NEFROLOGÍA. Vol. XX. Suplemento 1. 2000 Role of Angiotensin Receptor Antagonists in the treatment of Hypertension, Heart Failure, and Renal Disease C. M. Ferrario, MD, FACC Professor and Director. The Hypertension and Vascular Disease Center. Wake Forest University School of Medicine. Winston-Salem, North Carolina 27157. Clinical pharmacokinetics and metabolism studies show that losartan in an active, potent, and selective angiotensin II (Ang II) receptor blocker. It has a rapid and smooth onset on blood pressure, and is solely responsible for the beneficial antihypertensive effects on target organ damage. Losartan is converted to the active metabolite E3174 by the liver cytochrome P450 enzyme system. E-3174 exhibits significant biochemical activity and contributes to the duration of action as well as having a greater affinity for the AT1 receptor. Losartan, the active parent compound has a mean half-life of approximately 4-6 hrs. The active metabolite, E-3174, has a half-life exceeding 9 hrs. This dual antihypertensive action of both the parent compound and its metabolite may account for the prolonged and smooth blood pressure control observed in clinical trials and in the clinical experience gained in its worldwide application. Angiotensin II receptor blockers (ARBs) comprise a new class of antihypertensive agents whose action is mediated by prevention of binding of Ang II to the AT1 receptor. As an agent with indications for first line therapy of hypertension, ARBs controls blood pressure and retards Ang II related target organ damage. Within the family of therapeutic agents that antagonize the pathological actions of Ang II, ARBs differ from beta-blockers and angiotensin converting enzyme (ACE) inhibitors as illustrated in table 1. Clinically, the effects of ARBs are comparable to those obtained with ACE inhibitors, although additional long-term clinical studies are required to validate experimental conclusions. Nevertheless, the selective and specific action of ARBs in blocking the AT1 receptor affords a greater degree of specificity in preventing the pathological actions of Ang II. There are also opportunities to combine the beneficial aspects of ACE inhibitors and ARBs by the synergistic effect that is achieved from combining inhibition of Ang II formation and receptor activity. 22 Table I. Similarities and Differences Component Renin Activity Angiotensin I Converting Enzyme Angiotensin II -Blockers (-) (-/+) (-/+) (-) Angiotensin-(1-7) Kinins not determined (-/+) ACE Inhibitors (+++) (+++) (-) Acutely (-) Chronically (-/+) (+++) (+++) ARBs (++) (+) (-/+) (+) (++) (-) Keys: (-/+), no change; (+), increase; (-), inhibits. The assumption that ARBs may represent a single class of agents may require revision because there are differences in the action and effects between various receptor blockers. This is not unexpected since variations in the chemical structure of ARBs can have significant effects on their ability to bind to target organ receptors and access AT1 across the blood brain barrier or intracellularly. Tw o examples are illustrated. Losartan, the act i v e parent peptide has been observed to prod u c e a significant and sustained reduction in s e r u m uric acid, a factor associated with increa s e risk for cardiovascular disease and a troub l e s o m e co-morbid event in congestive heart f a i l u r e . Todate, other ARBs appear not to reduc e serum uric acid or augment urinary excret i o n of this factor. In animals, both losartan and E - 3 1 7 4 inhibit monocyte adhesion and act as c o m p e t i t i v e blockers of the thromboxane A 2 r e c e p t o r. Valsartan and candersartan, two other A n g II receptor blockers are weak antagonists at t h e thromboxane A 2 r e c e p t o r. T h u s , losartan and i t s active metabolite show additional beneficial a c t i o n s not shared by two other Ang II antago- ROLE OF ANGIOTENSIN RECEPTOR ANTAGONISTS IN THE TREATMENT OF HYPERTENSION n i s t s : reversal of hyper-uricacidemia and antit h r o m b o t i c effects. In keeping with these observ a t i o n s , losartan has been shown to retard the d e v e l o p m e n t of atherosclerosis in a non-human p r i m a t e model by inhibiting monocyte recruitm e n t and retarding the oxidation of LDL. T h e s e d i f f e r e n t i a l effects among ARBs posits a quest i o n as to whether small changes in their chem i c a l and pharmacokinetic profile may be ass o c i a t e d with important differences in their a b i l i t y to block the pathological actions of Ang I I , independent of their effects on blood pressure. REFERENCES 1. Li P, Ferrario CM, Brosnihan KB: Nonpeptide angiotensin II antagonist losartan inhibits thromboxane A2-induced contractions in canine coronary arteries J Pharmacol Exp Ther 281: 1065-1070, 1997. 2. Ferrario CM, Flack JM: Pathologic consequences of increased angiotensin II activity. Cardiovasc. Drugs Ther 10: 511-518, 1996. 3. Tallant EA, Ferrario CM: Drug evaluation: Cardiovascular & renal: Biology of angiotensin II receptor inhibition with a focus on losartan: a new drug for the treatment of hypertension. Exp Opin Invest Drugs 5: 1-14, 1996. 4. Azizi M, Guyene TT, Chatellier G, Wargon M, Menard J: Additive effects of losartan and enalapril on blood pressure and plasma active renin. Hypertension 29: 634-640, 1997. 23