Intestinal and hepatic metabolic activity of five cytochrome P450 enzymes: impact on prediction of first-pass metabolism.

The contribution of the gut is not routinely incorporated into in vitro-in vivo predictions of either clearance or drug-drug interactions, and this omission may partially explain the general underprediction trend often observed. In the current study, the metabolic ability of hepatic and intestinal pooled microsomes was compared for eight CYP3A substrates (midazolam, triazolam, diazepam, alprazolam, flunitrazepam, nifedipine, testosterone, and quinidine) and paclitaxel, tolbutamide, S-mephenytoin, and bufuralol as CYP2C8, CYP2C9, CYP2C19, and CYP2D6 probes, respectively. A general agreement in the type of kinetics was observed between the two systems for the substrates investigated. Of the 16 pathways investigated, 75% of K(m) (S(50)) values obtained in intestinal microsomes (5.9-769 microM) were within 2-fold of hepatic estimates. Irrespective of the cytochrome P450 (P450) investigated and normalization of V(max) values for the P450 abundance, clearance was 4.5- to 50-fold lower in intestinal microsomes (0.0005-0.51 microl/min/P450) compared with the hepatic estimates (0.002-5.8 microl/min/P450), whereas the rank order was consistent between the systems. Assessment of two enterocyte isolation methods (mucosal scraping or enterocyte elution) was performed at the substrate concentrations corresponding to the determined V(max) conditions for 11 pathways. The activity difference between the methods (3-29-fold) was P450-related in the following rank order: CYP2C19 > CYP3A4 > CYP2C9 approximately CYP2D6. After correction for the loss of activity between the methods, the intrinsic activities of hepatic and intestinal CYP3A4, CYP2C9, CYP2C19, and CYP2D6 were comparable for the 16 pathways. The implications of these findings on the prediction of intestinal first-pass metabolism are discussed.