Plasma protein binding of bepridil

The binding of the calcium-channel blocking agent, bepridil HCl (Vascor), to plasma proteins was investigated using radiolabeled bepridil and equilibrium dialysis. Greater than 99.7% of added bepridil-14C was found to freshly collected human plasma. The binding was characterized by a saturable high-affinity site (KD = 32 ng/mL = 87 nM) on alpha1-acid glycoprotein (AAG) or on an AAG-human serum albumin complex and lower affinity binding sites on albumin and other plasma macromolecules. Bepridil that is not bound to plasma proteins is extensively distributed into erythrocytes as evidenced by a red blood cell to free drug distribution coefficient of 71 +/- 7. Despite this high value, the blood to plasma ratio of bepridil averaged only 0.67 in humans, indicating that most of the circulating drug is bound to plasma proteins. Bepridil protein binding was not affected by additions of nonesterified fatty acids. Free fractions of bepridil were enhanced by addition of verapamil, nifedipine, diltiazem, disopyramide, and warfarin but only at concentrations above those achieved clinically. Bepridil was also displaced by the plasticizer, tris-(2-butoxyethyl)phosphate. Plasma obtained from a small number of angina patients prior to bepridil administration showed no differences in ability to bind bepridil compared with plasma obtained from healthy subjects.