From Discovery to Clinical Trial: YCT-529, an Oral NonHormonal Male Contraceptive Targeting the Retinoic Acid...

The retinoic acid receptor alpha (RARα) has emerged as a compelling genetically and pharmacologically validated target for nonhormonal male contraception due to its essential role in spermatogenesis. In the present study, a search for specific inhibitors of RARα utilized systematic linker bioisosterism, hydrophobic core modification, and iterative structure-activity relationship refinement, identified the acid 9, a pyrrole-linked analog that potently inhibits RARα (IC₅₀ = 1.2 nM) with >300-fold selectivity over RARβ and RARγ. Sprague-Dawley rat studies with the sodium salt of 9, YCT-529, showed good oral bioavailability and dose-proportional pharmacokinetics without drug accumulation after 28 days of dosing. Once-daily oral administration (0.75 mg/kg for 28 days) reversibly suppressed epididymal sperm counts and fertility in rats, with a no-observed-adverse-effect level at 30 mg/kg (highest dose tested), affording a ≥40-fold therapeutic window. These findings validated aromatic linker substitution as a powerful design strategy for identifying RARα antagonists and led to the clinical advancement of YCT-529 as a nonhormonal male contraceptive.