Abstract

Background: Antagonists to the H₃ receptor are considered to be potential drugs for the treatment of Alzheimer’s disease, attention deficit-hyperactive disorder, memory and learning deficits, and epilepsy. The initial development of potent H₃ receptor antagonists focused on extensive modification of the natural ligand histamine. However, it has appeared that imidazole-containing ligands are associated with inhibition of cytochrome P450 enzymes, caused by imidazole nitrogen complexation to heme iron in the active site of the enzyme. For these reasons, the development of potent non-imidazole H₃ receptor antagonists was eagerly awaited.

Objective: Previously, we reported the synthesis and pharmacological in vitro characterization of series of potent histamine H₃-receptor non-imidazole antagonists belonging to the class of substituted 2-thiazol-4-n-propylpiperazines. A lead compound 1 of this family was a derivative carrying the ethylaminomethylpropyl chain.

Methods: With the aim of increasing lipophilicity, that will help the ligands to cross the blood-brain barrier, we synthesized a series of new 2-thiazol-4-n-propylpiperazines where the ethylaminomethylpropyl moiety was replaced by a p-substituted-, an unsubstituted benzene ring, and ω-phenylalkyl substituent at positions 4 and 5 of thiazole ring, respectively. All compounds were tested for H₃ antagonistic effects in vitro using the electrically contracting guinea pig jejunum.

Results: The most active compounds of presented series 3d, 3e, and 3j showed lower affinity than the lead compound 1 and additionally, derivatives 3d and 3j possessed weak, competitive H₁- antagonistic activity. This is in contrast to the lead compound 1 that has no affinity at H₁ receptor.

Conclusion: We can conclude that a side chain in the 2-thiazol-4-n-propylpiperazine scaffold should contain a basic center and should be present at a favorable position 5 of thiazole ring.

Keywords: Histamine H₃-receptor/non-imidazole H₃-antagonists/ 1-(2-thiazol-4-yl)- and 1-(2-thiazol-5-yl)-4-npropylpiperazines.

Graphical Abstract