CRTH2-specific binding characteristics of [3H]ramatroban and its effects on PGD2-, 15-deoxy-Δ12, 14-PGJ2- and indomethacin-induced agonist responses

We previously showed that ramatroban (Baynas™), a thromboxane A2 (TxA2) antagonist, had inhibited prostaglandin D2 (PGD2)-stimulated human eosinophil migration mediated through activation of chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2). However, detailed pharmacological characterization of its inhibitory activity has not been described. In the present study, we showed that [3H]ramatroban bound to a single receptor site on CRTH2 transfectants with a similar Kd value (7.2 nM) to a TxA2 receptor (8.7 nM). We also demonstrated that ramatroban inhibited PGD2-, 15-deoxy-Δ12, 14-PGJ2 (15d-PGJ2)- and indomethacin-induced calcium responses on CRTH2 transfectants in a competitive manner with similar pA2 values (8.5, 8.5, and 8.6, respectively). This is the first report showing the evidence for direct binding of ramatroban to CRTH2, revealing its competitive inhibitory effects and another interesting finding that PGD2, indomethacin and 15d-PGJ2 share the same binding site with ramatroban on CRTH2.