DIFFERENTIAL INTERACTION OF GLIMEPIRIDE AND GLIBENCLAMIDE WITH THE BETA-CELL SULFONYLUREA RECEPTOR .1. BINDING CHARACTERISTICS

Glimepiride is a novel sulfonylurea drug for treatment of non-insulin-dependent diabetes mellitus with higher blood sugar lowering efficacy in diabetic patients than glibenclamide raising the question whether this characteristics is in line with different binding of glimepiride and glibenclamide to the beta-cell sulfonylurea receptor. Scatchard plot analysis of [H-3]sulfonylurea binding to membranes isolated from rat beta-cell tumors and (RINm5F) insulinoma cells and to RINm5F cells demonstrated that glimepiride has a 2.5-3-fold lower affinity than glibenclamide. This corresponded well to the 8-9-fold higher k(off) and 2.5-3-fold higher k(on) rates of glimepiride compared to glibenclamide as revealed by the dissociation and association kinetics of [H-3]sulfonylurea binding and the K-d values calculated thereof. In agreement, the concentrations required for half-maximal displacement of [H-3]sulfonylurea bound to beta-cell membranes were significantly higher for glimepiride compared to glibenclamide. However, the binding affinity of glimepiride measured by both equilibrium binding and kinetic binding studies upon solubilization of beta-cell tumor membranes and RINm5F cell membranes increased up to the value for glibenclamide. This was primarily based on a drastic decrease of the dissociation rate constant of glimepiride whereas the kinetics of glibenclamide binding remained largely unaffected upon solubilization. These data suggest that the K-d value alone is not sufficient for characterization of a sulfonylurea drug, since the kinetic binding parameters may also determine its acute blood sugar lowering efficacy.