Pharmacological Differences among Angiotensin II Receptor Antagonists

Angiotensin II AT1 receptor antagonists (AIIRAs) have demonstrated efficacy similar to other classes of antihypertensive agents as well as "placebo-level" tolerability at all doses. Pharmacokinetic and pharmacodynamic studies provide a framework for understanding important intra-class dissimilarities. Disparity in antagonistic effects may be determined by in vivo responses to challenges of exogenous angiotensin II (Ang II) and by ex vivo/in vitro responses to a drug's biological activity by radioligand receptor assay (RRA). Two independent studies have been conducted in which irbesartan exhibited a more pronounced and longer-lasting antagonism to the effects of exogenous Ang II than losartan and valsartan. Comparative trials have indicated that both irbesartan and candesartan show greater clinical efficacy in lowering blood pressure than losartan. Recently, we have compared the Ang II antagonistic properties of irbesartan 150 mg/day and candesartan 8 mg/day. Both drugs block AT1 receptors with "insurmountable" antagonism and demonstrate a long duration of action. While both irbesartan and candesartan showed a similar degree of antagonistic activity in vivo, distinctly higher antagonistic activity in plasma was found for irbesartan by RRA at all time-points. Furthermore, plasma renin activity during periods with high antagonistic activity was significantly higher, and aldosterone levels following Ang II stimulation were blunted to a greater extent, following administration of irbesartan. In summary, in the doses tested, irbesartan exhibits the strongest antagonism when compared with losartan, valsartan and candesartan. This finding may have clinical implications.