ABSTRACT:The potency of methylenedioxymethamphetamine (MDMA) as a mechanism-based inhibitor of CYP2D6 has been defined using microsomes prepared from yeast expressing the enzyme and from three human livers. The inhibitory effect was increased by preincubation through formation of a metabolic intermediate complex. MDMA (commonly known as "Ecstasy") is a synthetic amphetamine derivative that has become a popular recreational drug, particularly in the "rave" culture (Cole and Sumnall, 2003). Case reports of exaggerated responses and death associated with its use suggest that some individuals are at an increased risk of toxicity (Tucker et al., 1994). Hyperthermia, tachycardia, convulsions, rhabdomyolysis, and acute liver and renal failure following MDMA use in nightclubs are well documented, and longer-term use is linked to irreversible changes in serotoninergic function (Ramamoorthy et al., 2002). Although administration of MDMA and its metabolite methylenedioxyamphetamine to rats produces serotonergic nerve terminal degeneration, these compounds are not neurotoxic when injected directly into the brain. This suggests a requirement for their peripheral metabolism to neurotoxic metabolites (Easton et al., 2003) which, presumably, are then actively transported into the brain. These metabolites are believed to be downstream products formed after the opening of the methylendioxyphenyl ring, a process which is mainly mediated by CYP2D6 as a high-affinity enzyme (Tucker et al., 1994;Kreth et al., 2000), with low-affinity contributions from CYP1A2, CYP2B6, and CYP3A4 (Kreth et al., 2000).CYP2D6 is a polymorphic enzyme and a functional form is absent in 5 to 9% of Caucasians as a result of autosomal recessive inheritance of gene mutations (Zanger et al., 2004). Thus, it has been suggested that genetically deficient metabolism of MDMA may help to explain why some users of "Ecstasy" appear to be more sensitive to its acute effects (Tucker et al., 1994;Lin et al., 1997). However, recent studies have found no obvious link between inherited CYP2D6 deficiency and MDMA intoxication (O'Donohoe et al., 1998;Schwab et al., 1999;Gilhooly and Daly, 2002). Wu et al. (1997) reported that MDMA is a potent competitive inhibitor of CYP2D6 in human liver microsomes. In addition, they showed that the inhibitory effect of MDMA was increased following preincubation with human liver microsomes in the presence of NADPH. Later studies (Delaforge et al., 1999) confirmed that MDMA forms a metabolic intermediate complex (MIC) with human CYP2D6 expressed in yeast microsomes. However, a quantitative definition of the potency of MDMA as a mechanismbased inhibitor of CYP2D6 is lacking. Thus, the kinetics of the inhibitory effects of MDMA on CYP2D6 in human liver microsomes from extensive metabolizer (EM) subjects and microsomes prepared from yeast expressing the human enzyme were investigated in this study.
Materials and MethodsDrugs and Chemicals. Racemic MDMA was provided by Professor R. Forrest (Department of Forensic Pathology, Royal Hallamshire Hos...