Predicting the Contribution of Rat Cytochrome P-450 3A1, 3A2 and Human Cytochrome P-450 3A4, 3A5 to Territrem a 4beta-C Hydroxylation Using the Relative Activity Factor

The relative activity factor (RAF) was used to predict the contribution of different cytochrome P-450 (CYP) 3A isoforms (3A1 and 3A2 in rat liver microsomes and 3A4 and 3A5 in human liver microsomes) to 4beta-C hydroxylation of territrem A (TRA). Seven recombinant rat and eight recombinant human CYP450 isoforms, five rat liver microsomes, and seven human liver microsomes were assessed. In liver microsomes from five male Wistar rats, TRA 4beta-C hydroxylation activity significantly correlated with CYP3A1/2 activity, while, in liver microsomes from seven humans, there was marked correlation with CYP3A4 activity. Immunoinhibition confirmed that CYP3A2 and CYP3A4 were responsible for the hepatic metabolism of TRA 4beta-C hydroxylation. Using RAF, the percent contributions of CYP3A1 and CYP3A2 to 4beta-C hydroxylation of TRA in rat liver microsomes were estimated as 5 to 6 and 94 to 96, respectively, and those of CYP3A4 and CYP3A5 in human liver microsomes as 70 to 72 and 28 to 30%, respectively. These results suggest that CYP3A2 and CYP3A4 are the main form involved in the 4beta-C hydroxylation of TRA in rat and human liver microsomes.