Solvent degradation of cloxacillin in vitro: Tentative identification of degradation products using thermospray liquid chromatography-mass spectrometry

The determination of cloxacillin and the other penicillin antibiotics at trace levels in pharmaceutical samples and biological fluids and tissues is difficult. Degradation of penicillins occurs during exposure to chemicals and solvents used in sample extraction, storage of extracts and chemical analysis. One of the most important factors for the determination of cloxacillin in bovine milk and tissues is the choice of solvent used for extraction. Cloxacillin stored in different solutions underwent two types of degradation: hydrolysis with decarboxylation to yield cloxacillinpenilloic acids when water or aqueous solutions of acetonitrile or 2-propanol were used, and alcoholysis to form cloxacillinpenicilloic acid ester when methanol or ethanol was used. Cloxacillin stored in aqueous methanol or acetonitrile solution underwent faster degradation when stored in water or aqueous solutions of 2-propanol or ethanol. The solvent combinations that resulted in minimum sample degradation of cloxacillin were acetonitrile-ethanol-water (25:25:50) or ethanol-water (50:50). Degradation of cloxacillin was faster at the lower levels (500 and 50 ng/ml) of the antibiotic for all solvents tested. Nearly complete degradation of cloxacillin was observed at the 50 ng/ml level after 2 h at 20°C. Degradation was nearly non-existent at the 1 mg/ml level of the antibiotic in each solvent.