Blood pressure is regulated by a multitude of interrelated factors involving neural, vascular and volume-related effects. The renin-angiotensin system (RAS) is one of the important blood pressure regulating systems.
The RAS functions as shown in the scheme below. Low renal perfusion pressure stimulates the juxtaglomerular cells of the kidney to produce the proteolytic enzyme renin. This enzyme acts on a circulating protein, angiotensinogen, cleaving off a decapeptide angiotensin I. Angiotensin I is then cleaved to the octapeptide angiotensin II by angiotensin converting enzyme (ACE). Angiotensin II is the most powerful pressor substance in the RAS. Angiotenin II binds to vascular smooth muscle receptors and induces vasoconstriction, but has little or no stimulating action on the heart. ##STR2##
Inhibitors of renin (for example enalkiren) and inhibitors of ACE (for example, captopril and enalapril) have clinical efficacy in treating hypertension and congestive heart failure. ACE inhibitors, however, have reported side effects including cough and skin rash.
Peptidyl and non-peptidyl angiotensin II receptor antagonists are known. The peptidyl compound saralasin or [Sar.sup.1,Ala.sup.8 ] angiotensin II has been found to be a potent antagonist of the actions of angiotensin II. Saralasin, however, has several disadvantages. Because it is a peptide, saralasin has very poor oral bioavailability. The use of saralasin, therefore, is limited to administration to hospitalized patients by continuous intravenous infusion. Saralasin is also known to cause an initial increase in blood pressure after intravenous administration due to its activity as an angiotensin receptor agonist. Therefore, non-peptidyl angiotensin II receptor antagonists are preferred.