TY - JOUR T1 - Cytochrome <em>b</em><sub>5</sub> is a Major Determinant of Human Cytochrome P450 CYP2D6 &amp; CYP3A4 Activity In Vivo JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.114.097394 SP - mol.114.097394 AU - Colin J Henderson AU - Lesley A McLaughlin AU - Nico Scheer AU - Lesley Stanley AU - Charles Roland Wolf Y1 - 2015/01/01 UR - http://molpharm.aspetjournals.org/content/early/2015/02/04/mol.114.097394.abstract N2 - The cytochrome P450-dependent mono-oxygenase system is responsible for the metabolism and disposition of chemopreventive agents, chemical toxins and carcinogens and &gt;80% of therapeutic drugs. P450 activity is regulated transcriptionally and by the rate of electron transfer from P450 reductase. In vitro studies have demonstrated that cytochrome b5 (Cyb5) also modulates P450 function. We recently showed that hepatic deletion of Cyb5 in the mouse (HBN) markedly alters in vivo drug pharmacokinetics; a key outstanding question is whether Cyb5 modulates the activity of the major human P450s in drug disposition in vivo. To address this we crossed mice humanised for CYP2D6 or CYP3A4 with mice carrying a hepatic Cyb5 deletion. In vitro triazolam 4-hydroxylation (probe reaction for CYP3A4) was reduced by &gt;50% in hepatic microsomes from CYP3A4-HBN mice compared to controls. Similar reductions in debrisoquine 4-hydroxylation and metoprolol α-hydroxylation were observed using CYP2D6-HBN microsomes, indicating a significant role for Cyb5 in the activity of both enzymes. This effect was confirmed by the concentration-dependent restoration of CYP3A4-mediated triazolam turnover and CYP2D6-mediated bufuralol and debrisoquine turnover on addition of E. coli membranes containing recombinant Cyb5. In vivo, the Cmax and AUC0-8h of triazolam were increased 4- and 5.7-fold, respectively, in CYP3A4-HBN mice. Similarly, the pharmacokinetics of bufuralol and debrisoquine were significantly altered in CYP2D6-HBN mice, AUC0-8h being increased ~1.5-fold and clearance decreased by 40-60%. These data demonstrate that Cyb5 can be a major determinant of CYP3A4 and CYP2D6 activity in vivo, with potential impact on the metabolism, efficacy and side-effects of numerous therapeutic drugs. ER -