In vitro phamacology of MK‐996, a new potent and selective angiotensin II (AT1) 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 [125l]‐Sar1lle8‐All. MK‐996 also exhibited high affinity at All receptors in several tissues from different animal species (Ki = 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 (pA2 = 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 μM) 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 AT1 compared to AT2 receptor subtype (AT2 IC50 ≥ 2 μM). Direct binding studies using [3H]‐MK‐996 in rat adrenal indicated specific binding of [3H]‐MK‐996 is saturable and of high affinity (Kd = 0.47 nM). The specific [3H]‐MK‐996 binding in rat adrenal represents binding to pharmacologically relevant AT1 receptors as demonstrated by the similar Ki values for various All agonists and antagonists in inhibiting specific 3H‐MK‐996 and [125l]‐All binding to AT1 receptors. Dissociation rate studies of specific [3H]‐MK‐996 binding indicated a 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.