Hepatic Metabolism and Susceptibility to Ecstasy Toxicity

说明(申请人提供)：与使用3，4-亚甲基二氧基甲基苯丙胺(MDMA；摇头丸、XTC、E)有关的几种不良反应，最令人担忧的是对5-羟色胺能神经递质系统的长期毒性。因此，MDMA的使用和滥用有可能引起一个重大的公共卫生问题。MDMA的神经毒性作用与给药途径和给药频率有关。将MDMA或丙二醛直接注射到大脑中，未能复制外周给药后的神经毒性，表明母体苯丙胺不太可能是神经毒性的唯一责任。我们(和其他人)提出，MDMA和丙二醛的肝脏代谢物介导了神经毒性。我们假设，丙二醛和MDMA的数量较少但活性较强的肝脏代谢物有助于它们的神经毒性，其中一类代谢物是由N-甲基-β-甲基多巴胺(N-甲基-β-MEDA)和β-MEDA氧化产生的，然后用谷胱甘肽(GSH)清除邻苯二酚。此外，由于胺基上的碳原子a代表立体生成中心，MDMA可以存在于两种不同的三维镜像结构或对映体中：(R)-MDMA和(S)-MDMA。事实上，这些对映体的药理学特征不同，它们的相对神经毒性也不同。因此，在动物模型中，(S)-MDMA似乎是5-羟色胺能神经元退化的主要贡献者。由于MDMA的所有主要肝脏代谢物都保留了立体中心，因此所有的代谢物都以一对非对映异构体的形式存在(GSH也包含一个手性中心)。因此，MDMA的神经毒性不仅依赖于肝脏代谢，而且我们假设具有(S)-构型的代谢物将比相应的(R)-非对映异构体更有效。由于代谢对神经毒性的表达是必需的，因此MDMA的I相(P450)和II相(COMT)代谢的差异将是个体对MDMA神经毒性易感性的重要决定因素。在人类中，负责MDMA代谢的I相(CYP2D6)和II相(COMT)酶是多态的，表现出显著的个体差异。由于MDMA诱导的神经毒性易感性个体间差异的因素尚不清楚，而且神经毒性依赖于代谢，我们假设MDMA的I相和II相代谢的差异调节了个体对神经毒性的易感性。本申请中提出的研究旨在检验这些总体假设，并检查这些代谢物进入大脑的途径。因此，我们将测试以下假设：MDMA代谢物的神经毒性是立体选择性的(特定目标#1)，对神经毒性的敏感性受I期和II期肝脏代谢(特定目标#2)调节，并且由于神经毒性需要将水溶性代谢物摄取到大脑，毒性也受大脑摄取和大脑输出过程(第三阶段代谢)(特定目标#3)之间的平衡调节。总而言之，相对于MDMA的神经毒性，人们对其药效学和毒代动力学知之甚少，也不知道导致个体间易感性的因素，这种应用旨在解决我们所知的这些缺陷。与公共卫生相关：与使用和滥用3，4-亚甲基二氧基甲基苯丙胺(MDMA；摇头丸、XTC、E)有关的几种不良影响，最令人担忧的是对大脑中称为5-羟色胺能神经元的特殊细胞的长期损害。因此，MDMA的使用和滥用有可能引起一个重大的公共卫生问题。特别是，神经毒性的发展需要肝脏将MDMA代谢成代谢物，然后进入大脑。由于负责MDMA代谢和大脑摄取代谢物的蛋白质在不同的个体之间存在很大差异，了解这些不同过程对MDMA神经毒性的相对贡献将有助于我们识别哪些人可能更容易受到MDMA的不良影响(可能还有类似药物的影响)。

DESCRIPTION (provided by applicant): 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. PUBLIC HEALTH RELEVANCE: 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).