The binding affinities of ONO-9054 and ONO-AG-367 were evaluated in radiolabeled ligand-binding assays using membranes prepared from Chem-1 cells stably expressing human FP receptors and Chinese hamster ovary (CHO) cells stably expressing human EP1, human EP2, human EP3I, human EP4, mouse IP, mouse TP, and mouse DP receptors. Membranes expressing these receptors were incubated with a fixed concentration of radiolabeled ligands ([3H]-PGE2 for the EP1, EP2, EP3, and EP4 receptor assays; [3H]-PGF2α for the FP receptor assay; [3H]-Iloprost for the IP receptor assay; [3H]-SQ29548 for the TP receptor assay; and [3H]-prostaglandin D2 for the DP receptor assay) and the test compound in the following buffers: 10 mM KH2PO4-KOH buffer, pH 6.0, containing 1 mM EDTA, 10 mM MgCl2, and 100 mM NaCl for the EP1, EP2, EP3, EP4, and FP receptor assays; 25 mM HEPES-NaOH buffer, pH 7.4, containing 1 mM EDTA, 5 mM MgCl2, and 10 mM MnCl2 for the DP receptor assay; 10 mM Tris-HCl buffer, pH 7.4, containing 100 mM NaCl for the TP receptor assay; and 50 mM Tris-HCl buffer, pH 7.4, containing 1 mM EDTA and 10 mM MgCl2 for the IP receptor assay. After reaching equilibrium, the assays were terminated by rapid filtration through Whatman GF/B filters (Brandel Co., Gaithersburg, MD, USA). The filters were washed with the following ice-cold buffers: 10 mM KH2PO4-KOH buffer, pH 6.0, containing 100 mM NaCl for the EP1, EP2, EP3, EP4, and FP receptor assays; 10 mM Tris-HCl buffer, pH 7.4, containing 100 mM NaCl and 0.01 wt/vol% BSA for the DP receptor assay; and 10 mM Tris-HCl buffer, pH 7.4, containing 100 mM NaCl for the TP and IP receptor assays. The filters were then dried for 60 minutes at 60°C, and radioactivity on the filter was measured using a liquid scintillation counter (TRI-CARB2900TR; Perkin Elmer Japan, Kanagawa, Japan).
Nonspecific binding was defined by adding excess amounts of unlabeled ligands (unlabeled PGE2 for the EP1, EP2, EP3, and EP4 receptor assays; unlabeled PGD2 for the DP receptor assay; unlabeled SQ29548 for the TP receptor assay; unlabeled PGF2α for the FP receptor assay; or unlabeled Iloprost for the IP receptor assay). Specific binding was calculated by subtracting nonspecific binding from total binding. The dissociation constant (Kd) and maximum specific binding (Bmax) were determined by nonlinear regression analysis. The Bmax was defined as 100%, and 50% inhibitory concentration (IC50) values were calculated from inhibition rates (%) of the test compounds at each concentration. The IC50 values were converted to an absolute inhibition constant Ki by using the Cheng-Prusoff equation.