Calcium Channel Receptor Binding Studies for Diltiazem and Its Major Metabolites

Abstract

Pharmacologically distinct but allosterically interacting calcium channel antagonist binding sites have recently been identified using radiolabeled dihydropyridine derivatives (e.g., [3H]nitrendipine) and the benzothiazepine [3H]diltiazem. Whereas the functional significance of the dihydropyridine calcium channel antagonist receptor is well documented, it remains to be established whether drug interactions with the recognition site for [3H]diltiazem within the slow calcium channel or the allosteric interaction of the diltiazem binding site with the dihydropyridine receptor are of physiological significance. In a study of structure-activity relationships, we therefore examined the effects of diltiazem and five of its analogs on the binding of [3H]diltiazem and [3H]nitrendipine to the rat cerebral cortex. In parallel, we studied the effects of these drugs on the spontaneous myogenic contractions of the rat portal vein, a functional test of calcium antagonism. The diltiazem analogs used in this study correspond to its major metabolites in humans, i.e., N-desmethyl-(MA, desacetyl-(M1), N-desmethyl, desacetyl-(M2), O-desmethyl, desacetyl (M4), N-desmethyl, O-desmethyl, desacetyl-diltiazem (M6). Unlabeled diltiazem inhibited [3H]diltiazem binding at 37 degrees C with a pIC50 [-log IC50 (M)] of 6.87. pIC50 values for M1, MA, M2, M4, and M6 were 6.72, 6.49, 6.03, 5.51, and 5.33, respectively. pIC50 values for these drugs on [3H]diltiazem binding were significantly correlated (p less than 0.01) with their pEC50 values for enhancement of [3H]nitrendipine binding to cerebral cortical membranes at 37 degrees C. Maximal enhancement of [3H]nitrendipine binding by diltiazem, M1, MA, M2, M4, and M6 was 73, 50, 9.7, 11, 12, and 52%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)