A new class of calcium entry blockers defined by 1,3-diphosphonates. Interactions of SR-7037 (belfosdil) with receptors for calcium channel ligands.

Tetrabutyl-2(2-phenoxyethyl)-1,3-propylidene diphosphonate (SR-7037) completely displaced dihydropyridine [( 3H]PN200-110), phenylalkylamine [( 3H]D888), and benzothiazepine [( 3H]diltiazem) ligands from brain L-type calcium channels. Half-maximal inhibition of [3H]PN200-110 binding occurred at 19 nM with a Hill coefficient of 0.96. SR-7037 primarily decreased the affinity for [3H]PN200-110 with a small, but significantly, effect on the maximal binding capacity. Kinetic studies showed that this was due to an increased radioligand dissociation rate from 0.04 min-1 to 0.43 min-1 in the presence of the diphosphonate. Displacement of [3H]D888 by SR-7037 was biphasic with respective IC50 of 44 and 8400 nM. Likewise, unlabeled (-)-D888 identified two sites with IC50 values of 0.9 and 27 nM. Both SR-7037 (1000 nM) and D888 (200 nM) accelerated radioligand dissociation about 2-fold. [3H]Diltiazem binding was inhibited by SR-7037 with an IC50 value of 29 nM. The inhibition of dihydropyridine binding by SR-7037 is enhanced by most divalent cations at millimolar concentrations with the following potency: Mn2+ greater than Mg2+ greater than Ca2+ greater than Co2+. Barium has the opposite effect. The half-maximal effect of calcium occurred at 6 microM free ion. Specific binding of [3H]D888 was antagonized in the presence of 1 mM CaCl2. It is concluded that SR-7037 has allosteric interactions with the dihydropyridine receptor of the L-type calcium channel. The differential effect of Ca2+ on the potency of D888 and diltiazem relative to that of SR-7037 indicates that the three drugs may bind to nonequivalent sites. These results support specific calcium channel inhibition, possibly at a novel site, as the primary mechanism of the diphosphonate's pharmacological actions.