The elucidation of species-specific receptor pharmacology: A case study using subtype-selective para- and meta-carborane estrogen receptor agonists

Estrogen receptors (ERs) are essential pharmacological targets for treating hormonal disorders and estrogen-dependent malignancies. Selective activation of ERb is hypothesized to provide therapeutic benefit with reduced risk of unwanted estrogenic side-effects associated with ERa activity. However, activating ERb without activating ERa is challenging due to the high sequence and structural homology between the receptor subtypes. We assessed the impact of structural modifications to the parent compound OSU-ERb-12 on receptor subtype binding selectivity using cell-free binding assays. Functional selectivity was evaluated by transactivation in HEK-293 cells overexpressing human or murine ERs. In vivo selectivity was examined through the uterotrophic effects of the analogs after oral administration in estrogen-naïve female mice. Furthermore, we evaluated the in vivo pharmacokinetics of the analogs following single-dose intravenous and oral administration. Regarding selectivity, a single compound exhibited greater functional selectivity than OSU-ERb-12 for human ERb. However, like others in the meta-carborane series, its poor in vivo pharmacokinetics limit its suitability for further development. Surprisingly, and at odds with their pharmacokinetic and in vitro human activity data, most analogs potently induced uterotrophic effects in estrogen-naïve female mice. Further investigation of activity in HEK-293 cells expressing murine ERs revealed species-specific differences in the ER subtype selectivity of these analogs. Our findings highlight species-specific receptor pharmacology and the challenges it poses to characterizing developmental therapeutics in preclinical species. Significance Statement: This study investigates para- and meta-substituted carborane analogs targeting estrogen receptors (ERs), revealing the greater selectivity of carborane analogs for human ERb compared to the mouse ortholog. These findings shed light on the intricacies of using preclinical species in drug development to predict human pharmacology. The report also provides insights for the refinement and optimization of carborane analogs as potential therapeutic agents for estrogen-related disease states.