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The degradation kinetics of pivaloyloxymethyl (POM) esters of cephalosporins in phosphate buffer solution (pH 6-8) were investigated. The degradation of the starting delta 3 cephalosporin ester proceeded mainly via isomerization to the delta 2 ester and subsequent hydrolysis to the delta 2 acid. Hydrolysis to the delta 3 acid (the parent acid) was very slow. Analysis of the rate constants indicated that the isomerization rate k12 was approximately equal to the apparent degradation rate of the delta 3 ester kdeg, and slower than the hydrolysis rate of the delta 2 ester k24. The isomerization process to the delta 2 ester was found to be the rate-determining step in the degradation of cephalosporin esters. The substituent at the C-3 position of the cephalosporins affected the degradation kinetics. The degradation was accelerated by increase of pH, buffer concentration and added protein.

Studies on orally active cephalosporin esters. II. Chemical stability of pivaloyloxymethyl esters in phosphate buffer solution.