Differential Bonding Interactions of Inverse Agonists of Angiotensin II Type 1 Receptor in Stabilizing the Inactive State

Although the sartan family of angiotensin II type 1 (AT1) receptor blockers (ARBs), which includes valsartan, olmesartan, and losartan, have a common pharmacophore structure, their effectiveness in therapy differs. Although their efficacy may be related to their binding strength, this notion has changed with a better understanding of the molecular mechanism. Therefore, we hypothesized that each ARB differs with regard to its molecular interactions with AT1 receptor in inducing inverse agonism. Interactions between valsartan and residues Ser105, Ser109, and Lys199 were important for binding. Valsartan is a strong inverse agonist of constitutive inositol phosphate production by the wild-type and N111G mutant receptors. Substituted cysteine accessibility mapping studies indicated that valsartan, but not losartan, which has only weak inverse agonism, may stabilize the N111G receptor in an inactive state upon binding. In addition, the inverse agonism by valsatan was mostly abolished with S105A/S109A/K199Q substitutions in the N111G background. Molecular modeling suggested that Ser109 and Lys199 bind to phenyl and tetrazole groups of valsartan, respectively. Ser105 is a candidate for binding to the carboxyl group of valsartan. Thus, the most critical interaction for inducing inverse agonism involves transmembrane (TM) V (Lys199) of AT1 receptor although its inverse agonist potency is comparable to olmesartan, which bonds with TM III (Tyr113) and TM VI (His256). These results provide new insights into improving ARBs and development of new G protein-coupled receptor antagonists.