Alpha1-Adrenoceptor Antagonists. 4. Pharmacophore-based Design, Synthesis, and Biological Evaluation of New Imidazo-, Benzimidazo-, and Indoloarylpiperazine Derivatives

As a part of a program aimed at discovering compounds endowed with α1-adrenoceptor (AR) blocking properties, in this paper we describe the synthesis and biological characterization of the compounds designed to fully match a three-dimensional pharmacophore model for α1-AR antagonists previously developed by our research group. Accordingly, the structure of trazodone (1), identified during a database search performed by using the model as a 3D query, was chosen as the starting point for this study and modified following suggestions derived from a literature survey. In particular, the triazolopyridine moiety of trazodone was replaced with different heteroaromatic rings (such as imidazole, benzimidazole, and indole), and a pyridazin-3(2H)-one moiety was inserted into the scaffold of the new compounds to increase the overall length of the molecules and to allow for a complete fit into all the pharmacophore features. Our aim was also to study the influence of the position of both the chloro and the methoxy groups on the piperazine phenyl ring, as well as the effect of the lengthening or shortening of the polymethylene spacer linking the phenylpiperazine moiety to the terminal heterocyclic portion. Compounds obtained by such structural optimization share a 6-(imidazol-1-yl)-, 6-(benzimidazol-1-yl)-, or 6-(indol-1-yl)pyridazin-3(2H)-one as a common structural feature that represents an element of novelty in the SAR of arylpiperazine compounds acting toward α1-AR. Biological evaluation by radioligand receptor binding assays toward α1-AR, α2-AR, and 5-HT1A serotoninergic receptors indicated compounds characterized by very good α1-AR affinity and selectivity. Very interestingly, chemical features (such as the o-methoxyphenylpiperazinyl moiety and an alkyl spacer of three or four methylene units) that generally do not allow for 5-HT1A/α1 selectivity led to compounds 2c and 6c with a 5-HT1A/α1 ratio of 286 and 281, respectively. Finally, compounds with the best α1-AR affinity profile (2c, 5f, and 6c) were demonstrated to be α1-AR antagonists.