INTERACTION OF BUPRENORPHINE AND ITS METABOLITE NORBUPRENORPHINE WITH CYTOCHROMES P450 IN VITRO WENJIANG ZHANG, YAMINI RAMAMOORTHY, RACHEL F. TYNDALE, AND EDWARD M. SELLERS Departments of Pharmacology (W.Z., Y.R., R.F.T., E.M.S.), Psychiatry (E.M.S.), and Medicine (E.M.S.), University of Toronto, Ontario, Canada; Center for Addiction and Mental Health, Toronto, Ontario, Canada (E.M.S.); and Sunnybrook and Women’s Health Science Centre, Toronto, Ontario, Canada (E.M.S.) (Received July 25, 2002; accepted March 13, 2003) This article is available online at http://dmd.aspetjournals.org ABSTRACT: Buprenorphine is a thebaine derivative used in the treatment of heroin and other opiate addictions. In this study, the selective probe reactions for each of the major hepatic cytochromes P450 (P450s) were used to evaluate the effect of buprenorphine and its main metabolite norbuprenorphine on the activity of these P450s. The index reactions used were CYP1A2 (phenacetin O-deethyla- tion), CYP2A6 (coumarin 7-hydroxylation), CYP2C9 (diclofenac 4- hydroxylation), CYP2C19 (omeprazole 5-hydrxoylation), CYP2D6 (dextromethorphan O-demethylation), CYP2B6 (7-ethoxy-4-triflu- oromethyl-coumarin 7-deethylation), CYP2E1 (chlorzoxazone 6-hydroxylation), and CYP3A4 (omeprazole sulfoxidation). Bu- prenorphine exhibited potent, competitive inhibition of CYP2D6 (K i 10  2 M and 1.8  0.2 M) and CYP3A4 (K i 40  1.6 M and 19  1.2 M) in microsomes from human liver and cDNA-expressing lymphoblasts, respectively. Compared with buprenorphine, norbu- prenorphine demonstrated a lower inhibitory potency with CYP2D6 (22.4% inhibition at 20 M norbuprenorphine) and CYP3A4 (13.6% inhibition at 20 M) in microsomes from human cDNA-expressing lymphoblast cells. Furthermore, buprenorphine was shown to be a substrate of CYP2D6 (K m  600 M; V max  0.40 nmol/min/mg protein) and CYP3A4 (K m  36 M; V max  0.19 nmol/min/mg protein). The present in vitro study suggests that buprenorphine and its major metabolite norbuprenorphine are inhibitors of CYP2D6 and CYP3A4; however, at therapeutic concentrations they are not predicted to cause potentially clinically important drug interactions with other drugs metabolized by major hepatic P450s. Buprenorphine (BUP 1 ) is a semisynthetic, highly lipophilic opioid derivative of the morphine alkaloid thebaine. As a partial opioid agonist, BUP has been used in the treatment of cocaine, heroin, and other opiate addictions (Kosten et al., 1989; Lewis et al., 1992; Mello et al., 1993; Vignau et al., 1998). Kosten et al. (1991) demonstrated a 33% reduction in illicit opioid use in opioid-dependent outpatients treated with BUP, with 72% treatment retention (Kosten et al., 1991). Preclinical studies in both primates and rodents have also indicated that BUP may reduce cocaine self-administration and attenuate place preference for cocaine (Kosten et al., 1992). The apparent reduced tendency to elicit withdrawal effects suggests that BUP may be an effective long-term agent in the treatment of cocaine, opioid, and perhaps other drug addictions. The positive utility of BUP, however, is tempered with its potential for abuse mainly in combination with benzodiazepines (Singh et al., 1992; Reynaud et al., 1998). The abuse of BUP is owing to its potent analgesic effects, which are 30 times that of morphine (Cowan et al., 1977). Fatalities have been reported in abusers of a high-dose, sublingual BUP formulation recently marketed in France for the substitutive therapy of opiate addictions (Tracqui et al., 1998), as well as with concomitant BUP-benzodiazepine misuse (Reynaud et al., 1998). In clinical practice, BUP is administered to drug-addicted patients concurrently receiving other drugs such as cocaine, morphine, ben- zodiazepines, and cyclizine (Singh et al., 1992; Teoh et al., 1993; Niv et al., 1998), several of which interact with cytochrome P450 (P450) enzymes. There is, therefore, a theoretical risk of drug interactions with respect to both inhibition of BUP metabolism and inhibition of metabolism of other drugs by BUP, which may lead to serious adverse consequences. The principal route of BUP metabolism in humans is via the N-dealkylation of its N-cyclopropylmethyl group to norbu- prenorphine (Nor-BUP) (Iribarne et al., 1997). CYP3A4 has been shown to be the main enzyme responsible for BUP N-dealkylation in vitro in microsomes from human liver and cDNA-expressing cells (Iribarne et al., 1997; Kobayashi et al., 1998). At the time of this study, detailed inhibition profiles of either BUP or Nor-BUP with all of the major P450s had yet to be determined. During the course of preparation of this report, a study by Umehara et al. (2002) on the effect of BUP on the metabolism of P450 probe substrates was published, allowing comparisons to be made to the results of the present study. Hepatic P450s 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4 are the most important P450 forms involved in drug metabolism in humans (Shimada et al., 1994). The metabolism of many therapeuti- cally important drugs and endogenous compounds is mediated pri- marily by these enzymes. It is necessary to investigate the interactions of BUP and its main metabolite Nor-BUP with the major hepatic Supported in part by National Cancer Institute Grant CA-12010 and National Institute on Drug Abuse Grants DA-03817 and DA-06889. 1 Abbreviations used are: BUP, buprenorphine; P450, cytochrome P450; Nor- BUP, norbuprenorphine. Address correspondence to: Dr. Edward M. Sellers, 340 College Street Suite 400, Toronto, ON M5T 3A9 Canada. E-mail: e.sellers@utoronto.ca 0090-9556/03/3106-768–772$7.00 DRUG METABOLISM AND DISPOSITION Vol. 31, No. 6 Copyright © 2003 by The American Society for Pharmacology and Experimental Therapeutics 881/1068328 DMD 31:768–772, 2003 Printed in U.S.A. 768 at ASPET Journals on April 15, 2017 dmd.aspetjournals.org Downloaded from