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

Diphtheria toxoid (DT) was encapsulated in microparticles prepared from polylactide-co-glycolide (PLG) polymers using a solvent evaporation technique. Combinations of small and large sized microparticles with controlled release characteristics were used to immunize Sprague Dawley rats and the antibody responses were monitored for one year. For comparison, control groups of rats were immunized at 0, 1 and 2 months with DT adsorbed to alum. The antibody responses generated by the microparticles were comparable to the alum immunized control groups from 32 weeks. Microparticles with a single entrapped antigen (DT) induced better antibody responses than microparticles with two antigens entrapped simultaneously (DT + TT). Microparticles prepared from a single polymer were less effective for long term antibody induction than a combination of microparticles prepared from three different polymers. A combination vaccine consisting of antigen absorbed to alum and also entrapped in microparticles gave the best response. In an inhibition assay designed to determine the relative binding of antisera to the antigen, the sera from the microparticle and the alum immunized animals showed comparable binding. An intradermal challenge study was performed in rabbits, which showed similar levels for the alum and the microparticle immunized animals at 4, 12 and 32 weeks after immunization.