title: Drug interaction of efavirenz and midazolam: efavirenz activates the CYP3A-mediated midazolam 1'-hydroxylation in vitro.
abstract: CYP3A4 and CYP3A5 are the most important drug-metabolizing enzymes. For several drugs, heteroactivation of CYP3A-mediated reactions has been demonstrated in vitro. In vivo data suggested a possible acute activation of CYP3A4-catalyzed midazolam metabolism by efavirenz. Therefore, we aimed to investigate the effect of efavirenz on the in vitro metabolism of midazolam. The formation of 1'-hydroxymidazolam was studied in pooled human liver microsomes (HLM) and recombinant human CYP3A4 and CYP3A5 (rCYP3A4 and rCYP3A5) in the presence of efavirenz (0.5, 1, and 5 μM). Product formation rates (V(max)) increased with increasing efavirenz concentrations (∼1.5-fold increase at 5 μM efavirenz in HLM and ∼1.4-fold in rCYP3A4). The activation in rCYP3A4 was dependent on cytochrome b5, and the activating effect was also observed in rCYP3A5 supplemented with cytochrome b5, where Vmax was ∼1.3-fold enhanced. Concomitant inhibition of CYP3A activity with ketoconazole in HLM abolished the increase in the 1'-hydroxymidazolam formation rate, further confirming involvement of CYP3A. The results of this study represent a distinct acute activation of midazolam metabolism and support the in vivo observations. Moreover, only efavirenz, but not its major metabolite 8-hydroxyefavirenz, was responsible for the activation. The increase in 1'-hydroxymidazolam formation may have been caused by binding of efavirenz to a peripheral site of the enzyme, leading to enhanced midazolam turnover due to changes at the active site.
authors.affiliation: Department of Clinical Pharmacology and Pharmacoepidemiology, University of Heidelberg, Heidelberg, Germany.
authors.forename: Anja
authors.initials: A
authors.lastname: Keubler
authors.forename: Johanna
authors.initials: J
authors.lastname: Weiss
authors.forename: Walter E
authors.initials: WE
authors.lastname: Haefeli
authors.forename: Gerd
authors.initials: G
authors.lastname: Mikus
authors.forename: Jürgen
authors.initials: J
authors.lastname: Burhenne
mesh_terms: D000480:Alkynes; D048588:Benzoxazines / Q000378:metabolism*; D003521:Cyclopropanes; D051544:Cytochrome P-450 CYP3A / Q000502:physiology*; D004347:Drug Interactions / Q000502:physiology; D004789:Enzyme Activation / Q000187:drug effects / Q000502:physiology; D006801:Humans; D006900:Hydroxylation / Q000187:drug effects / Q000502:physiology; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D008874:Midazolam / Q000378:metabolism*
chemical_list: D000480:Alkynes; D048588:Benzoxazines; D003521:Cyclopropanes; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C098320:efavirenz; D008874:Midazolam