Structural insights into nonpeptide antagonist inhibition of somatostatin receptor subtype 5.

The somatostatin receptor subtype 5 (SSTR5) is a critical pharmacological target involved in neuroendocrine signaling, metabolic regulation, and tumorigenesis. Despite its therapeutic potential, the structural mechanisms underlying SSTR5 inhibition by nonpeptide antagonists remain largely unresolved. In this study, we present high-resolution cryoelectron microscopy structures of human SSTR5 in complex with two selective small-molecule antagonists, antagonist 1 and S5A1, revealing the receptor's inactive conformation. Both antagonists induce a unique remodeling of extracellular loop 2, which adopts a capping architecture that sterically occludes the orthosteric site, thus preventing agonist access and stabilizing receptor inactivation. Structural analyses and functional experiments elucidate how distinct molecular moieties of the antagonists differentially contribute to inhibitory efficacy, and subtype selectivity. Furthermore, we delineate rational avenues for molecular optimization to enhance therapeutic index and mitigate off-target liabilities. These findings, complementing our previous agonist-bound structures, establish a comprehensive structural foundation for developing improved nonpeptidic SSTR5 antagonists with potential therapeutic applications for type 2 diabetes and related endocrine disorders.