Cyanoindole derivatives as highly selective dopamine D(4) receptor partial agonists: solid-phase synthesis, binding assays, and functional experiments.
Traceless linking of diethoxymethyl (DEM)-protected 5- and 6-cyanoindoles and subsequent incorporation of phenylpiperazine derivatives led to the 2- and 3-piperazinylmethyl-substituted cyanoindoles 3a-m. Dopamine receptor binding studies on the final products 3a-m clearly indicated strong and selective recognition of the D(4) subtype which is known as a promising target for the treatment of neuropsychiatric disorders. The most interesting binding properties were observed for the 2-aminomethyl-5-cyanoindoles FAUC 299 (3f) and FAUC 316 (3j) (K(i) = 0.52 and 1.0 nM, respectively) when the fluoro derivative 3j proved extraordinary selectivity over D(1), D(2long), D(2short), and D(3) (>8600). To determine ligand efficacy, mitogenesis experiments were performed indicating partial agonist effects for the test compounds 3f,j (35% and 30%, when compared to the full agonist quinpirole).