Rationale for the chemical development of angiotensin ii receptor antagonists

Abstract

The renin-angiotensin system (RAS) has been demonstrated to be a key element in blood pressure regulation and fluid volume homeostasis. Since angiotensin II (All) is the effector molecule of the RAS, the most direct approach to block this system is to antagonize All at the level of its receptor. Therefore, at Du Pont Merck the working hypothesis has been that the identification of metabolically stable and orally effective All-receptor antagonists would constitute a new and superior class of agents useful in treating hypertension and congestive heart failure. Our program began with a detailed pharmacologic evaluation of some simple N-benzylimida- zoles, originally described by Takeda Chemical Industries in Osaka, Japan. They were found to be a series of weak but selective All-receptor antagonists with a competitive mode of action. We embarked on a program aimed to design and synthesize more potent and orally effective nonpeptide antagonists, while attempting to preserve their selective affinity for the All receptor. The first major breakthrough in our efforts to increase the potency of these compounds came with the development of a series of N-benzylimidazole phthal- amic acid derivatives. Although effective at lowering blood pressure when administered intravenously, the phthalamic acids were devoid of oral activity. The first orally active All antagonists came with the discovery of the biphenyl carboxylic acids. Although these compounds are absorbed after oral dosing, their bioavailability was less than desired. In the hope of improving the oral absorption of these biphenyls, we investigated a variety of acidic groups as bioisosteric replacements for the carboxylic acid. The key to the discovery of nonpeptide All-receptor antagonists with improved oral activity and duration of action resulted from replacing the carboxylic acid group with the isosteric but more lipophilic tetrazole ring. Hence, our efforts culminated in the discovery of losartan (2-fi-butyl- 4-chloro-5-hydroxymethyl-l-[(2'-(lH-tetrazol-5-yl) biphenyl-4-yl)methyl]imidazole, potassium salt), a highly potent angiotensin type 1 (AT selective receptor antagonist with a long duration of action. Losartan is currently undergoing clinical investigation for the treatment of hypertension. The history, including the rationale for the design of the compounds, and ensuing structure-activity relationships of losartan and related analogs will be described. Many of the newer compounds exceed the potency of losartan, and the best compounds in the series rival the affinity of the endogenous ligand, All, for its receptor. Over the past 2 years, most of the major pharmaceutical companies have instituted substantial programs targeted at All-receptor antagonists. The structural features of the compounds currently undergoing clinical evaluation will also be briefly reviewed. Am J Hypertens 1992;5:209S-220S. © 1992 by the American Journal of Hypertension, Inc.