Nonpeptidic urotensin-II receptor antagonists I:: in vitro pharmacological characterization of SB-706375

1 SB-706375 potently inhibited [I-125] hU- II binding to both mammalian recombinant and ' native' UT receptors (K-i 4.7 +/- 1.5 to 20.7 +/- 3.6 nM at rodent, feline and primate recombinant UT receptors and Ki 5.4 +/- 0.4 nM at the endogenous UT receptor in SJRH30 cells). 2 Prior exposure to SB-706375 (1 mu M, 30 min) did not alter [I-125] hU-II binding affinity or density in recombinant cells (K-D 3.1 +/- 0.4 vs 5.8 +/- 0.9 nM and B-max 3.1 +/- 1.0 vs 2.8 +/- 0.8 pmol mg(-1)) consistent witha reversible mode of action. 3 The novel, nonpeptidic radioligand [H-3] SB-657510, a close analogue of SB-706375, bound to the monkey UT receptor (K-D 2.6 +/- 0.4 nM, B-max 0.86 +/- 0.12 pmol mg(-1)) in a manner that was inhibited by both U- II isopeptides and SB- 706375 (K-i 4.6 +/- 1.4 to 17.6 +/- 5.4 nM) consistent with the sulphonamides and native U-II ligands sharing a common UT receptor binding domain. 4 SB-706375 was a potent, competitive hU- II antagonist across species with pK(b) 7.29 - 8.00 in HEK293-UT receptor cells (inhibition of [Ca2(+)] (i)- mobilization) and pK(b) 7.47 in rat isolated aorta (inhibition of contraction). SB-706375 also reversed tone established in the rat aorta by prior exposure to hU-II (K-app similar to 20 nM). 5 SB-706375 was a selective U- II antagonist with >= 100-fold selectivity for the human UT receptor compared to 86 distinct receptors, ion channels, enzymes, transporters and nuclear hormones (K-i/IC50 > 1 mM). Accordingly, the contractile responses induced in isolated aortae by KCl, phenylephrine, angiotensin II and endothelin-1 were unaltered by SB-706375 (1 mM). 6 In summary, SB-706375 is a high-affinity, surmountable, reversible and selective nonpeptide UT receptor antagonist with cross-species activity that will assist in delineating the pathophysiological actions of U-II in mammals.