BIODEGRADABLE MICROSPHERES AS CONTROLLED-RELEASE TETANUS TOXOID DELIVERY SYSTEMS

Purified tetanus toroid, a high-molecular-M eight protein, was entrapp ed within poly(L-lactic acid) (PLA) and poly(D,L-lactic/glycolic acid) (PLGA) microspheres prepared by either a solvent extraction or a solv ent evaporation method carried out in a multiple emulsion system (wate r-in-oil-in-water). The physical integrity and antigenicity of the pro tein treated under different processing conditions were investigated. A reduction of antigenicity that was related to the percentage of aggr egated protein was noticed under some experimental conditions. This pa rtial loss of antigenicity was associated with the lyophilization proc ess and affected by the nature of the organic solvent. All types of mi crospheres prepared with different molecular weight PLA and PLGA displ ayed a high protein-loading efficiency (>80%) but their size was stron gly influenced by polymer molecular weight (3000 versus 100 000). Prot ein release pattern was influenced by both polymer molecular weight an d composition (PLA versus PLGA). A constant release pattern after an i nduction period of 10 days was observed for microspheres composed of h igh-molecular-weight polymers (PLA and PLGA). The release rate was low er from PLA microspheres than from PLGA microspheres. In contrast, a c ontinuously increasing release rate preceded by a burst was observed f or low-molecular-weight (3000) PLGA microspheres. Microencapsulated te tanus toroid was significantly more immunogenic in mice than fluid tor oid as determined by IgG anti-tetanus antibody levels and neutralizing antibodies. However, the magnitude and duration of the antibody respo nse did not differ significantly from a similar dose of aluminium phos phate-adsorbed toroid. We conclude that microencapsulated tetanus toro id shows significant adjuvant activity. Further improvements in the fo rmulation of microspheres which result in the release of higher concen trations of antigenically active tetanus toxoid for more prolonged per iods may result in higher and more sustained antibody levels.