Synthesis and biological evaluation of piperazinyl heterocyclic antagonists of the gonadotropin releasing hormone (GnRH) receptor

A series of bicycloheterocyclic replacements for the previously described benzimidazole template were prepared and tested for GnRH activity. Antagonism of the gonadotropin releasing hormone (GnRH) receptor has resulted in positive clinical results in reproductive tissue disorders such as endometriosis and prostate cancer. Following the recent discovery of orally active GnRH antagonists based on a 4-piperazinylbenzimidazole template, we sought to investigate the properties of heterocyclic isosteres of the benzimidazole template. We report here the synthesis and biological activity of eight novel scaffolds, including imidazopyridines, benzothiazoles and benzoxazoles. The 2-(4- tert-butylphenyl)-8-(piperazin-1-yl)imidazo[1,2- a]pyridine ring system was shown to have nanomolar binding potency at the human and rat GnRH receptors as well as functional antagonism in vitro. Additional structure–activity relationships within this series are reported along with a pharmacokinetic comparison to the benzimidazole-based lead molecule.