Ecstasy [(±)-3.4-methylenedioxymethamphetamine, MDMA, XTC, X, E] is one of the most popular drugs of abuse in the world (Capela et al., 2009, p. 211). Often used in social settings, such as the so-called 'raves' or all night dance parties, ecstasy has been reported to lower barriers to intimacy, increase the pleasure derived from friendships, enhance social interactions, and increase energy (euphoria) (Peters and Kok, 2009, p. 242).
In the U.S., MDMA is classified as a schedule 1 drug due its addictive potential, lack of therapeutic utility, dubious safety profile, and neurotoxic potential (Capela et al., 2009, p. 212) and its use has been illegal since 1985. The safety concerns of MDMA include the potential for a negative therapeutic outcome (Parrott, 2007) and its neurotoxicity (Capela et al., 2009). Apparently, the use of MDMA in a psychotherapy setting can produce a negative outcome that can persist for years (Parrott, 2007). In addition, psychiatric patients may be more susceptible to having a negative experience when treated with MDMA.
With respect to neurotoxicity, research studies have shown that a number of neurotransmitter pathways are affected in humans and primates, including reduced serotonin (5-HT) production and thus the main metabolite of serotonin, 5-hydroxyindoleacetic acid (5-HIAA), reduced serotonin transporter (SERT) levels, and lower levels of tryptophan hydroxylase (Verrico, Miller, and Madras, 2007, p. 490). In addition, MDMA induces the degeneration of serotonergic axonal projections and nerve terminals (Thomasius et al., 2006, p. 212). Such changes would explain the persistence of low moods and positive/negative experiences after MDMA treatment; however, the proposed mechanisms underlying MDMA-induced neurotoxicity remain controversial (Verrico, Miller, and Madras, 2007, p. 490). The most common proposed mechanisms include reduced metabolic clearance, the production of toxic metabolites, oxidative stress, hyperthermia, apoptosis, a rise in extracellular concentrations of serotonin and dopamine, carrier-dependent MDMA transport, and induced release of serotonin via SERT.
One of the more widely tested theories for MDMA-induced neurotoxicity is how well the liver metabolizes the drug (Capela et al., 2009, p. 216-225). In particular, how genetics influences the rate of metabolic clearance. This report will examine the evidence for the neurotoxic effects of MDMA use and the proposed mechanisms, with a focus on the influence of genetics on MDMA metabolism.
MDMA is typically ingested, where it is metabolized by the liver (Esse, Fossati-Bellani, Traylor, and Martin-Schild, 2011, p. 48). Peak serum levels are reached approximately 2 hours later. Around 20% is excreted in the urine, with a half-life of 6-9 hours (Capela et al., 2009, p. 218). Renal clearance is also enantioselective, such that (S)-MDMA has a shorter half-life of 4.8 hours and (R)-MDMA has a half-life of 14.8 hours. This distinction is important because (S)-MDMA has been associated with subjective and psychomotor effects and (R)-MDMA associated with altering mood and cognition.
At doses below 150 mg, MDMA serum levels after 24 hours reflected dosage, but at 150 mg, serum levels were no longer proportional (Yang et al., 2006, p. 845). Doses above 150 mg may therefore saturate the metabolic pathway for MDMA, and accordingly the liver metabolic enzyme cytochrome P450 2D6 (CYP2D6) has been shown to be inhibited by MDMA in vitro (Capela et al., 2009, p. 216-217). In addition, MDMA is not bound by serum proteins and it can readily diffuse across lipid barriers into tissues, organs, and cells. For this reason, bioavailability is expected to be high. Consistent with this finding, concentrations in the brain and serotonergic neurons are higher than would be expected given serum levels.
The enzymes believed to be involved in MDMA metabolism in the liver are CYP2D6, CYP1A2, CYP2B6, and CYP3A4, while enantioselectivity depends on CYP2C19 and CYP2D6 (Capela et al., 2009, p. 218). The main metabolic steps are O-demethylation to 3,4-dihydroxymethamphetamine [HHMA, N-methyl-?-methyldopamine (N-Me-?-MeDA)] via CYP2D6, and to a lesser extent CYP2B6 and CYP3A4, and O-methylation to 4-hydroxy-3-methoxymethamphetamine (HMMA, 3-O-methyl-N-methyl-?-methyldopamine) via catechol-O-methyltransferase (COMT). Importantly, administration of the CYP2D6 inhibitor paroxetine significantly increased MDMA plasma levels in human subjects (Segura et al., 2005). CYP2D6 availability and activity may therefore influence MDMA-induced neurotoxicity.
Genetic Influence of MDMA Metabolism
Genetic polymorphisms in rat CYP isoenzymes generate sex and strain differences in MDMA metabolism (Capela et al., 2009, p. 217). For example, the rat equivalent of CYP2D6 is absent in Dark Agouti female rats and brain concentrations of MDMA were found to be relatively high. Naturally occurring genetic variability may therefore play a significant role in MDMA-mediated neurotoxicity.
A number of genetic polymorphisms have been identified in the human gene encoding CYP2D6 (Carmo et al., 2006, p. 790). Polymorphisms represent low frequency differences (>1%) in the protein-coding DNA sequence or in non-coding DNA sequence controlling the expression of a particular gene. By 2006, more than 100 variations and 58 distinct cyp2d6 alleles had been identified that produce a wide range of activities and expression levels. Some of these polymorphisms are found only in some ethnic groups and almost 1% of Asians and 10% of Caucasians produce a non-functional protein. These individuals have been called poor metabolizers. By contrast, between 10% and 15% of the European population are predicted to be intermediate metabolizers. At the upper extreme, ultrarapid metabiolizers include individuals that have additional copies of the gene, which are very prevalent in Ethiopian and Saudi Arabian populations and are represented in the European population at a relatively low 5%.
The genetic differences that determine CYP2D6 activity or expression levels are predicted to have an impact on MDMA metabolism rates. When serum levels of MDMA were compared between poor and ultrarapid metabolizers, a 100 mg dose induced a persistent 36% increase in serum MDMA levels in poor metabiolizers (Yang et al., 2006, p. 846). The authors of this study suggested that this difference is too small to explain a possible association between CYP2D6 polymorphisms and MDMA-induced neurotoxicity, a conclusion that agrees with the findings of some clinical studies.
A possible explanation for MDMA-induced toxicity was suggested by an in vitro study examining cytotoxicity in a mammalian cell line transfected with five different CYP2D6 alleles and CYP3A4 (Carmo et al., 2006, p. 793). The most prevalent genotype (wild-type, CYP2D6*1) induced a 39% to 50% decrease in cell survival after treatment with MDMA. The only other CYP2D6 allele that had a significant negative impact on cell survival was CYP2D6*10, producing a 28% to 39% decrease. All other CYP2D6 alleles, and CYP3A4, failed to have an impact on cell survival. An analysis of the culture supernatants identified HHMA as the primary metabolite following MDMA treatment, but the highest concentrations were produced by the cells expressing CYP2D6*1 (1.0 mmol/million cells). No other metabolites reached detection levels.
When the cytotoxicity of HHMA was assessed in vitro, it was found that this metabolite was at least 100-fold more toxic than MDMA (Carmo et al., 2006, p. 793). Cytotoxicity was detected at concentrations between 5 and 10 umol/L HHMA and by 20 umol/L it was 100% efficient in killing cells. Carmo and colleagues (2006) argue that ultrarapid metabolizers would therefore be particularly susceptible to HHMA-induced cytotoxicity. In other words, rather than poor metabolizers being most susceptible to the toxic effects of MDMA, it is instead the ultrarapid metabolizers. This finding is consistent with a clinical study showing that three ecstasy users admitted to the emergency department for hepatotoxicity were found to be ultrarapid metabolizers (Carmo et al., 2006, p. 795-796). However, this mechanism may not explain the neurotoxic effects of MDMA.
The DNA sequence encoding the COMT enzyme is also highly variable, resulting in lower or higher activity than the wild-type form (Capela et al., 2009, p. 223). Specifically, the valine108methyonine and valine158methyonine polymorphisms result in an enzyme with lower activity and this combination is prevalent in Caucasians, but not in African and Asian populations. Decreased COMT activity could have dramatic effects on endogenous catecholamine clearance, especially after MDMA-induced release of serotonin, norepinephrine, and dopamine. The neurotoxic effects of MDMA could therefore result from a buildup of HHMA, sustained catecholamine activity, and the reduced clearance of autoxidated catechols. In support of this possibility, individuals homozygous for the met/met COMT phenotype were susceptible to the neurotoxic effects of D-amphetamine.
The mechanisms underlying MDMA-induced neurotoxicity remain unknown; however, research in this area continues to uncover interesting phenomenon that may one day provide an answer. CYP2D6 genetic variants provide a wide variety of activities and expression levels, from producing an inactive protein fragment, to having multiple copies of the wild-type gene, or having internal variations that reduce its enzymatic activity. The number of cyp2d6 variants is quite large, but can be grouped into the functional classifications of poor, intermediate, wild-type, and ultrarapid metabiolizers. Poor metabolizers result in higher serum concentrations of MDMA, which in turn drives higher concentrations of the drug in tissues and organs. At the other extreme, the metabolic conversion of MDMA to HHMA renders these individuals susceptible to acute liver toxicity. Whether this latter mechanism also contributes to serotonergic neurotoxicity is unknown.
Many of the clinical studies that have investigated whether poor metabolizers are susceptible to…