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. In an attempt to design a novel class of D3 ligands with which to study this receptor system, a series of chemically divergent compounds that possessed various structural features that exist within several classes of reputed D3 agents was screened and compared to the recently reported D3 antagonist, NGB 2904. Based on these results, a novel series of compounds was designed that included functional moieties that were required for high affinity and selective D3 receptor binding. All the compounds in this series included an aryl-substituted piperazine ring, varying alkyl chain linker (C3-C5) and a terminal aryl amide. These novel compounds were synthesized, purified, chemically characterized and evaluated in vitro for binding in CHO cells transfected with human D2, D3, or D4 receptor cDNAs. D3 binding affinities ranged from Ki=1.4-1460 nM. The most potent analog in this series, demonstrated a D3/D2 selectivity of 64 and a D3/D4 selectivity of 1300. Structure-activity relationships demonstrated that the 2,3-dichloro-substituted phenylpiperazine was required for high affinity binding at D3, wherein every compound with this substituent was more potent than its unsubstituted homologue. The optimum alkyl chain length, between the amido-aryl function and the phenylpiperazine was four carbons. Although the 5-carbon linked compounds exhibited reasonably high affinity for D3, D2 affinity was also very high. The position of the amide-linkage on the fluorenyl ring appears to be optimal at either the 2- or 4-positions. Although some of the compounds displayed moderate to high affinity for D2 receptors, none of the compounds displayed appreciable affinity for D4. The most potent and selective compounds, of this series, were synthesized in multigram quantities and were evaluated in several animal models of cocaine and methamphetamine abuse. However, the high lipophilicity of the most potent agents appeared to limit bioavailability of these compounds. Hence, novel ligands have now been designed and synthesized with the goal of retaining high affinity binding for D3 receptors, but improving physico-chemical properties that will provide a more favorable pharmacokinetic/bioavailability profile than the currently existing D3 agents. Toward this goal, a series of heterocyclic analogues of our lead compound, as well as a series of structurally rigid analogs, in which the alkyl linking chain was saturated with either cis or trans olefins or an alkyne group, were prepared. These compounds were evaluated for binding in rat cloned D2 and D3 receptors stably transfected in Sf9 cells. This series of compounds, demonstrated that the fluorenyl ring could readily be substituted with less lipophilic heterocyclic ring systems and retain high affinity binding and selectivity for D3. The most potent and selective agent in this series had a Ki=0.6 nM at D3 and was 100-fold selective over D2. Furthermore, the lipophilic and sterically bulky fluorenyl group could be replaced with a simple phenyl ring, when the trans olefin replaced the saturated linking chain. Hence these compounds retain high affinity and selective D3 binding and may be more bioavailable, due to their improved physicochemical properties. Scale-up and behavioral evaluation of the most potent and selective analogues is underway as well as the design and synthesis of novel compounds that combine the optimum features discovered for D3 binding. Although clinical efficacy of these agents has yet to be substantiated, the development of highly selective and potent molecular probes will prove useful in the elucidation of the role D3 receptors play in the psychomotor stimulant and reinforcing properties of cocaine and methamphetamine.
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