Pharmacological Activity of Retinoic Acid Receptor Alpha-Selective Antagonists in Vitro and in Vivo

Oral administration of a retinoic add receptor (RAR) pan-antagonist reversibly inhibits spermatogenesis. Given the importance of RAR alpha in regulating spermatogenesis, we identified two RAR alpha-selective antagonists by transactivation and transactivation competition assays and asked whether they effectively inhibit spermatogenesis. Although these two antagonists were potent in vitro, they displayed poor in vivo activity in mice when administered orally. Testicular weights were normal, and morphological analysis revealed normal spermatid alignment and sperm release. In vitro drug property analyses were performed with one of these antagonists and compared with the pan-antagonist. We showed that the discrepancies may be explained by several factors, including high plasma protein binding, faster hepatic metabolism relative to the pan-antagonist, and only moderate permeability. The condusion of poor oral bioavailability was supported by more pronounced defects in mice when the antagonist was administered intravenously versus intraperitoneally. These results are crucial for designing new RAR alpha-selective antagonists for pharmaceutical application.