PSB-16133 is a Potent and Selective Antagonist for the UTP-Activated P2Y4 Receptor. PSB-16133 exhibited an IC50 value of 233 nM, selectivity versus other P2Y receptor subtypes, and is thought to act as an allosteric antagonist. A receptor homology model was built and docking studies were performed to analyze ligand-receptor interactions. Compound 64 represents the most potent and selective P2Y4 receptor antagonist known to date and is therefore anticipated to become a useful tool for studying this scarcely investigated receptor.