GW 627368X is a potent and selective competitive antagonist of the EP4 receptor with additional human TP receptor affinity. In competition radioligand bioassays, GW 627368X had affinity for human EP4 and TP receptors with Ki values of 100 nM and 158 nM, respectively. Affinity for all other human prostanoid receptors is >5.0 µM. In human washed platelets, GW 627368X produced 100% inhibition of U-46619 (EC100)-induced aggregation at a concentration of 10 µM.