New Inactivators of GABA Aminotransferase for Addiction

描述(申请人提供)：这项研究的长期目标是设计新的化合物来使GABA转氨酶(GABA-AT)失活，GABA-AT是一种分解抑制性神经递质GABA的酶，用于治疗化学成瘾和癫痫。抑制GABA-AT可提高GABA水平，已被证明在不影响基础神经元放电的情况下有效地抑制过度的神经活动。增加GABA水平可以阻止大鼠的可卡因、尼古丁、甲基苯丙胺、酒精和海洛因成瘾，也可以阻止人类的可卡因成瘾，而不会影响对食物的渴望。此外，当大脑中的GABA浓度降低到阈值以下时，就会导致抽搐；提高大脑中的GABA水平可以终止癫痫发作。对滥用药物的神经化学反应是伏隔核中多巴胺水平的急剧增加，这会激活负责愉悦和奖赏反应的神经元。增加GABA的浓度可以拮抗多巴胺和相关行为的增加。Vigabatrin(商标为SabrilTM)是一种不可逆转的GABA-AT抑制剂，目前作为儿科患者的单一疗法和成人难治性癫痫发作的辅助疗法而上市。在灵长类动物身上的正电子发射断层扫描(PET)显示，Vigabatrin抑制这些可卡因诱导的多巴胺的增加。然而，由于长期服用大量Vigabatrin后25%-50%的患者存在视野缺陷(VFD)，人们对Vigabatrin用于治疗癫痫和兴奋剂成瘾的接受度受到了阻碍；典型的剂量为每天1-3克。导致VFD的机制尚不清楚；然而，如果普遍认为VFD是由于长期接触大剂量药物而产生的是正确的，并且如果可以使用低得多的药物剂量，则可能不会有不良后果。一种新的GABA-AT失活剂，(1S，3S)-3-氨基-4-二氟亚甲基-1-环戊酸(2)，在调节成瘾物质引起的多巴胺增加和逆转可卡因成瘾方面比Vigabatrin强300倍。由于2的效力，可以使用比Vigabatrin低得多的剂量，这可能会防止VFD。这一建议的一个重要目的是阐明2的失活机理，这将对未来的抑制剂设计非常有利。将进行2和磷酸吡哆醛(PLP)辅酶的同位素标记研究。另一个目标是合成新的GABA-AT失活剂并研究其失活机理；建议的化合物之一应该通过一种途径进行，以避免可能产生VFD的Vigabatrin的潜在有毒副产物。其他新化合物与2的结构有关，以进一步增强药效。第三个目标是确定失活剂附着在酶上的位置。第四个目标将涉及合作者斯蒂芬·杜威博士利用正电子发射计算机断层扫描技术研究这些新化合物对大鼠大脑中多巴胺释放的影响以及它们对大鼠成瘾的影响。最后一个目的是确定这些失活剂相对于其他依赖PLP的酶对GABA-AT的选择性。 与公共健康相关：这项研究的目的是设计新的化合物来阻断GABA转氨酶，这种酶催化破坏抑制性神经递质GABA，从而提高大脑中GABA的水平。增加GABA水平可以阻止大鼠的可卡因、尼古丁、甲基苯丙胺、酒精和海洛因成瘾，也可以阻止人类的可卡因成瘾，而不会影响对食物的渴望。此外，当大脑中的GABA浓度降低到阈值以下时，就会导致抽搐；提高大脑中的GABA水平，例如通过抑制GABA转氨酶，可以终止癫痫发作。

DESCRIPTION (provided by applicant): The long-term goal of this research is to design new compounds to inactivate GABA aminotransferase (GABA- AT), the enzyme that catabolizes the inhibitory neurotransmitter GABA, for the treatment of chemical addiction and epilepsy. Inhibition of GABA-AT, which raises GABA levels, has been shown to effectively dampen excessive neural activity without affecting basal neuronal firing. Increasing GABA levels blocks cocaine, nicotine, methamphetamine, alcohol, and heroin addiction in rats and cocaine addiction in humans without affecting the craving for food. Also, when the concentration of GABA diminishes below a threshold level in the brain, convulsions result; raising the brain GABA levels terminates the seizure. The neurochemical response to drugs of abuse is a sharp increase in dopamine levels in the nucleus accumbens, which activates the neurons responsible for pleasure and reward responses. The rise in dopamine and associated behaviors can be antagonized by an increase in the concentration of GABA. Vigabatrin (trade name SabrilTM), an irreversible inhibitor of GABA-AT, is currently marketed as a monotherapy for pediatric patients and as an adjunctive therapy for adults with refractory seizures. It was shown by positron emission tomography (PET) in primates that vigabatrin inhibits these cocaine-induced dopamine increases. The acceptance of vigabatrin for the treatment of both epilepsy and stimulant addiction, however, has been hampered by concerns about visual field defects (VFDs) in 25-50% of patients following chronic administration of large amounts of vigabatrin; the typical dose is 1-3 grams a day. The mechanism leading to the VFDs is not known; nonetheless, if the prevailing belief that VFDs arise from prolonged exposure to large doses of drug is correct, and if much lower doses of a drug can be used, there may be no untoward consequences. A new GABA-AT inactivator, (1S,3S)- 3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (2), was found to be 300 times more potent than vigabatrin in modulation of the dopamine increase induced by addictive substances and in reversal of cocaine addiction in rats. Because of the potency of 2, much lower doses can be used than those with vigabatrin, which may prevent the VFDs. An important aim of this proposal is to elucidate the inactivation mechanism of 2, which will be very beneficial to future inhibitor design. Studies involving isotopic labeling of 2 and of the pyridoxal phosphate (PLP) coenzyme will be carried out. Another aim is to synthesize new inactivators of GABA-AT and study their inactivation mechanisms; one of the proposed compounds should proceed by a pathway that avoids a potential toxic by-product of vigabatrin that might produce the VFDs. Other new compounds are related to the structure of 2 to enhance potency further. A third aim is to determine the site of inactivator attachment on the enzyme. A fourth aim will involve studies by collaborator Dr. Stephen Dewey on the effect of these new compounds on dopamine release in rat brains using PET and their effect on addiction in rats. The last aim is to determine the selectivity of these inactivators for GABA-AT relative to other PLP-dependent enzymes. PUBLIC HEALTH RELEVANCE: The aim of this research is to design new compounds to block GABA aminotransferase, the enzyme that catalyzes the destruction of the inhibitory neurotransmitter GABA, thereby increasing the brain levels of GABA. Increasing GABA levels blocks cocaine, nicotine, methamphetamine, alcohol, and heroin addiction in rats and cocaine addiction in humans without affecting the craving for food. Also, when the concentration of GABA diminishes below a threshold level in the brain, convulsions result; raising the brain GABA levels, for example by inhibiting GABA aminotransferase, terminates the seizure.