MK-3901

MK-3901 is a **preclinical small-molecule P2X3 receptor antagonist** that has been reported as an early Merck-discovered program for pain research. Publicly available medicinal chemistry and review sources describe it as a potent, selective, and orally active antagonist of the purinergic **P2X3 receptor**, with activity in a rat Complete Freund's Adjuvant inflammatory pain model. Published secondary sources also note liabilities including **UGT1A1 inhibition**, **CYP2C9 inhibition**, **PXR activation**, and associated **hyperbilirubinemia risk**, which appear to have limited further advancement and motivated follow-on second-generation P2X3 antagonist discovery efforts. No approved brand name, INN, or confirmed clinical-stage development record was identified for this exact compound.