New Inactivators of GABA Aminotransferase for Addiction

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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.