BMS-180560, an insurmountable inhibitor of angiotensin II-stimulated responses: comparison with losartan and EXP3174.

1. This study compares the activity of BMS-180560 (2-butyl-1-chloro-1-[[1-[2-(2H-tetrazol-5-yl)phenyl]-1H-indol-4- yl]methyl]-1H-imidazole-5-carboxylic acid), an insurmountable angiotensin II (AII) receptor antagonist, with that of losartan and EXP3174 in functional and biochemical models of AII-receptor activation. 2. BMS-180560 selectively inhibited [125I]-Sar1Ile8AII ([125I]SI-AII) binding to rat aortic smooth muscle (RASM) cell and rat adrenal cortical AT1 receptors (Ki = 7.6 +/- 1.2 and 18.4 +/- 3.9 nM respectively) compared to adrenal cortical AT2 receptors (Ki = 37.6 +/- 1.3 microM). The Ki values of BMS-180560 and EXP3174, but not losartan, varied as a function of the BSA concentration used in the assays, indicating that the diacid drugs bound to albumin. 3. BMS-180560 (3-300 nM) increased the KD of SI-AII for RASM cell AT1 receptors. Only at high concentrations of BMS-180560 (300 nM) were Bmax values decreased. 4. BMS-180560 inhibited AII-stimulated contraction of rabbit aorta with a calculated KB = 0.068 +/- 0.048 nM and decreased maximal AII-stimulated contraction at 1 nM BMS-180560 by 75%. In the presence of 0.1% BSA, a higher KB value (5.2 +/- 0.92 nM) was obtained. Losartan behaved as a competitive antagonist with a KB = 2.6 +/- 0.13 nM. Contraction stimulated by endothelin-1, noradrenaline, KCl, or the TXA2 receptor agonist U-46619 were unaffected by BMS-180560 (1 nM). 5. AII stimulated the acidification rates of RASM cells as measured by a Cytosensor microphysiometer with an EC50 of 18 nM. Losartan (30 nM) shifted the AII concentration-effect curves in a competitive manner whereas BMS-180560 (0.01 and 0.1 nM) decreased the maximum responses by 60 and 75% respectively. Inhibition by losartan and BMS-180560 could be reversed following washout although recovery took longer for BMS-180560. 6. In [3H]-myoinositol-labelled RASM cells, losartan (30 and 200 nM), shifted the EC50 for AII-stimulated [3H]-inositol monophosphaste formation to higher values, with no change in the maximal response. By contrast, EXP3174 (0.1 to 1 nM) decreased the maximal response in a concentration-dependent manner (17-55%). BMS-180560 (3 and 10 nM) increased the EC50 for AII and decreased the maximum response by 30 and 80% respectively. The inhibition by EXP3174 and BMS-180560 could be reversed by inclusion of losartan (200 nM) indicating that the inhibition was not irreversible. 7. In conclusion, BMS-180560 is a potent, specific, predominantly competitive, reversible All receptor antagonist, which displays insurmountable receptor antagonism. At concentrations of BMS-180560 which have no effect on receptor number, BMS-180560 produced insurmountable antagonism of AII-stimulated second messenger formation, extracellular acidification, and smooth muscle contraction.