Dopamine has been implicated as the primary neurotransmitter associated with the psychomotor stimulant and reinforcing effects of cocaine. These findings have resulted in intensive efforts to characterize and elucidate the roles of the various dopamine receptor subtypes in the pharmacology and abuse liability of this drug of abuse. In this pursuit, the dopamine D3 receptor subtype has been recently targeted. However, definitive behavioral investigations have been hampered by the lack of highly selective D3 agonists and antagonists. We have prepared the known D3 antagonist NGB 2904 in multi-gram quantities for acute and chronic behavioral testing. This compound demonstrated in vivo D3 antagonist actions and attenuated cocaine self-administration under a progressive ratio schedule. NGB 2904 also blocked cocaine-induced reinstatement of drug seeking behavior, an animal model of relapse. We have also designed and synthesized novel series of compounds, based on NGB 2904. All the compounds included a 2,3-dichloro-substituted phenylpiperazine, a four carbon linking chain with varying saturation (butyl and trans butenyl) and a terminal aryl amide. Evaluation for in vitro binding in HEK 293 cells transfected with human D2, D3, or D4 receptor cDNAs resulted in D3 binding affinities ranging from Ki=0.5-500 nM. The most potent analogs in this series, demonstrated D3/D2 selectivity of >130 and a D3/D4 selectivity of >1000. Functional evaluation in vitro using a mitogenesis assay in D3 or D2 receptor transfected CHO cells demonstrated that these compounds were either potent antagonists or partial agonists at D3 receptors and were selective over D2 receptors in this function. Structure-activity relationships demonstrated that the trans-butenyl linker provided additional D3 selectivity as compared to the other linking chains. Further, replacement of the sterically bulky aryl ring system with various heteroaryl groups served to retain high affinity and selectivity for D3, while decreasing lipophilicity, as compared to the parent compound NGB 2904. The latter goal of reducing lipophilicity of the most potent agents was to improve physico-chemical properties that would provide a more favorable pharmacokinetic/bioavailability profile than the currently existing D3 agents. Two of the most potent and selective compounds of this series, CJB 090 and PG 01037, were synthesized in multi-gram quantities and are currently being evaluated in several animal models of cocaine and methamphetamine abuse, in both rodents and primates. Thus far, both analogues attenuated dopamine-agonist-induced yawning in rats or monkeys in a dose dependent manner, indicating selective D3 antagonist action in vivo. Further, phMRI studies have shown that PG01037 localizes in D3 receptor rich regions of the living rat brain and functions as an antagonist by inducing positive cerebral blood volumes, in those regions. Chronic studies in these and additional rodent and monkey models of drug abuse and schizophrenia are underway with PG01037. Additional analogues of this new lead compound are also being prepared to further increase D3 receptor selectivity and determine structural requirements for D3 efficacy and improved bioavailability.
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