NPC 16377, a potent and selective sigma-ligand. I. Receptor binding, neurochemical and neuroendocrine profile.

6-\[6-(4-Hydroxypiperidinyl)hexyloxy]-3-methylflavone HCI (NPC 16377), a structurally novel compound, was found to be a highly potent and selective ligand for sigma-sites. Although 5-fold less potent than haloperidol and 2-fold less potent than ifenprodil to inhibit 1,3-di-o-tolylguanidine binding, NPC 16377 (IC50 = 36 nM) was more potent than alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazinyl butanol (BMY 14802), rimcazole and the atypical antipsychotic, clozapine. A similar rank order of potency was observed when [3H\](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperdine was used as the radioligand. Like BMY, rimcazole and clozapine, NPC 16377 (IC50 = 2671 nM) had low affinity for dopamine type 2 receptors. Additionally, the compound was only weakly active in 35 additional receptor binding assays including those for serotonin2 and serotonin1C receptors. In vivo, NPC 16377 potently inhibited the binding of \[3H]-(+)-N-allylnormetazocine to sigma sites after both intraperitoneal and oral administration. At doses 30-fold in excess of the ID50 to inhibit [3H\](+)N-allylnormetazocine, NPC 16377 failed to displace [3H]raclopride from dopamine type 2 binding sites. Unlike haloperidol, BMY 14802, ifenprodil and clozapine, behaviorally effective doses of NPC 16377 did not increase dopamine turnover in the frontal cortex, nucleus accumbens or corpus striatum of rats. In contrast, each of these agents increased circulating levels of both adrenocorticotropin and corticosterone, but only NPC 16377 decreased circulating plasma levels of prolactin. The results of the current study are consistent with the notion that NPC 16377 is a potent, selective and orally active sigma site ligand. At behaviorally relevant doses the compound produces neuroendocrine effects both similar to, and different from, neuroleptics, other sigma-ligands and atypical antipsychotics, while having no effect on dopamine turnover. Given these data, NPC 16377 should prove to be a useful compound to explore further the physiological and functional significance of sigma-sites in brain.