGH), and human chorionic gonadotropin-based antifertility vaccine (beta-hCG-TT) with DCM during primary emulsification stages of particle formulation led to the precipitation of the proteins at the aqueous organic interface with concomitant reduction in their immunoreactivity. On the other hand, the B subunit of E. coli enterotoxin (LTB) was found to be comparatively stable toward the denaturing action of DCM. Attempts were made to overcome the DCM-induced denaturation by incorporation of stabilizers during the primary emulsification step of the particle formulation. Of the many additives tested to overcome the DCM-induced denaturation of proteins, serum albumins and polyvinyl alcohol (PVA) showed promising results in terms of retention of the immunoreactivity of the protein. TT stabilized by the incorporation of serum albumin during the primary emulsification step not only showed immunoreactivity in vitro, but also invoked antibody titers in rats comparable to those obtained for the native protein molecules. Incorporation of 2.5% of serum albumins in the internal aqueous phase not only protected the protein from the degradative action of DCM but also led to stabilized primary emulsion, which is necessary for uniform entrapment of protein drugs in the polymer matrix.

Recombinant human growth hormone poly(lactic-co-glycolic acid) microsphere formulation development

The development of a sustained release formulation of recombinant human growth hormone (rhGH) has focused on a depot preparation using the biodegradable polymer, poly(lactic-co-glycolic acid) (PLGA), for microsphere production. These formulations have been designed to assure the maintenance of protein integrity both during the microencapsulation process and upon subsequent release in vitro and in vivo. In addition, animal models were developed to assess both the in vivo release kinetics and the potency of the released protein. These studies emphasized the importance of obtaining a correlation between the in vivo and in vitro release at an early stage of development. Juvenile rhesus monkey studies revealed that continuous rhGH administration resulted in a greater total insulin-like growth factor-I (IGF-I) response than daily rhGH administration, indicating that a continuous rhGH dose may provide comparable efficacy to daily dosing at a lower total dose of rhGH. The use of a conventional water-in-oil-in-water process yielded a triphasic release of biologically active and non-immunogenic rhGH, while the novel cryogenic process achieved a continuous release of rhGH that is biologically active and non-immunogenic. The rhGH PLGA formulation produced by the novel cryogenic process was manufactured under aseptic GMP conditions and was shown to be safe in growth hormone-deficient adults. This protein and these studies should serve as a model for the future development of PLGA formulations for therapeutic proteins.

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.

Protein delivery from biodegradable microspheres

The key components to the successful development of a biodegradable microsphere formulation for the delivery of proteins are polymer chemistry, engineering, and protein stability. These areas are intricately related and require a thorough investigation prior to embarking on the encapsulation of proteins. While each of these components is important for the development of a biodegradable microsphere formulation for protein delivery, other critical issues should also be considered. In particular, preclinical studies in the appropriate animal model are usually necessary to assess the potential feasibility of a continuous-release dosage form. These studies should be performed at the earliest possible stage of development to validate the feasibility of a controlled release formulation. After the utility of a controlled release formulation has been demonstrated, the polymer matrix should be chosen and bench-scale production of microspheres initiated. The only polymers presently approved for human use for controlled delivery are the polylactides [poly(lactic acid), poly(glycolic acid), and poly(lactic-coglycolic) acid]. These polymers require multiphase processes involving several steps to produce microspheres containing the desired protein. A thorough review of previous work on encapsulation with these polymers should provide some insight into conditions to be assessed in developing a process. Once a process is chosen, it must be optimized to provide the highest possible yield of microspheres with the desired characteristics (e.g., loading, release, size, etc.). Finally, the final aseptic process should be validated and methods generated to assess the final product. The clinical studies should then start upon approval of the IND application. In the future, the biotechnology industry, and the pharmaceutical industry in general, will be seeking new methods to improve the delivery of therapeutic agents such as proteins and peptides. Formulations like biodegradable microspheres significantly reduce health-care costs since fewer administrations are needed, and they provide a competitive advantage in markets with several competing products (e.g., LHRH agonist market). Further, many new indications such as neurological diseases may require a long-term delivery system. The future success of biodegradable microsphere formulations will primarily depend on the commitment of the pharmaceutical and biotechnology industries to the development of this technology.

Drug delivery issues in vaccine development

Although significant headway has been made in vaccine development, there are several delivery-related issues that must be overcome to advance tomorrow's candidate vaccines. Some of these are in the areas of: single-shot subunit vaccines, therapeutic vaccines for cancer, the use of cytokines as vaccine adjuvants, DNA-based vaccines, and the development of vaccines that provide sterilizing immunity, as might be required for an affective HIV-1 prophylactic vaccine. The hurdles for vaccine advancement in these areas are briefly described.

Pulsed controlled-released system for potential use in vaccine delivery

Biodegradable polymeric devices intended to provide a viable route for single-dose vaccination were developed using controlled-release technology. One of the main challenges in the development of these devices was to overcome several water-mediated inactivation processes that occur in conventional polymeric systems. Our strategy was focused on the prevention of antigen exposure to environmental conditions. For this purpose a microencapsulation process was designed and optimized to provide an inert and insulated environment for the bioactive material inside controlled-release systems. Tetanus toxoid (TT) was used as a model antigen. The systems consist of core-wall microcapsule structures in which the antigenic protein is entrapped into oil-based cores of TT surrounded by outer polymer shells made of poly(D,L-lactide-co-glycolide), thus potentially protecting the bioactive material against deleterious conditions. Furthermore, using these microcapsules, pulses of immunochemically detected TT were programmed to release at two different times (3 and 7 weeks), as corroborated by in vitro release studies. The engineering of these specific antigen release properties was possible by careful selection of the copolymer composition and molecular weight. The final formulations were characterized with respect to morphology, structure, size distribution, and amount of immunochemically detected TT encapsulated. The new systems offer the potential to control the manner and timing of delivery. Over 92% of the TT released over a 63 day period from these microcapsules was immunochemically detected.