Vol. 296, Issue 2, 351-358, February 2001

Modulation of P450 CYP3A4-Dependent Metabolism by P-glycoprotein: Implications for P450 Phenotyping

Jens M. Baron, Lay Beng Goh, Denggao Yao, C. Roland Wolf and Thomas Friedberg

Biomedical Research Centre, University of Dundee, Ninewells Hospital and Medical School, Dundee, United Kingdom (L.B.G., C.R.W., T.F.); and Department of Dermatology, University Hospital, Rheinland-Westfälische Technische Hochschule (RWTH), Aachen, Germany (J.M.B.)

Some compounds used for phenotyping of cytochrome P450s are substrates of P-glycoprotein (pgp). It is likely that in these cases, the level of pgp modulates the metabolism of in vivo probes. To address this important issue, we have analyzed the effects of pgp on CYP3A4-mediated reactions in two newly established cell lines (3A4/HR/MDR and 3A4/HR/MDR⁺), which express CYP3A4 in the absence and presence of pgp, respectively. In cultured cells, the presence of pgp increased the apparent K_m for the 6-hydroxylase activity of CYP3A4 toward testosterone and cortisol by a factor of 1.7 and 4, respectively. These steroids are poor and good substrates of pgp, respectively, and cortisol 6-hydroxylase has been frequently used as an in vivo probe for CYP3A4. Interestingly, we also found that pgp modulated the inhibition of CYP3A4-mediated metabolism by several compounds in intact cells. Although quinidine inhibited testosterone 6-hydroxylase activity in membranes or in intact cells that expressed recombinant CYP3A4 in the absence of pgp, low concentrations of this compound increased CYP3A4 activity in intact cells that expressed pgp. These results imply that pharmacokinetic drug-drug interactions involving CYP3A4 can be influenced by pgp.