NEW STRATEGIES FOR THE MICROENCAPSULATION OF TETANUS VACCINE

The progress toward the development of a single dose tetanus vaccine h as been limited by the poor stability of the protein antigen, tetanus toroid (TT), during its encapsulation in, and release from, biodegrada ble polymer microspheres. To investigate alternative microencapsulatio n approaches that may improve the stability of TT under these conditio ns, a two-step microencapsulation method has been devised to form micr ocapsules which consist of: (a) forming microcores of TT in a hydrophi lic support matrix by spray-congealing, followed by (b) coating the mi crocores with poly(lactide-co-glycolide) (PLGA) by an oil-in-oil solve nt extraction method. Several protein stabilizers including gelatin (w ith or without poloxamer 188), dextran, sodium glutamate, and polyethy lene glycol were examined as potential core-materials. Among them, gel atin was superior in its ability to impart stability to TT against hea t and moisture-induced inactivation. Microcores of this latter stabili zer and TT were encapsulated in PLGA using the foregoing technique, wh ich exposed the dry antigen to minimal water in order to present its i rreversible inactivation during exposure to the organic solvent. The m icroencapsulation method resulted in minimal loss of antigenically act ive TT (similar to 10-20%). Microscopic analysis of the microcapsules following preparation showed the microcores to be fully encapsulated. However, microcapsules containing TT and gelatin released the active a ntigen nearly completely within one day. Fluorescence confocal microsc opy revealed that the swelling of the hydrophilic core-material was re sponsible for the burst-release behaviour. Manipulation of the polymer coating could nor slow down this 'explosion' of the microcapsules. TT -containing PLGA microcapsules have been prepared using a novel microe ncapsulation method, which retains an extremely high fraction of antig enically active TT. Hence, these mechanistic approaches may be useful in the development of effective single-dose vaccines.