Several adverse effects are associated with the use of 3,4-(?)-methylenedioxymethamphetamine (MDMA;Ecstasy, XTC, E), the most worrisome of which is long-term toxicity to the serotonergic neurotransmitter system. MDMA use and abuse therefore has the potential to give rise to a major public health problem. The neurotoxic effects of MDMA are dependent on the route and frequency of drug administration. Direct injection of either MDMA or MDA into the brain fails to reproduce the neurotoxicity following peripheral administration, indicating that the parent amphetamines are unlikely to be solely responsible for the neurotoxic effect. We (and others) have proposed that liver-derived metabolites of MDMA and MDA mediate the neurotoxicity. We hypothesize that quantitatively minor, yet reactive hepatic metabolites of MDA and MDMA contribute to their neurotoxicity, and that one such class of metabolites arise from the oxidation of N-methyl-?-methyldopamine (N-methyl-?-MeDA) and ?-MeDA, followed by scavenging of the ortho-quinones with glutathione (GSH). Moreover, because the carbon atom a to the amine group represents a stereogenic center, MDMA can exist in two different three-dimensional mirror-image structures, or enantiomers: (R)-MDMA and (S)-MDMA. Indeed, the pharmacological profiles of the enantiomers differ, as does their relative neurotoxicity. Thus, in animal models, (S)-MDMA appears to be the major contributor to the degeneration of serotonergic neurons. Because all the principal hepatic metabolites of MDMA retain the stereogenic center, it follows that all of the metabolites exist as a pair of diastereoisomers (GSH also contains a chiral center). Therefore, not only is the neurotoxicity of MDMA dependent upon hepatic metabolism, but we hypothesize that metabolites possessing the (S)-configuration will be more potent than the corresponding (R)-diastereoisomers. Because metabolism is necessary for the expression of neurotoxicity, differences in the Phase I (P450) and Phase II (COMT) metabolism of MDMA will be important determinants of individual susceptibility to the neurotoxicity of MDMA. In humans, the phase I (CYP2D6) and phase II (COMT) enzymes responsible for MDMA metabolism are polymorphic, exhibiting significant inter-individual differences. Since the factors that contribute to the inter-individual variability in susceptibility to MDMA induced neurotoxicity are not known, and since neurotoxicity is dependent upon metabolism, we hypothesize that differences in the phase I and phase II metabolism of MDMA modulate individual susceptibility to neurotoxicity. Studies proposed in this application are designed to test these overall hypotheses and to examine the pathways by which such metabolites gain access to the brain. We will therefore test the hypotheses that the neurotoxicity of MDMA metabolites is stereoselective (Specific Aim #1), that susceptibility to neurotoxicity is modulated by both Phase I and Phase II hepatic metabolism (Specific Aim #2) and that because neurotoxicity requires the uptake of water soluble metabolites into brain, toxicity is also regulated by the balance between brain uptake and brain export processes (Phase III metabolism) (Specific Aim #3). In summary, little is known about the pharmacodynamics and toxicokinetics of MDMA relative to its neurotoxicity, nor of the factors that contribute to inter-individual susceptibility, and this application is designed to address these deficits in our knowledge.
Several adverse effects are associated with the use and abuse of 3,4-(?)-methylenedioxymethamphetamine (MDMA;Ecstasy, XTC, E), the most worrisome of which is long-term damage to specialized cells within the brain, known as serotonergic neurons. MDMA use and abuse therefore has the potential to give rise to a major public health problem. In particular, the development of neurotoxicity requires metabolism of MDMA by the liver into a metabolite that subsequently enters the brain. Because the proteins responsible for both the metabolism of MDMA and the brain uptake of the metabolites vary considerably between different individuals, knowledge of the relative contribution of these different processes to the neurotoxicity of MDMA will assist in our ability to identify those individuals who may be more susceptible to the adverse effects of MDMA (and perhaps to similar drugs).
