| Abstract
| - Dopamine D3 receptor subtypes have been hypothesized to play a pivotal role in modulatingthe reinforcing and drug-seeking effects induced by cocaine. However, definitive pharmacologicalinvestigations have been hampered by the lack of highly D3 receptor selective compounds thatcan be used in vivo. To address this problem, the potent and D3-receptor-selective antagonistNGB 2904 (1, 9H-fluorene-2-carboxylic acid {4-[(2,3-dichlorophenyl)-piperazin-1-yl]-butyl}-amide, Ki (hD3) = 2.0 nM, Ki (hD2L) = 112 nM, D2/D3 selectivity ratio of 56) was chosen as alead structure for chemical modification in an attempt to reduce its high lipophilicity (c log D= 6.94) while optimizing D3 receptor binding affinity and D2/D3 selectivity. A series of >30novel analogues were synthesized, and their binding affinities were evaluated in competitionbinding assays in HEK 293 cells transfected with either D2L, D3, or D4 human dopaminereceptors using the high affinity, selective D2-like receptor antagonist 125I-IABN. Structuraldiversity in the aryl amide end of the molecule was found to have a major influence on(sub)nanomolar D3 receptor affinity and D2/D3 selectivity, which was optimized using a morerigid trans-butenyl linker between the aryl amide and the piperazine. Several analoguesdemonstrated superior D3 receptor binding affinities and selectivities as compared to the parentligand. Compound 29 (N-{4-[4-(2,3-dichlorophenyl)-piperazin-1-yl]-trans-but-2-enyl}-4-pyridine-2-yl-benzamide) displayed the most promising pharmacological profile (Ki (hD3) = 0.7 nM, Ki(hD2L) = 93.3 nM, D2/D3 selectivity ratio of 133). In addition, this ligand inhibited quinpirolestimulation of mitogenesis at human dopamine D3 receptors transfected into Chinese hamsterovary (CHO) cells, with an EC50 value of 3.0 nM. Compound 29 was a nearly 5 times morepotent antagonist at the D3 receptor than 1 (EC50 = 14.4 nM). Moreover, a decrease in c log Dvalue of ∼2 orders of magnitude was determined for this novel D3-receptor-preferring ligand,compared to 1. In summary, chemical modification of 1 has resulted in compounds with highaffinity and selectivity for D3 receptors. The most promising candidate, compound 29, iscurrently being evaluated in animal models of cocaine abuse and will provide an importanttool with which to elucidate the role of D3 receptors in drug reinforcement in vivo.
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