Physicochemical and immunological studies on the stability of free and microsphere-encapsulated tetanus toxoid in vitro.

The stability of tetanus toxoid released from experimental, slow-release microsphere vaccines was compared with that of free toxoid under different conditions over a 3 month period. The amount of antigenicity remaining, as measured by ELISA, correlated well with loss of toxoid structure, as determined by circular dichroism and fluorescence spectroscopy. At 37 degrees C and low pH, pH 2.5 for free toxoid, or under the acidic conditions generated by the hydrolysis of fast-release microspheres, a gradual unfolding of the polypeptide chain was observed within the first few weeks with more rapid denaturation beyond 30 days.