Evaluation of biodegradable microspheres as vaccine adjuvant for hepatitis B surface antigen

Biodegradable microspheres were evaluated as vaccine adjuvants based on their ability to provide prolonged release of incorporated agents. Hepatitis B surface antigen (HBSA) prepared by recombinant DNA technology was chosen as a model antigen and encapsulated into polyglycolic acid (PGA) by solvent extraction and solvent evaporation techniques. Five microsphere formulations were prepared to evaluate effect of microsphere size and the presence of immunostimulants such as muramyl dipeptide (MDP) or aluminum hydroxide. The microspheres were characterized for size distribution, surface morphology and antigenicity. Guinea pigs were chosen as the animal model for evaluation of antigenicity of the formulations. The animals were divided into seven groups of four animals each and the microsphere formulations were injected intraperitoneally, using alum adsorbed HBSA as positive control and placebo microspheres as negative control. Blood samples were withdrawn from the animals by toe clipping at two, four, six and sixteen weeks and plasma was analyzed for antibodies against hepatitis B by an enzyme linked immunoassay. At sixteen weeks, the animals were reinjected and evaluated for antibody response at two, four and six weeks post second injection. Antibody response to the microspheres was higher than control. Smaller size microspheres elicited earlier antibody response while the larger size microspheres provided delayed and longer duration of antibody production. Microspheres with MDP potentiated the antibody response. The results demonstrate the applicability of biodegradable microspheres for immunization against hepatitis B.