Non-Imidazole Histamine H₃ Ligands. Part VII. Synthesis, In Vitro and In Vivo Characterization of 5-Substituted-2-thiazol-4-n-propylpiperazines.

H-3 receptors present on histaminergic and non-histaminergic neurons, act as autoreceptors or heteroreceptors controlling neurotransmitter release and synthesis. Previous, studies have found that the compound N-methyl-N-3-phenylalkyl-2-[ 2-(4-n-propylpiperazin-1-yl)-1,3-thiazol-5-yl] ethan-1 -amine (ADS-531, 2c) exhibits high in vitro potency toward H3 guinea pig jejunal receptors, with pA(2) = 8.27. To optimize the structure of the lead compound ADS-531, a series of 5-substituted-2- thiazol-4-n-propylpiperazines 3 were synthesized and subjected to in vitro pharmacological characterization; the alkyl chain between position 2 of the thiazole ring and the terminal secondary N-methylamino function was elongated from three to four methylene groups and the N-methylamino functionality was substituted by benzyl-, 2-phenylethyl-, and 3-phenyl-propyl-moieties. SAR studies on novel non-imidazole, 5-substituted-2-thiazol-4-n-propyl-piperazines 3 showed that the most active compound 3a (pA(2) = 8.38), additionally possessed a weak competitive H-1-antagonistic activity. Therefore, compound ADS-531, which did not exhibit any H1-antagonistic activity, was chosen for further evaluation for its affinity to the recombinant rat and human histamine H-3 receptors (rH(3)R and hH(3)R, respectively). ADS-531 exhibited nanomolar affinity for both rH(3)R and hH(3)R receptors. It was also shown that, ADS-531 given subchronically to rats (s.c. 3 mg/kg, 5 days) penetrated the brain, where it affected dopamine, noradrenaline and serotonin concentration; however, it did not affect histamine concentration nor feeding behavior.