Studies on the cytochrome P450 (CYP)-mediated metabolic properties of miocamycin: Evaluation of the possibility of a metabolic intermediate complex formation with CYP, and identification of the human CYP isoforms

Some macrolide antibiotics cause clinical drug interactions, resulting in a ltered metabolism of concomitantly administered drugs, via the formation of a metabolic intermediate (MI) complex with cytochrome P450 (CYP), or compe titive inhibition of CYP. In this study, the possibility of MI complex form ation by miocamycin (MOM) was assessed first. CYP contents and activities i n rat liver microsomes were not affected and there were no detectable MI co mplexes after administration of MOM for either 3 or 10 days to rats. Furthe rmore, MOM did not form MI complexes in vitro even with microsomes from hum ans or dexamethasone-pretreated rats. Second, in vitro studies were conduct ed to identify the human CYP isoforms involved in four 14-hydroxylation rea ctions in the MOM metabolic pathway. The results showed that it was most li kely CYP3A4 involved in the hydroxylations: 1) each hydroxylation in human liver microsomes from 10 different donors strongly correlated with testoste rone 6 beta-hydroxylation; 2) each hydroxylation was essentially inhibited by ketoconazole and troleandomycin; 3) only cDNA-expressed CYP3A4 and CYP3A 5 catalyzed the hydroxylations, and the activities of CYP3A5 were below 5% of those of CYP3A4; and 4) the apparent K-M values obtained with native hum an liver microsomes were comparable with those obtained with cDNA-expressed CYP3A4. In conclusion, MOM is not an inhibitor of CYP via the formation of an MI complex. Moreover, CYP3A4 is mainly responsible for catalyzing the h ydroxylation of MOM metabolites. Because CYP3A4 is the most abundant form o f CYP in the liver and intestine, this isoform probably accounts for the ma jority of drug-MOM interactions observed in clinical practice.