NONPEPTIDE ENDOTHELIN RECEPTOR ANTAGONISTS .1. EFFECTS ON BINDING AND SIGNAL-TRANSDUCTION ON HUMAN ENDOTHELIN(A) AND ENDOTHELIN(B) RECEPTORS

The effect of a series of endothelin (ET) receptor antagonists, including the novel nonpeptide receptor antagonist, SB 209670, on [I-125]ET-1 binding to human ET receptors (ET(A) and ET(B)) cloned and stably expressed in Chinese hamster ovary cells was studied. SB 209670 was found to compete for [I-125]ET-1 binding with apparent K-i values of 0.43 +/- 0.09 and 14.7 +/- 3.0 nM for ET(A) and ET(B) receptors, respectively. This inhibition was competitive because addition of SB 209670 in saturation binding experiments resulted in decreased affinity, with no change in maximum binding. In addition, SB 209670 inhibited ET-1-mediated accumulation of inositol phosphates and intracellular calcium release in a concentration-dependent manner. The racemic mixtures, (+/-)-SB 209670 and (-)-SB 209670, were similar to 1.5 and at least 200-fold less potent than SB 209670. The binding affinity of (+/-)-SB 209670 therefore resides in (+)-antipode. The peptide antagonists, BQ123 (ET(A)-selective) and RES 701 (ET(B)-selective), were 40- and 6-fold less potent than SB 209670 in inhibition of [I-125] ET-1 binding to ET(A) and ET(B) receptors, respectively. The nonselective peptide antagonist, PD 142893, was 75- and 10-fold less potent than SB 209670, whereas the nonpeptide antagonist, bosentan, was similar to 80- and similar to 30-fold less potent than SB 209670 in inhibition of [I-125]ET-1 binding to ET(A) and ET(B) receptors, respectively. Thus, the present studies indicate clearly that SB 209670 is the most potent nonpeptide ET receptor antagonist yet described.