title: Isoniazid mediates the CYP2B6*6 genotype-dependent interaction between efavirenz and antituberculosis drug therapy through mechanism-based inactivation of CYP2A6.
abstract: Efavirenz is commonly used to treat patients coinfected with human immunodeficiency virus and tuberculosis. Previous clinical studies have observed paradoxically elevated efavirenz plasma concentrations in patients with the CYP2B6*6/*6 genotype (but not the CYP2B6*1/*1 genotype) during coadministration with the commonly used four-drug antituberculosis therapy. This study sought to elucidate the mechanism underlying this genotype-dependent drug-drug interaction. In vitro studies were conducted to determine whether one or more of the antituberculosis drugs (rifampin, isoniazid, pyrazinamide, or ethambutol) potently inhibit efavirenz 8-hydroxylation by CYP2B6 or efavirenz 7-hydroxylation by CYP2A6, the main mechanisms of efavirenz clearance. Time- and concentration-dependent kinetics of inhibition by the antituberculosis drugs were determined using genotyped human liver microsomes (HLMs) and recombinant CYP2A6, CYP2B6.1, and CYP2B6.6 enzymes. Although none of the antituberculosis drugs evaluated at up to 10 times clinical plasma concentrations were found to inhibit efavirenz 8-hydroxylation by HLMs, both rifampin (apparent inhibition constant [Ki] = 368 μM) and pyrazinamide (Ki = 637 μM) showed relatively weak inhibition of efavirenz 7-hydroxylation. Importantly, isoniazid demonstrated potent time-dependent inhibition of efavirenz 7-hydroxylation in both HLMs (inhibitor concentration required for half-maximal inactivation [KI] = 30 μM; maximal rate constant of inactivation [kinact] = 0.023 min(-1)) and recombinant CYP2A6 (KI = 15 μM; kinact = 0.024 min(-1)) and also formed a metabolite intermediate complex consistent with mechanism-based inhibition. Selective inhibition of the CYP2B6.6 allozyme could not be demonstrated for any of the antituberculosis drugs using either recombinant enzymes or CYP2B6*6 genotype HLMs. In conclusion, the results of this study identify isoniazid as the most likely perpetrator of this clinically important drug-drug interaction through mechanism-based inactivation of CYP2A6.
authors.affiliation: Individualized Medicine Program, Department of Veterinary Clinical Sciences, Washington State University College of Veterinary Medicine, Pullman, Washington, USA michael.court@vetmed.wsu.edu.
authors.forename: Michael H
authors.initials: MH
authors.lastname: Court
authors.affiliation: Department of Integrative Physiology and Pathobiology, Tufts University School of Medicine, Boston, Massachusetts, USA Program in Pharmacology and Experimental Therapeutics, Sackler School of Graduate Biomedical Sciences, Tufts University, Boston, Massachusetts, USA.
authors.forename: Fawziah E
authors.initials: FE
authors.lastname: Almutairi
authors.affiliation: Department of Integrative Physiology and Pathobiology, Tufts University School of Medicine, Boston, Massachusetts, USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Greenblatt
authors.affiliation: Department of Integrative Physiology and Pathobiology, Tufts University School of Medicine, Boston, Massachusetts, USA.
authors.forename: Suwagmani
authors.initials: S
authors.lastname: Hazarika
authors.affiliation: Individualized Medicine Program, Department of Veterinary Clinical Sciences, Washington State University College of Veterinary Medicine, Pullman, Washington, USA.
authors.forename: Hongyan
authors.initials: H
authors.lastname: Sheng
authors.affiliation: Dr. Margarete Fischer-Bosch-Institute of Clinical Pharmacology, Stuttgart, and University of Tübingen, Tübingen, Germany.
authors.forename: Kathrin
authors.initials: K
authors.lastname: Klein
authors.affiliation: Dr. Margarete Fischer-Bosch-Institute of Clinical Pharmacology, Stuttgart, and University of Tübingen, Tübingen, Germany.
authors.forename: Ulrich M
authors.initials: UM
authors.lastname: Zanger
authors.affiliation: BD Biosciences, Discovery Labware, Woburn, Massachusetts, USA.
authors.forename: Joanne
authors.initials: J
authors.lastname: Bourgea
authors.affiliation: BD Biosciences, Discovery Labware, Woburn, Massachusetts, USA.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Patten
authors.affiliation: Warren Alpert Medical School of Brown University and The Miriam Hospital, Providence, Rhode Island, USA.
authors.forename: Awewura
authors.initials: A
authors.lastname: Kwara
mesh_terms: D000480:Alkynes; D000995:Antitubercular Agents / Q000494:pharmacology / Q000627:therapeutic use; D048588:Benzoxazines / Q000097:blood / Q000378:metabolism*; D003521:Cyclopropanes; D065637:Cytochrome P-450 CYP2A6 / Q000378:metabolism*; D065702:Cytochrome P-450 CYP2B6 / Q000235:genetics / Q000378:metabolism*; D004347:Drug Interactions; D004789:Enzyme Activation / Q000187:drug effects*; D004977:Ethambutol / Q000494:pharmacology; D006801:Humans; D006900:Hydroxylation / Q000187:drug effects; D007538:Isoniazid / Q000494:pharmacology*; D008862:Microsomes, Liver / Q000378:metabolism; D011718:Pyrazinamide / Q000494:pharmacology; D012293:Rifampin / Q000494:pharmacology
chemical_list: D000480:Alkynes; D000995:Antitubercular Agents; D048588:Benzoxazines; D003521:Cyclopropanes; D011718:Pyrazinamide; D004977:Ethambutol; C585710:CYP2A6 protein, human; C585599:CYP2B6 protein, human; D065637:Cytochrome P-450 CYP2A6; D065702:Cytochrome P-450 CYP2B6; C098320:efavirenz; D007538:Isoniazid; D012293:Rifampin