Here we report the design, synthesis, and 5-HT(7) receptor affinity of a set of 1-(3-biphenyl)- and 1-(2-biphenyl)piperazines. The effect on 5-HT(7) affinity of various substituents on the second (distal) phenyl ring was analyzed. Several compounds showed 5-HT(7) affinities in the nanomolar range and >100-fold selectivity over 5-HT(1A) and adrenergic α(1) receptors. 1-[2-(4-Methoxyphenyl)phenyl]piperazine (9a) showed 5-HT(7) agonist properties in a guinea pig ileum assay but blocked 5-HT-mediated cAMP accumulation in 5-HT(7)-expressing HeLa cells.

Serotonin 7 (5-hydroxytryptamine7 or 5-HT7) is the most recently identified serotonin receptor. It is involved in mood disorders and is studied as a target for antidepressants. Here, we report on the structural manipulation of the 5-HT7 receptor ligand 4-[2-(3-methoxyphenyl)ethyl]-1-(2-methoxyphenyl)piperazine (1a) aimed at obtaining 5-HT7 receptor ligands endowed with good in vitro metabolic stability. A set of N-[3-methoxyphenyl)ethyl-substituted] 1-arylpiperazine, 4-arylpiperidine and 1-aryl-4-aminopiperidine was synthesized and tested in radioligand binding assays at human cloned 5-HT7 and 5-HT1A receptors. In vitro metabolic stability of the target compounds was assessed after incubation with rat hepatic S9 microsomal fraction. Among the new compounds, 1-(2-biphenyl)-4-[2-(3-methoxyphenyl)ethyl]piperazine (1d) and 4-(2-biphenyl)-1-[2-(3-methoxyphenyl)ethyl]piperidine (2d) showed a good compromise between affinity at 5-HT7 receptor (K i = 7.5 nM and 13 nM, respectively) and in vitro metabolic stability (26 and 65 % recovery of parent compound, respectively) but were poorly selective over 5-HT1A receptor.