Vaccine formulation development : towards addressing major limitations of vaccines that are adjuvanted with aluminum salts
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Many vaccines require an adjuvant to induce a strong immune response. Aluminum–containing adjuvants have been approved by the United States Food and Drug Administration for human use for many years. There are two main aluminum-containing adjuvants, aluminum hydroxide and aluminum phosphate. Due to their favorable safety profile, aluminum-containing adjuvants have been widely used in human vaccines for decades. Many currently licensed and commercially available vaccines contain aluminum-containing adjuvants. However, aluminum-containing vaccine adjuvants suffer from two major limiting factors: (1) aluminum-containing adjuvants can only weakly or moderately potentiate antigen-specific antibody responses and are generally considered incapable of inducing cellular immune responses; (2) vaccines that contain aluminum-containing adjuvants require cold-chain refrigeration for storage and distribution, and may not be frozen, because freezing of the vaccine in dispersion causes irreversible coagulation that damages vaccines (e.g., loss in potency and stability). In this dissertation, the first limitation was addressed by reducing the size of the aluminum hydroxide from micrometers (3-10 micrometer) to nanometers of less than 200 nm, and the second limitation mentioned above was addressed by freeze-drying vaccines that contain aluminum salts as adjuvants into a dry powder using thin-film freeze-drying. In addition, using an improved experimental design, the vaccine adjuvant activities of nanoparticles of around 200 nm was compared to that of the nanoparticles of around 700 nm. The smaller 200 nm nanoparticles showed a more potent adjuvant activity than the larger nanoparticles. When dispersed in an aqueous medium, both aluminum hydroxide and aluminum phosphate are physically 1–20 micrometer particulates. There are data showing that particulate vaccine carriers of around 200 nm (or less) may be optimal in potentiating the immunogenicity of vaccines. Based on this finding, aluminum hydroxide nanoparticles of 112 nm were synthesized, and its adjuvant activity was compared to that of the traditional aluminum hydroxide adjuvant, which have particulates of 3-20 micrometer. Using ovalbumin and Bacillus anthracis protective antigen protein as model antigens, it was found that protein antigens adsorbed on the aluminum hydroxide nanoparticles induced stronger antigen-specific antibody responses than the same protein antigens adsorbed on the traditional aluminum hydroxide microparticles of around 9.3 µm. Importantly, the inflammation reactions induced by aluminum hydroxide nanoparticles in the injection sites were milder than that induced by microparticles. Simply reducing the particle size of the traditional aluminum hydroxide adjuvant in suspension from micrometers into nanometers represents a novel and effective approach to improve its potency. The second limitation was addressed by converting vaccines that contain an aluminum salt as an adjuvant from an aqueous dispersion into a dried powder using thin-film freeze-drying. There is evidence that aluminum-containing vaccines can be lyophilized to dry powders using high speed freezing methods. Thin-film freezing is a high speed freezing method with a freezing rate between 100 to 10,000 K/s, but the feasibility of using thin-film freeze-drying to freeze-dry vaccines that contain aluminum salts as adjuvants has not been tested before. In this dissertation, Using ovalbumin as a model protein antigen and aluminum hydroxide or aluminum phosphate as an adjuvant, it was confirmed that vaccines that are adjuvanted with aluminum hydroxide or aluminum phosphate can be freeze-dried with as low as 2% (w/v) of trehalose as a cryoprotectant by thin-film freeze-drying without causing vaccine aggregation while preserving the immunogenicity of the vaccine. Finally, the feasibility of using the thin-film freeze-drying method to freeze-drying vaccines that contain aluminum salts as adjuvants was further confirmed by drying a commercial aluminum salt-adjuvanted tetanus toxoid vaccine. Vaccines that contain aluminum salts as adjuvants may be converted to a dry powder using the thin-film freeze-drying method to avoid loss of potency due exposure to freezing conditions during transport and storage.