PHOTOISOMERIZATION KINETICS OF CEFUROXIME AXETIL AND RELATED-COMPOUNDS

The photoisomerization kinetics of aqueous solutions of cefuroxime axe til under irradiation at 254 nm was investigated by HPLC. The overall degradation is the result of a competition between the isomerization o f the alkoxyimino group and the photolysis of the beta-lactam ring. Ce furoxime axetil exists as a mixture of two diastereomers which are sho wn to react at different rates. This is true not only for the photoiso merization step but also for ground-state hydrolysis in alkaline condi tions. Photoisomerization of the alkoxyimino group is also observed fo r the anti isomer of cefuroxime axetil and for some of its degradation products. The quantum yields for all these photoisomerizations are al ways lower than 1%, which explains the relative importance of the phot olysis step. A stationary syn to anti ratio of 1 is measured for cefur oxime axetil and of 2.1 for cefuroxime. From this and previous studies , it appears that cefuroxime axetil is the most sensitive under irradi ation at 254 nm when compared to other antibiotics bearing the alkoxyi mino group. Aztreonam is the most stable followed by cefotaxime, cefur oxime, and cefuroxime axetil.