Multiple binding of bepridil in human blood

Using radiolabelled bepridil, equilibrium dialysis experiments and direct ligand-binding studies were conducted to characterize the binding of bepridil to some human serum proteins, and to blood cells and platelets, respectively. Bepridil was bound to serum albumin with K = 26 +/- 1.6 X 10(3) mol-1 X 1, n = 1.07 +/- 0.04; to alpha 1-acid glycoprotein with K = 442 +/- 68 X 10(3) mol-1 X 1, n = 0.90 +/- 0.04; and with a lesser extent to lipoproteins and gamma-globulins. Bepridil, propranolol, quinidine and erythromycin were found to share the same site on alpha 1-acid glycoprotein. The binding of bepridil to both RBC and WBC was nonspecific with the following parameters: NK = 10.55 +/- 0.10 per 5 X 10(6) RBC/mm3 and NK = 0.34 +/- 0.01 per 10(3) WBC/mm3, whereas the binding to platelets was saturable with N = 10.97 +/- 1.06 mumol/l per 108 +/- 10(3) platelets/mm3 and K = 40 +/- 8 X 10(3) mol-1 X 1. The binding parameters were used to simulate the distribution of bepridil between serum proteins, blood cells and platelets over the therapeutic range and showed that serum albumin, alpha 1-acid glycoprotein and RBC were the major binding components.