Abstract
The species of enzymes involved in human metabolism of quazepam and its metabolites M4 and M6 were clarified, and the effects of these compounds on activities of major human P450 isozymes were also evaluated. To identify the CYP isozymes involved, nine kinds of cDNA-expressed human CYPs (CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP4A11) were used to clarify each metabolic reaction. Moreover, inhibitory study was performed to identify the major CYP isozymes responsible for the metabolism of each compound, using human liver microsomes as the enzyme sources, anti-P450 antibodies and anti-P450 antiserum. In addition, flavin-containing monooxygenase (FMO) was examined for its involvement in the metabolism of quazepam to M4. The results revealed that the metabolism of quazepam to M4 was catalyzed by two CYP isozymes, i.e. CYP2C9 and CYP3A4. FMO was found not to be involved in this reaction. The metabolism from M4 to M5 and from M4 to M6 was catalyzed by CYP3A4 and mainly by CYP2C9 and CYP3A4, respectively. Furthermore, M6 was found to be metabolized to M7 by CYP3A4. Next, quazepam, M4, and M6 were examined for their inhibitory effects on the metabolic activities of major human drug metabolizing CYPs (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, and CYP3A4). Here, cDNA-expressed human CYPs as enzyme sources and typical substrates for each CYP isozymes were used. The results indicated that each of these compounds has little effects on the metabolic activities of all the CYP isozymes studied.
