In-Vitro Pharmacology Of Mk-996, A New Potent And Selective Angiotensin-Ii (At(1)) Receptor Antagonist

MK-996 (N-((4'-((5,7-Dimethyl-2-ethyl-3H-imidazo[4,5-b]pyridin-3-yl)methyl)(1,1'-biphenyl)-2-yl) sulfonylbenzamide) interacted in a competitive manner with rabbit aortic angiotensin II (All) receptors as determined by Scatchard analysis of specific binding of [I-125]-Sar(1)Ile(8)-All. MK-996 also exhibited high affinity at All receptors in several tissues from different animal species (K-i = 0.1-0.4 nM). In vitro functional assays utilizing All-induced aldosterone release in rat adrenal cortical cells demonstrated further that MK-996 acts as a competitive, high affinity antagonist of All (pA(2) = 10.3) and lacks agonist activity. MK-996 also potently inhibited All-induced contractile response in isolated rabbit aorta and pulmonary artery with a reduction in maximal response. The specificity of MK-996 for All receptors was demonstrated by its lack of activity (IC50 > 1 mu M) in several other receptor binding assays and its inability to affect in vitro functional responses produced by other agonists. MK-996 demonstrated a very high selectivity for the AT(1) compared to AT(2) receptor subtype (AT(2) IC50 greater than or equal to 2 mu M). Direct binding studies using [H-3]-MK-996 in rat adrenal indicated specific binding of [H-3]-MK-996 is saturable and of high affinity (K-d = 0.47 nM). The specific [3H]-MK-996 binding in rat adrenal represents binding to pharmacologically relevant AT(1) receptors as demonstrated by the similar K-i values for various All agonists and antagonists in inhibiting specific H-3-MK-996 and [I-125]-All binding to AT(1) receptors. Dissociation rate studies of specific [H-3]-MK-996 binding indicated at t1/2 of 103 min. This slow dissociation may account for the reduction in maximal responses to All in MK-996 treated isolated blood vessels. (C) 1994 Wiley-Liss, Inc.