Stabilization of tetanus toxoid in poly(DL-lactic-co-glycolic acid) microspheres for the controlled release of antigen.

Sustained release of antigenically active tetanus toxoid (TT) for several weeks from biodegradable poly (DL-lactic-co-glycolic acid) (PLGA) microspheres is very important for the development of a single dose vaccine. With a view to developing such a formulation, we evaluated in the vitro release of antigenic TT from TT-containing PLGA films and microspheres stabilized with gelatin or human serum albumin. Without stabilizer, TT-containing PLGA films released low levels of antigenic TT whereas TT-containing PLGA microspheres (average size 50 microMs) released 18% of antigenic TT in 4 weeks. Inclusion of gelatin and human serum albumin increased the amount of antigenic TT released from PLGA films and microspheres. As the concentration of gelatin increased from 0.08% to 2.2% in PLGA films, release of antigenic TT also increased. Similar results were found with gelatin-stabilized TT-containing microspheres. But the significant increase by higher concentration of gelatin was on the initial burst during the first week. A concentration of 0.2% gelatin was found suitable for getting approximately 40% of antigenic TT in 4 weeks. The proportion of antigenic TT to total protein was higher during the first 2 weeks of the release studies (50% during the first week and 23% during the second week). After 2 weeks, the proportion of antigenic TT was approximately 10% for the next 2 weeks. TT-containing microspheres stabilized with gelatin and human serum albumin showed stability of antigenic TT release when these microspheres were stored at 37 degrees C for 4 weeks. Gelatin appears to be a good stabilizer for TT-containing microspheres, but further studies are required to increase the proportion of antigenic TT beyond 2 weeks.