Identification of lactams as in vitro metabolites of piperidine-type phenothiazine antipsychotic drugs

The metabolism of the piperidine-type phenothiazine antipsychotic agents thioridazine, mesoridazine and sulforidazine was studied in vitro with 10 000g liver supernatants obtained from rats and dogs. After incubations at 37°C for different time intervals, the incubates were extracted with dichloromethane and the isolated compounds analyzed by HPLC, direct probe MS and on-line HPLC-MS. Five lactam metabolites of these three drugs were unequivocally identified in the rat in vitro system, but none was found in dog preparations; at least one lactam metabolite was identified for each drug in the rat. The lactams of thioridazine and thioridazine ring sulfoxide were characterized as metabolites of thioridazine for the first time in any system. The other three lactam metabolites, namely the lactams of mesoridazine, sulforidazine and mesoridazine ring sulfoxide, were found in vitro for the first time, although they have been previously reported as in vivo metabolites of these drugs. The results indicate that rat would be a more suitable animal model than dog for further studies on the formation of lactam metabolites of these drugs.