In vitro pharmacology of an angiotensin AT1 receptor antagonist with balanced affinity for AT2 receptors

Abstract L-163,017 (6-[benzoylamino]-7-methyl-2-propyl-3-[[2′-( N -(3-methyl-1-butoxy) carbonylaminosulfonyl)[1,1′]-biphenyl-4-yl]methyl]-3 H -imidazo[4,5- b ]pyridine) inhibited specific 125 I-[Sar 1 ,Ile 8 ]angiotensin II binding to angiotensin AT 1 receptor ( K i = 0.11−0.20 nM) in rabbit aorta, rat adrenal and human angiotensin AT 1 receptor in CHO (Chinese hamster ovary transformed) cells and to AT 2 receptor ( K i = 0.14−0.23 nM) in rat adrenal and brain receptors. L-163,017 also had a high affinity in the presence of bovine serum albumin (2 mg/ml), for angiotensin AT 1 and AT 2 receptors on human adrenal ( K i 3.9 and 4.3 nM), aorta ( K i 0.45 and 0.96 nM) and kidney ( K i 3.6 and 2.3 nM). The much higher K i values in human tissues were likely due to the presence of bovine serum albumin in the binding assay buffer since L-163,017 had K i values of 0.13 ± 0.04 and 2.0 ± 0.04 nM in the absence and presence of bovine serum albumin, respectively, in inhibiting 125 I-[Sar 1 ,Ile 8 ]angiotensin II binding to angiotensin AT 1 receptor in rat adrenal membranes. Scatchard analysis of 125 I-[Sar 1 ,Ile 8 ]angiotensin II binding in the presence of bovine serum albumin (2 mg/ml) in rabbit aorta and bovine cerebellum indicated a competitive interaction of L-163,017 with angiotensin AT 1 and AT 2 receptors ( K i values 2.5 and 2.1 nM respectively). L-163,017 inhibited angiotensin II-induced aldosterone release in rat adrenal demonstrating that L-163,017 acted as a competitive antagonist (pA 2 = 9.9) and lacked agonist activity. L-163,017 also inhibited angiotensin II responses in rat vascular tissues. The specificity of L-163,017 was shown by its lack of activity on the above functional responses produced by other agonists and in several binding assays.