Structure-Guided Discovery of Highly Selective, Gut-Restricted RXFP4 Agonists for the Treatment of Constipation

The relaxin family peptide receptor 4 (RXFP4) is a promising therapeutic target for constipation owing to its critical role in regulating gastrointestinal motility. However, the development of selective small-molecule RXFP4 agonists remains challenging because of the high structural homology between RXFP4 and RXFP3 and the difficulty of achieving gut-restricted exposure. Herein, guided by the cryo-electron microscopy structure of RXFP4, we report the structure-guided design and optimization of a novel series of tetrahydroisoquinoline-based RXFP4 agonists. Structure−activity relationship studies identified compound 44 as a potent RXFP4 agonist with excellent selectivity over RXFP3 and a gut-restricted pharmacokinetic profile with an approximately 500-fold ileum-to-plasma exposure ratio. In a loperamide-induced constipation mouse model, oral administration of compound 44 significantly enhanced colorectal propulsion and alleviated constipation-related symptoms. Collectively, this work establishes a structure-guided strategy for optimizing receptor subtype selectivity and gut-restricted exposure, providing a promising lead and a framework for the development of RXFP4-targeted therapeutics.