PT - JOURNAL ARTICLE AU - Pan-Fen Wang AU - Alicia Neiner AU - Evan D. Kharasch TI - Efavirenz metabolism: Influence of polymorphic CYP2B6 variants and stereochemistry AID - 10.1124/dmd.119.086348 DP - 2019 Jan 01 TA - Drug Metabolism and Disposition PG - dmd.119.086348 4099 - http://dmd.aspetjournals.org/content/early/2019/07/18/dmd.119.086348.short 4100 - http://dmd.aspetjournals.org/content/early/2019/07/18/dmd.119.086348.full AB - Efavirenz (more specifically the S-enantiomer) is a cornerstone antiretroviral therapy for treatment of HIV infection. The major primary metabolite is S-8-hydroxyefavirenz, which does not have antiretroviral activity but is neurotoxic. Cytochrome P4502B6 (CYP2B6) is the major enzyme catalyzing S-8-hydroxyefavirenz formation. CYP2B6 genetics and drug interactions are major determinants of clinical efavirenz disposition and dose adjustment. In addition, as a prototypic CYP2B6 substrate, S-efavirenz and analogs can inform on the structure, activity, catalytic mechanisms, and stereoselectivity of CYP2B6. Metabolism of R-efavirenz by CYP2B6 remains unexplored. This investigation assessed S-efavirenz metabolism by clinically relevant CYP2B6 genetic variants. This investigation also evaluated R-efavirenz hydroxylation by wild-type CYP2B6.1 and CYP2B6 variants. S-efavirenz 8-hydroxylation exhibited positive cooperativity and apparent substrate inhibition, for wild-type CYP2B6.1 and variants. Based on intrinsic clearances, relative activities for S-efavirenz 8-hydroxylation were in the order CYP2B6.4>CYP2B6.1>CYP2B6.5≈CYP2B6.17>CYP2B6.6≈CYP2B6.7≈CYP2B6.26>CYP2B6.9≈CYP2B6.19>>CYP2B6.16 and CYP2B6.18, which showed minimal activity. Rates of R-efavirenz metabolism were approximately one-tenth those of S-efavirenz, for wild-type CYP2B6.1 and variants. Based on intrinsic clearance values, there was 16-fold enantioselectivity (S>R-efavirenz) for wild type CYP2B6.1, and 6- to 21-fold differences for other CYP2B6 variants. These results show that both CYP2B6 516G>T (CYP2B6*6 and CYP2B6*9), and 983T>C (CYP2B6*16 and CYP2B6*18) polymorphisms cause canonical loss of function variants for S-efavirenz 8-hydroxylation, provide a mechanistic basis for known clinical pharmacogenetic differences in efavirenz disposition, and may predict additional clinically important variant alleles. Efavirenz is the most stereoselective CYP2B6 drug substrate yet identified and may be a useful probe for the CYP2B6 active site and catalytic mechanisms.SIGNIFICANCE STATEMENT N/A