Degradation kinetics and mechanisms of a new cephalosporin, cefixime, in aqueous solution.

Hydrolysis of cefixime in buffer solutions (pH 1-9) at 25 degrees C and a constant ionic strength of 0.3 was investigated using ion-pair reversed-phase HPLC. Hydrolysis rates followed pseudo first-order kinetics; the rate of hydrolysis of cefixime was very slow at pH 4-7, slightly faster at lower pH, and quite rapid at higher pH. In the early stages of hydrolysis, six major degradation products were isolated and identified: a beta-lactam ring-opened product and a 7-epimer (basic conditions), three lactones derived from intramolecular cyclization between the 2-carboxyl and 3-vinyl groups (acidic conditions), and an aldehyde derivative involving a 7-acyl moiety (neutral conditions). Principal degradation pathways for cefixime were found to involve initial cleavage of the beta-lactam ring.