Controlled release microparticles as a single dose diphtheria toxoid vaccine: immunogenicity in small animal models

Injectable controlled-release systems for the prevention and treatment of infectious diseases

Pharmaceutical drugs, including vaccines, pre- and post-exposure prophylactics, and chronic drug therapies, are crucial tools in the prevention and treatment of infectious diseases. These drugs have the ability to increase survival and improve patient quality of life; however, infectious diseases still accounted for more than 10.2 million deaths in 2019 before the COVID-19 pandemic. High mortality can be, in part, attributed to challenges in the availability of adequate drugs and vaccines, limited accessibility, poor drug bioavailability, the high cost of some treatments, and low patient adherence. A majority of these factors are logistical rather than technical challenges, providing an opportunity for existing drugs and vaccines to be improved through formulation. Injectable controlled-release drug delivery systems are one class of formulations that have the potential to overcome many of these limitations by releasing their contents in a sustained manner to reduce the need for frequent re-administration and improve clinical outcomes. This review provides an overview of injectable controlled drug delivery platforms, including microparticles, nanoparticles, and injectable gels, detailing recent developments using these systems for single-injection vaccination, long-acting prophylaxis, and sustained-release treatments for infectious disease.

Engineered drug delivery devices to address Global Health challenges

There is a dire need for innovative solutions to address global health needs. Polymeric systems have been shown to provide substantial benefit to all sectors of healthcare, especially for their ability to extend and control drug delivery. Herein, we review polymeric drug delivery devices for vaccines, tuberculosis, and contraception.

Virus-Like Particle, Liposome, and Polymeric Particle-Based Vaccines against HIV-1

It is acknowledged that vaccines remain the best hope for eliminating the HIV-1 epidemic. However, the failure to produce effective vaccine immunogens and the inability of conventional delivery strategies to elicit the desired immune responses remains a central theme and has ultimately led to a significant roadblock in HIV vaccine development. Consequently, significant efforts have been applied to generate novel vaccine antigens and delivery agents, which mimic viral structures for optimal immune induction. Here, we review the latest developments that have occurred in the nanoparticle vaccine field, with special emphasis on strategies that are being utilized to attain highly immunogenic, systemic, and mucosal anti-HIV humoral and cellular immune responses. This includes the design of novel immunogens, the central role of antigen-presenting cells, delivery routes, and biodistribution of nanoparticles to lymph nodes. In particular, we will focus on virus-like-particle formulations and their preclinical uses within the HIV prophylactic vaccine setting.

Immunogenicity of pulsatile-release PLGA microspheres for single-injection vaccination

The World Health Organization's Expanded Programme on Immunization has led to a dramatic rise in worldwide vaccination rates over the past 40 years, yet 19.4 million infants remain underimmunized each year. Many of these infants have received at least one vaccine dose but may remain unprotected because they did not receive subsequent booster doses due to logistical challenges. This study aimed to develop injectable controlled release microparticles with kinetics that mimic common vaccine dosing regimens consisting of large antigen doses administered periodically over the course of months in order to eliminate the need for boosters. Sixteen poly(lactic-co-glycolic acid) (PLGA) microsphere formulations containing bovine serum albumin (BSA) as a model vaccine antigen were screened in vitro to determine their respective release kinetics. Three formulations that exhibited desirable pulsatile release profiles were then selected for studying immunogenicity in mice. Two low-dose microsphere formulations induced peak anti-BSA IgG antibody titers of 13.9 ± 1.3 and 13.7 ± 2.2 log₂ compared to 15.5 ± 1.5 log₂ for a series of three bolus injections delivered at 0, 4, and 8 weeks with an equivalent cumulative dose. Similarly, high-dose formulations induced peak antibody titers that were 16.1 ± 2.1 log₂ compared to 17.7 ± 2.2 log₂ for controls. All three microparticle formulations studied in vivo induced peak antibody titers that were statistically similar to bolus controls. These results suggest that pulsatile antigen release from polymeric microparticles is a promising approach for single-injection vaccination, which could potentially reduce the logistical burden associated with immunization in the developing world.

Macromolecular systems for vaccine delivery

Vaccines have helped considerably in eliminating some life-threatening infectious diseases in past two hundred years. Recently, human medicine has focused on vaccination against some of the world's most common infectious diseases (AIDS, malaria, tuberculosis, etc.), and vaccination is also gaining popularity in the treatment of cancer or autoimmune diseases. The major limitation of current vaccines lies in their poor ability to generate a sufficient level of protective antibodies and T cell responses against diseases such as HIV, malaria, tuberculosis and cancers. Among the promising vaccination systems that could improve the potency of weakly immunogenic vaccines belong macromolecular carriers (water soluble polymers, polymer particels, micelles, gels etc.) conjugated with antigens and immunistumulatory molecules. The size, architecture, and the composition of the high molecular-weight carrier can significantly improve the vaccine efficiency. This review includes the most recently developed (bio)polymer-based vaccines reported in the literature.

The preparation and characterization of PLG nanoparticles with an entrapped synthetic TLR7 agonist and their preclinical evaluation as adjuvant for an adsorbed DTaP vaccine

The design of safe and potent adjuvants able to enhance and modulate antigen-specific immunity is of great interest for vaccine research and development. In the present study, negatively charged poly(lactide-co-glycolide) (PLG) nanoparticles have been combined with a synthetic immunepotentiator molecule targeting the Toll-like receptor 7. The selection of appropriate preparation and freeze-drying conditions resulted in a PLG-based adjuvant with well-defined and stable physico-chemical properties. The adjuvanticity of such nanosystem has later been evaluated in the mouse model with a diphtheria-tetanus-pertussis (DTaP) vaccine, on the basis of the current need to improve the efficacy of acellular pertussis (aP) vaccines. DTaP antigens were adsorbed onto PLG nanoparticles surface, allowing the co-delivery of TLR7a and multiple antigens through a single formulation. The entrapment of TLR7a into PLG nanoparticles resulted in enhanced IgG and IgG2a antibody titers. Notably, the immune potentiator effect of TLR7a was less evident when it was used in not-entrapped form, indicating that co-localization of TLR7a and antigens is required to adequately stimulate immune responses. In conclusion, the rational selection of adjuvants and formulation here described resulted as a highly valuable approach to potentiate and better tailor DTaP vaccine immunogenicity.

The impact of size on particulate vaccine adjuvants

Particulate adjuvants have been successful at inducing increased immune responses against many poorly immunogenic antigens. However, the mechanism of action of these adjuvants often remains unclear. As more potential vaccine targets are emerging, it is becoming necessary to broaden our knowledge on the factors involved in generating potent immune responses to recombinant antigens with adjuvants. While composition of adjuvants is integral in defining the overall performance of an adjuvant, some physical parameters such as particle size, surface charge and surface modification may also contribute to the potency. In this review, we will try to highlight the role of particle size in controlling the immune responses to adjuvanted vaccines, with a focus on insoluble aluminum salts, oil-in-water emulsions, polymeric particles and liposomes.

Single-injection vaccines: Progress, challenges, and opportunities

Currently, vaccination is the most efficient and cost-effective medical treatment for infectious diseases; however, each year 10 million infants remain underimmunized due to current vaccination schedules that require multiple doses to be administered across months or years. These dosing regimens are especially challenging in the developing world where limited healthcare access poses a major logistical barrier to immunization. Over the past four decades, researchers have attempted to overcome this issue by developing single-administration vaccines based on controlled-release antigen delivery systems. These systems can be administered once, but release antigen over an extended period of time to elicit both primary and secondary immune responses resulting in antigen-specific immunological memory. Unfortunately, unlike controlled release systems for drugs, single-administration vaccines have yet to be commercialized due to poor antigen stability and difficulty in obtaining unconventional release kinetics. This review discusses the current state of single-administration vaccination, challenges delaying the development of these vaccines, and potential strategies for overcoming these challenges.

Next generation vaccines: single-dose encapsulated vaccines for improved global immunisation coverage and efficacy

OBJECTIVES

Vaccination is considered the most successful health intervention; yet incomplete immunisation coverage continues to risk outbreaks of vaccine preventable diseases worldwide. Vaccination coverage improvement through a single-dose prime-boost technology would revolutionise modern vaccinology, impacting on disease prevalence, significantly benefiting health care and lowering economic burden of disease.

KEY FINDINGS

Over the past 30 years, there have been efforts to develop a single-dose delayed release vaccine technology that could replace the repeated prime-boost immunisations required for many current vaccines. Biocompatible polymers have been employed to encapsulate model vaccines for delayed delivery in vivo, using either continuous or pulsed release. Biomaterial considerations, safety aspects, particle characteristics and immunological aspects of this approach are discussed in detail.

SUMMARY

Despite many studies showing the feasibility of vaccine encapsulation for single-dose prime-boost administration, none have been translated into convincing utility in animal models or human trials. Further development of the encapsulation technology, through optimising the particle composition, formulation, antigen loading efficacy and stability, could lead to the application of this important approach in vaccine deployment. If successful, this would provide a solution to better global vaccination coverage through a reduction in the number of immunisations needed to achieve protection against infectious diseases. This review provides an overview of single-dose vaccination in the context of today's vaccine needs and is derived from a body of literature that has not been reviewed for over a decade.

The long-term potential of biodegradable poly(lactide-co-glycolide) microparticles as the next-generation vaccine adjuvant

Biodegradable polymeric microparticles of poly(lactide-co-glycolide) (PLG) have been extensively evaluated for drug delivery and vaccine applications over the last three decades. Despite a wealth of studies on the use of PLG microparticles in vaccines through controlled release of antigens, there is no commercial PLG-based vaccine as yet. The key challenge that prevented the development of PLG microparticles as commercial vaccines was the instability of encapsulated antigen. Over the years, advancements were made towards maintaining antigen integrity during PLG microparticle preparation and sterilization. In parallel and independently, development of PLG microparticles as therapeutic commercial products established PLG with an excellent safety record in humans, and as a suitable candidate for next-generation vaccines. Through the combination of Toll-like receptor agonist encapsulation and surface adsorption of antigen, PLG microparticles can be used as a vaccine adjuvant to address unmet medical needs, such as vaccines against HIV, malaria and TB. With strategic development of PLG-based vaccines, PLG microparticles can offer advantages over the conventional vaccine adjuvants allowing commercial development of this adjuvant.

A novel vaccine delivery system: biodegradable nanoparticles in thermosensitive hydrogel

In this work, a novel vaccine delivery system, biodegradable nanoparticles (NPs) in thermosensitive hydrogel, was investigated. Human basic fibroblast growth factor (bFGF)-loaded NPs (bFGF-NPs) were prepared, and then bFGF-NPs were incorporated into thermosensitive hydrogel to form bFGF-NPs in a hydrogel composite (bFGF-NPs/hydrogel). bFGF-NPs/hydrogel was an injectable sol at ambient temperature, but was converted into a non-flowing gel at body temperature. The in vitro release profile showed that bFGF could be released from bFGF-NPs or bFGF-NPs/hydrogel at an extended period, but the release rate of bFGF-NPs/hydrogel was much lower. In vivo experiments suggested that immunogenicity of bFGF improved significantly after being incorporated into the NPs/hydrogel composite, and strong humoral immunity was maintained for longer than 12 weeks. Furthermore, an in vivo protective anti-tumor immunity assay indicated that immunization with bFGF-NPs/hydrogel could induce significant suppression of the growth and metastases of tumors. Thus, the NPs/hydrogel composite may have great potential application as a novel vaccine delivery system.

Single shot tetanus vaccine manufactured by a supercritical fluid encapsulation technology

Single shot vaccines of tetanus toxoid (TT) were manufactured using the NanoMix process - a low temperature solvent free encapsulation technology using supercritical fluids. The formulations were injected into mice, and compared to multiple injections of a commercially available alum adsorbed TT vaccine. After 5 months the antibody titres were found to be similar for both the alum adsorbed and microparticle formulations, demonstrating for the first time the potential of formulating antigens in PLA microparticles using the supercritical fluid (NanoMix) technique to produce single shot vaccines. The results are likely to be due to the maintenance of toxoid bioactivity and some degree of sustained release of the encapsulated antigens, resulting in repeated stimulation of antigen presenting cells eliminating the need for multiple immunisations. This demonstrates the potential of this supercritical fluid processing technique to reduce the need for booster doses in a vaccine regimen.

Pathogen-associated molecular patterns on biomaterials: a paradigm for engineering new vaccines

Vaccine development has progressed significantly and has moved from whole microorganisms to subunit vaccines that contain only their antigenic proteins. Subunit vaccines are often less immunogenic than whole pathogens; therefore, adjuvants must amplify the immune response, ideally establishing both innate and adaptive immunity. Incorporation of antigens into biomaterials, such as liposomes and polymers, can achieve a desired vaccine response. The physical properties of these platforms can be easily manipulated, thus allowing for controlled delivery of immunostimulatory factors and presentation of pathogen-associated molecular patterns (PAMPs) that are targeted to specific immune cells. Targeting antigen to immune cells via PAMP-modified biomaterials is a new strategy to control the subsequent development of immunity and, in turn, effective vaccination. Here, we review the recent advances in both immunology and biomaterial engineering that have brought particulate-based vaccines to reality.

Potentiation of Immune Response from Polymer-Entrapped Antigen: Toward Development of Single Dose Tetanus Toxoid Vaccine

Poly(lactide) (PLA) polymer particles entrapping immunoreactive tetanus toxoid (TT) were used for generation of immune response using single point immunization. Immunization with different sizes of polymer particles encapsulating immunoreactive TT elicited anti-TT antibody titers that persisted for more than 5 months. However, antibody response generated by single point immunization of either nanoparticles or microparticles were lower than the conventional two doses of alum adsorbed TT. To overcome this limitation, alum was used with particles that improved anti-TT antibody response. Immunization with nanoparticles along with alum resulted in very high and early immune response: high anti-TT antibody titers were detected as early as 15 days postimmunization. However anti-TT antibody titers declined rapidly with time. Immunization with admixture of microparticles and alum elicited higher antibody titers than the particles alone and the antibody titers were high particularly during the later part of the postimmunization period. Single point immunization with admixture of PLA microparticles and alum resulted in an antibody response very close to that achieved by two injection of alum-adsorbed TT. Physical mixture of both a nano- and microparticles along with alum resulted in sustained anti-TT antibody response from very early days of postimmunization until 150 days. The antibody titers were maintained around 50 microg/ml for more than 5 months. These results indicated that immune response from polymer particles can be further improved by use of additional adjuvant. Furthermore, using various size particles or physical mixture of different size particles along with alum, it is possible to modulate the kinetics of immune response using polymer particles based immunization.

Role of alum in improving the immunogenicity of biodegradable polymer particle entrapped antigens

This study was aimed at understanding the role of alum in improving the immunogenicity of biodegradable polymer particle entrapped antigens. Presence of alum formed a fine network around PLA particles holding them together and promoted attachment of higher number of particles on macrophage surface for a considerable period of time. Use of alum lowered the burst release of the entrapped antigen from particles and thereafter also reduced the cumulative release of antigen from particles. Apart from this, PLA microparticles alone induced macrophages to release TNF-alpha similar to that induced by alum. However admixture of PLA particles and alum enhanced the secretion of TNF-alpha from 876pg/ml at 6h to 3500pg/ml at 24h which was higher than that induced by alum adsorbed TT. Immunization with admixture of antigen loaded polylactide (PLA) microparticles (2-8microm) and alum improved the antibody titers almost twice than that achieved from particle alone in experimental animals. Single point immunization with particle entrapped antigens and alum also elicited antibody titers comparable to two doses of alum adsorbed tetanus toxoid (TT) or diphtheria toxoid (DT). Our results suggest that presence of alum acts in multiple ways to improve the antibody titers of polymer particles entrapped antigens. Such co-operative adjuvant action of alum and polymer particles can be exploited to improve the immunogenicity of other antigens.

Design opportunities for actively targeted nanoparticle vaccines

Vaccines for many infectious diseases are poorly developed or simply unavailable. There are significant technological and practical design issues that contribute to this problem; thus, a solution to the vaccine problem will require a systematic approach to test the multiple variables that are required to address each of the design challenges. Nanoparticle technology is an attractive methodology for optimizing vaccine development because design variables can be tested individually or in combination. The biology of individual components that constitute an effective vaccine is often well understood and may be integrated into particle design, affording optimal immune responses to specific pathogens. Here, we review technological variables and design parameters associated with creating modular nanoparticle vaccine systems that can be used as vectors to protect against disease. Variables, such as the material and size of the core matrix, surface modification for attaching targeting ligands and routes of administration, are discussed. Optimization of these variables is important for the development of nanoparticle-based vaccine systems against infectious diseases and cancer.

Microparticle-based technologies for vaccines

Microparticles have been effectively used for many years as delivery systems for drugs and therapeutic proteins. Their application to the delivery of vaccines is not as extensive, but is growing. Utility has been demonstrated for the delivery of various types of vaccines (e.g., recombinant proteins, plasmid DNA, and peptides) and other vaccine components (e.g., immune potentiators). With respect to delivery of immune potentiators, synergistic effects are often observed whereby much more potent immune responses are induced with a combination than with either component alone. Hence, the prospects for broad application of microparticle-based delivery systems for vaccines are excellent.

Effect of radio-opaque filler on biodegradable stent properties

The effect of the addition of a radio-opaque filler, barium sulfate (BaSO(4)), on the mechanical properties of a biodegradable amorphous polymer film (poly-lactic-co-glycolic acid, PLGA) was studied, as a function of degradation. With up to about 18% loading (v/v), the modulus of the filled polymer increases; beyond this concentration, agglomerates are formed. The filled systems are also radio-opaque, over a thickness range of 0.07-0.19 mm in stent form (helicoidal). These stents were then immersed in phosphate buffer pH 7.4 at 37(o)C for 2 weeks. The radial strength of stent was measured by using a compression test. It was found that filler-loaded stent (FS) increased in radial strength by about 4 times (14.95 +/- 1.20 N/mm) compared to the unfilled stent (UFS). However, both samples lost radial strength as the polymer degraded in buffer, but FS retained 60% (9.05 +/- 0.07 N/mm) of its strength after 2 weeks whereas only 36% (1.39 +/- 1.04 N/mm) was retained for UFS. Moreover, UFS lost its helical shape after 3 weeks. The findings have implications for optimization of degradable stent formulations.

Mucosal and Systemic Adjuvant Effects of Cholera Toxin and Cry1Ac Protoxin on the Specific Antibody Response to HIV-1 C4/V3 Peptides Are Different and Depend on the Antigen Co-administered

Evidence from several sources support the assertion that cholera toxin (CT) is a potent immunogen and mucosal adjuvant; however there are also reports showing its lack of adjuvanticity against some antigens. Cry1Ac protoxin also exerts adjuvant effects in the antibody responses to proteins and polysaccharides but its adjuvanticity with regard to peptide vaccines had not been tested. To probe whether the adjuvant effects of these proteins varied depending on the antigen co-administered, we evaluated antipeptide antibody responses in serum and mucosal samples (vaginal, intestinal, and pulmonary) of mice that were immunized by intranasal or intraperitoneal routes with one of two distinct hybrid C4/V3 HIV peptides, either alone or with CT or Cry1Ac. The tested HIV 1 peptides differed in two aminoacids, T1SP10MN(A) was modified at the SP10 region by the substitution of the isoleucines 12 and 14 for cysteines and was called modified (m)T1SP10MN(A). Our data indicate that the adjuvant effects of CT and Cry1Ac are different. In addition they vary depending on the antigen co-administered and the number of antigen doses, because after three doses moderate adjuvant effects of CT and Cry1Ac on anti-peptide serum and mucosal antibody responses were observed only against the mT1SP10MN(A). In contrast, to attain significant adjuvant effects against the T1SP10MN, four doses were required. Interestingly we found that modification of the HIV peptide increases its immunogenicity without altering the principal neutralizing determinant (SP10).

Starch microparticles as vaccine adjuvant

The demand for new vaccine adjuvants is well documented. New purified antigens from parasites, bacterial or viral pathogens, as well as recombinant subunit antigens and synthetic peptides, are often inherently weak immunogens; therefore, they need some kind of adjuvant to help initiate an immune response. In addition, there are very few adjuvants using the potential of the mucosal immune system, which may play an important role in the defence against air- and food-borne infections. Starch is a natural biocompatible and biodegradable polymer that is suitable for the production of various particulate adjuvant formulations, which can induce mucosal as well as systemic immune responses. This review gives an account of the different starch adjuvants used in immunisation studies. In particular, the properties of polyacryl starch microparticles as an oral vaccine adjuvant that induce protective immune responses in mice challenge experiments are summarised. In addition, a diphtheria booster vaccine has been proposed to be used to proving the concept in man and the possibilities to design an efficient vaccine formulation for human use are discussed.

Recent advances in immunological adjuvants: the development of particulate antigen delivery systems

New generation vaccines, including those based on recombinant proteins, are safer than traditional vaccines, but are less immunogenic. Therefore, there is an urgent need for the development of new and improved vaccine adjuvants. A number of potent immunostimulatory molecules obtained from bacterial cells or plants have been extensively evaluated as adjuvants. However, a number of these molecules have displayed significant toxicity, both in preclinical animal models and in human clinical trials. An alternative approach to the development of novel adjuvants involves the preparation of particulate antigen delivery systems of similar dimensions to natural pathogens. In the absence of additional immunostimulatory molecules, emulsion droplets and microparticles have been shown to be potent adjuvants for the induction of both humoral and cell-mediated immune responses following systemic administration. Moreover, particulate delivery systems have been shown to display an acceptable toxicity profile in a number of clinical trials. Particulate antigen delivery systems also have the potential to function as potent adjuvants following administration by mucosal routes, including oral and intranasal. An alternative approach to the mucosal delivery of vaccines involves the use of genetically detoxified mutant toxins, e.g., LT-K63, as mucosal adjuvants. The use of novel adjuvants and antigen delivery systems is likely to extend the use of vaccines into the area of therapeutics, involving the eradication of infectious diseases and cancers, or the amelioration of autoimmune disorders.

The immunogenicity-enhancing effect of emulsion vaccine adjuvants is independent of the dispersion type and antigen release rate--a revisit of the role of the hydrophile-lipophile balance (HLB) value

Effective antigen delivery is one of the most important issues in vaccine development. It has been suggested that adjuvant action results from a depot effect by prolonging the duration of the interaction between antigen and cells, and thus is related to the antigen-releasing properties of emulsion adjuvants. The objective of this study was to investigate the effect of the dispersion properties of emulsion-type vaccine adjuvants on the immune response with the aim of optimizing vaccine adjuvant formulation. Emulsion-type adjuvants with various dispersion properties of either the oil-in-water or water-in-oil type were prepared using emulsifiers with various hydrophilic-hydrophobic balance (HLB) values. The physicochemical properties of the emulsions, including the conductivity and viscosity, and antigen release rates were then determined. Cell death induced by the vaccine adjuvants was examined in EL4 cells by Annexin V/propidium iodide (PI) staining and flow cytometric analysis. Mice were immunized with or without the adjuvants and the immunogenicity-enhancing effect of the adjuvants determined by measuring antibody production using an enzyme linked immunosorbent assay. The conductivity, viscosity, and antigen release rates varied widely among emulsions containing emulsifiers with different HLB values. However, the magnitude of the antigen-specific antibody response was similar in most emulsions adjuvants containing Spans or Tweens. L121-adjuvant, the control adjuvant inducing the strongest apoptosis in vitro, was shown to stimulate the highest antibody response in vivo. The results obtained in this study indicate that the immunogenicity-enhancing effect of emulsion adjuvants is independent of the dispersion type and the antigen release rate of the vaccine delivery system.

Novel biocompatible anionic polymeric microspheres for the delivery of the HIV-1 Tat protein for vaccine application

Two novel classes of biocompatible core-shell anionic microspheres, composed of an inner hard insoluble core, either made of poly(styrene) (PS) or poly(methyl methacrylate) (PMMA), and a soft outer tentacular shell made of long soluble negatively charged arms derived from the steric stabilizer, hemisuccinated poly(vinyl alcohol) or Eudragit L100/55, respectively, were prepared by dispersion polymerization and characterized. Five types of these novel microspheres, two made of poly(styrene) and hemisuccinated poly(vinyl alcohol) (A4 and A7), and three made of poly(methyl methacrylate) and Eudragit L100/55 (1D, 1E, H1D), differing for chemical composition, size, and surface charge density were analyzed for the delivery of the HIV-1 Tat protein for vaccine applications. All microspheres reversibly adsorbed the native biologically active HIV-1 Tat protein preventing Tat from oxidation and maintaining its biological activity, therefore increasing the shelf-life of the Tat protein vaccine. The microspheres efficiently delivered Tat intracellularly, and were not toxic in vitro nor in mice, even after multiple administrations. These results indicate that these novel microparticles are safe and represent a promising delivery system for vaccination with Tat, as well as for other subunit vaccines, particularly when a native protein conformation is required.

An experimental divalent vaccine based on biodegradable microspheres induces protective immunity against tetanus and diphtheria

In an endeavor towards development of multivalent vaccines based on biodegradable microspheres, we tested the immunologic performance of several divalent microsphere formulations against tetanus and diphtheria. Microspheres were made by separate microencapsulation of tetanus and diphtheria toxoid in poly(lactide-co-glycolide) by either spray-drying or coacervation. Guinea pigs were subcutaneously immunized by a single injection of the divalent vaccines or, for control, an equivalent dose of a licensed vaccine containing both antigens adsorbed on aluminium hydroxide. All microsphere formulations were strongly immunogenic, irrespective of particle size and hydrophobicity. End point titers of ELISA antibodies, mainly of the IgG1 subtype, were comparable to those obtained after immunization with the licensed vaccine. The microspheres provided increasing levels of antibodies, during the 16 weeks of testing, and the antibodies were weakly polarized towards tetanus. The induced antibodies were also toxin neutralizing, as determined for both diphtheria (1-4 IU/mL) and tetanus (5-9 IU/mL) 8 weeks after immunization. These neutralization levels were several orders of magnitude above the level considered minimum for protection (0.01 IU/mL). When the animals were challenged with tetanus or diphtheria toxins 6 weeks after immunization, microsphere vaccines produced protective immunity that was comparable to or better than that induced by the licensed divalent vaccine. In conclusion, this study showed that a single administration of biodegradable microsphere vaccines provided protective immunity against diphtheria and tetanus, and that this immunization approach might be feasible for multivalent vaccines.

On technological and immunological benefits of multivalent single-injection microsphere vaccines

PURPOSE

With the aim of developing multivalent vaccines for single-injection, we examined the feasibility of combining antigens in biodegradable microspheres. Such vaccines are expected to improve vaccination coverage by reducing the number of vaccination sessions required to generate immunity.

METHODS

Mono- and multivalent vaccines of Haemophilus influenzae type b (Hib) conjugate, diphtheria toxoid (DT), tetanus toxoid (TT), and pertussis toxin (PT) in poly (lactic acid) and poly(lactic-coglycolic acid) microspheres were prepared by spray drying, and the influence of coencapsulated antigens and excipients on antigen loading, release, and stability was examined. Two tetravalent formulations were tested in guinea pigs.

RESULTS

Monovalent Hib and PT vaccines showed loading efficiencies of 10% (Hib) and 30% (PT) in both polymers. The loading efficiencies increased upon addition of trehalose and, even more, when the antigens were coencapsulated in di- and trivalent combinations. Highest loading efficiencies (> 80%) were achieved with trivalent formulations (DT + PT + Hib) that also contained coencapsulated albumin. The percentage of antigen released during 24 h of incubation was typically 10-40% and decreased as loading efficiency increased. Enzyme-linked immunosorbent assay (ELISA) data revealed that TT, DT, and PT remained antigenic throughout the encapsulation and subsequent release processes. Finally, all antigens maintained their immunogenicity, since strong and sustained antibody responses were elicited after a single injection of tetravalent microsphere vaccines (DT + TT + PT + Hib) in guinea pigs.

CONCLUSIONS

This study reveals technologic benefit as well as an immunological potential of multivalent single-injection microsphere vaccines. The results support our hypothesis that coencapsulation of several antigens may intrinsically improve entrapment of antigenic and immunogenic antigen probably by virtue of increased protein concentration during microencapsulation leading to mutual stabilization of the components.

Continuous antigen delivery from controlled release implants induces significant and anamnestic immune responses

Two continuous delivery injectable silicone implants were tested to determine if they were capable of delivering vaccines in a single shot. The Type A implant delivers antigen in vitro over a 1-month-period and the Type B over several months. Vaccination studies in sheep were designed to compare the responses induced by the Type A and B implants, Alzet mini-osmotic pumps and conventional antigen delivery. A model antigen, avidin, was used along with IL-1beta or alum as adjuvants. Sheep were immunised with various formulations and the titre and isotype of the antigen specific antibodies monitored. The Type B implant induced antibody (Ab) titres of greater magnitude and duration than soluble vaccines or the Type A implant with adjuvant, but only if IL-1beta was included in the formulation. Both implants induced antibodies of IgG1 and IgG2 isotype. A memory response to soluble antigen challenge was induced by the Type B+IL-1beta implant, which was predominantly of an IgG1 isotype.

Collagen minipellet as a controlled release delivery system for tetanus and diphtheria toxoid

The use of biodegradable polymer matrices as a single-dose vaccine delivery system was investigated using tetanus toxoid (TT) and diphtheria toxoid (DT). BALB/c mice were immunized with TT or DT in different formulations including individual, in minipellet and aluminum hydroxide (alum), and the antibody responses were monitored for 48 weeks. Antigens entrapped in minipellet elicited higher antibody responses compared to those obtained with individual antigens and antigens adsorbed to alum and the antibody levels remained elevated over 48 weeks. In addition, minipellet formulations induced the same subclasses of antibodies induced by alum formulations. These results raise the possibility to obtain optimal and long-lasting immune responses by a single administration of minipellet formulations.

Comparison of in vitro and in vivo methods to study stability of PLGA microencapsulated tetanus toxoid vaccines

The purpose of this study was to investigate the utility of various in vitro and in vivo methods to assess the stability of experimental vaccines containing tetanus toxoid (TT) within PLGA microspheres. In vitro, the breakdown of the encapsulating polymers into their acid components led to changes in the structure of TT, as determined by the physico-chemical methods, rendering it undetectable by capture ELISA and altering its structural integrity. The changes in TT were directly related to increasing acidity of the vaccine supernate. Purified toxoid (not encapsulated) exposed to low pH (2.5) underwent similar changes but re-neutralisation of buffer containing free toxoid, even after one week at pH 2.5 led to some re-folding of protein as determined by fluorescence spectroscopy and gel filtration chromatography. The microencapsulated vaccines were still able to generate an antibody response in mice even after prolonged pre-incubation at 37 degrees C and the apparent absence of detectable toxoid in the vaccine supernate. Electron microscopy demonstrated differences in the amount of degradation between different formulations of microspheres. Vaccines that had retained their spherical morphology after incubation in vitro for up to 28 days were able to induce protective antibodies response equal to that of freshly prepared vaccines, which indicates that the toxoid within intact microspheres remained immunogenic. Immunochemical and physico-chemical detection methods, performed on antigen released from PLGA vaccines in vitro, are valuable in providing information on product characteristics but may not be able to predict effectiveness and should be used with in vivo methods to evaluate the stability of such formulations.

Formulation of poly(D,L-lactic-co-glycolic acid) microparticles for rapid plasmid DNA delivery

An optimised water-in-oil-in-water double emulsion process for the microencapsulation of plasmid DNA in poly(D,L-lactic-co-glycolic acid) (PLGA) was used to prepare microparticles from a range of different PLGA formulations. This process has been developed using pharmaceutically accepted solvents and is potentially scaleable. Incorporation of plasmid DNA in the microparticles of up to 11 microg/mg was obtained and the retention of plasmid DNA integrity was considerably greater than previously reported. Microparticle structure was determined, by scanning electron microscopy, to be hollow and size distribution characteristics were found to be independent of polymer formulation. The ability to vary the plasmid DNA release profile by changing the PLGA formulation and polymer concentration used in the encapsulation process was also demonstrated. This ability to control the release profile of the microparticles was shown to be especially important as the physical integrity of the encapsulated plasmid DNA was found to deteriorate with extended release times in vitro.

Potential of polymeric lamellar substrate particles (PLSP) as adjuvants for vaccines

In recent years microspheres or microparticles produced from biodegradable polymers such as poly(D,L-lactide) (PLA) and poly(D, L-lactide-co-glycolide) (PLGA) containing encapsulated vaccine antigens have been investigated for administration via parenteral, oral, and intranasal routes. These microparticles allow the controlled release of vaccines with an aim to reduce the number of doses for primary immunisation or to develop single dose vaccines. The polymer materials have been widely regarded as being of minimal toxicity. Evaluation of candidate systems in animal studies have shown antibody levels and cell responses similar to or greater than those observed with adjuvants such as alum. However, there are concerns regarding the integrity and immunogenicity of the antigen during the encapsulation process when the antigen is exposed to organic solvents, high shear stresses and the exposure of antigen to low pH which is caused by polymer degradation. An alternative approach would be to adsorb antigens to the surface of biodegradable polymer particles. Polymeric lamellar substrate particles (PLSP), produced by a simple precipitation of PLA, are suitable for this purpose. The adsorption of antigens onto these particles is a simple procedure. It avoids pH changes due to bulk polymer degradation and the use of solvents and therefore will be less damaging to the vaccine. Moreover, such systems will be much easier to scale up for a clinical study and eventual manufacture. The aim of this article is to discuss the preparation and physical characteristics of PLSP, antigen adsorption, in vivo efficacy of PLSP antigen systems and to consider the potential of PLSP as controlled release adjuvants for protein, peptide or viral vaccines.

Maintaining and enhancing vaccine immunogenicity

This contribution highlights factors involved with maintaining and enhancing antigen delivery or immunogenicity. Areas discussed include the cold chain, adjuvants, recombinant vectors for antigen delivery, routes for antigen delivery, and edible plant vaccines. It is doubtless that the technological understanding that underlies these advances is about to revolutionize vaccinology in the near future.