Degradation of acylaminopenicillins with regard to their pH dependency

Determination of antibiotic concentration is performed in many biological fluids and tissues which all have different pH values. Therefore, we investigated the in vitro stability of three acylaminopenicillins (azlocillin, mezlocillin and piperacillin) in borate buffer by the HLPC technique with regard to pH dependency. HPLC allows the detection of all three substances together with their metabolites, penicilloate and penilloate, within 15 min. Decomposition was monitored at 37 degrees C during a 24 h incubation period (pH values ranged between pH 3.0 and 10.0). The highest degradation rates were observed with a buffer solution of pH = 10.0: 50% of the azlocillin and 83% of the mezlocillin were decomposed after 8 h while under the same conditions, piperacillin was completely decomposed already after 1 h. The highest stability was detected in borate buffer at pH values of 4.0, 5.0, and 6.0. At pH = 3.0, degradation was determined as follows: 31% of the piperacillin, 39% of the mezlocillin, and 45% of the azlocillin were decomposed after 24 h. Penilloic acid was identified as the main metabolite in contrast to buffer solutions with higher pH values which only revealed negligible amounts of this compound.