Candesartan, an Insurmountable Antagonist of Angiotensin II-mediated Contractile Effects in Isolated Vascular Preparations: Comparison with Irbesartan, Losartan and its Active Metabolite (EXP-3174)

Candesartan is a new angiotensin II type 1 (AT 1 ) receptor blocker that produces more effective 24-h blood pressure lowering than losartan in hypertensive patients. In in vitro tissue preparations, candesartan displays insurmountable antagonism of the responses to angiotensin II. Both irbesartan and EXP-3174, the active metabolite of losartan, have also previously been described in some studies as insurmountable AT 1 -receptor blockers, whereas losartan exhibits surmountable blockade of the AT 1 -receptor. We compared the properties of candesartan, irbesartan, losartan and EXP-3174 in isolated vascular preparations of rat portal vein and rabbit aortic strips. The concentrations of the different AT 1 -receptor antagonists that were effective in these in vitro preparations were also correlated to the non-protein bound plasma concentrations obtained in clinical use. Preparations of the rabbit aorta and the rat portal vein were dissected, mounted on a force-displacement transducer and submerged in oxygenated Krebs' buffer at 37°C. The vessel strips were pre-stretched to a passive force of 5 and 20 mN for portal vein and aorta, respectively. The response to angiotensin II, measured as the mean force development in response to increasing concentrations of angiotensin II, was recorded in the absence and presence of candesartan, 0.003-10 nmol/l, irbesartan, 1-100 nmol/l, losartan, 1-100 nmol/l, and EXP-3174, 0.01-10 nmol/l, for a period of 90 min. In rabbit aortic strips, candesartan caused a non-parallel shift and suppression of the angiotensin II concentration-response curve, with complete suppression of the response to angiotensin II at a dose of candesartan of 1 nmol/l. In contrast, irbesartan, losartan and EXP-3174 all caused a parallel shift of the concentration-response curve. No suppression of the angiotensin II response was seen with losartan, while its active metabolite caused saturable suppression of the maximal response at higher concentrations. For irbesartan, some degree of suppression of the maximal response could not be excluded at the highest concentration studied. Similar concentration-response curves were obtained in rat portal vein. Data on protein binding for the different AT 1 -receptor blockers are variable in the literature. Plasma protein binding for the different AT 1 -receptor blockers was determined (in triplicate) by liquid chromatography with fluorescence detection after equilibrium dialysis (6 h) of cold drug at a concentration of 1500 nmol/l. Protein binding was high (see Table) and, for candesartan, losartan and its active metabolite EXP-3174, in accordance with previously reported levels. For irbesartan, a large discrepancy in protein binding between previously reported and the present experimental data, obtained from two different non-associated laboratories, was found. The higher plasma protein binding for irbesartan found in the present study may explain why high doses of irbesartan seem to be needed for clinical efficacy. It appears unlikely that losartan exerts any significant inhibitory effect at therapeutic plasma levels, and the main AT 1 -blocking effect observed after oral losartan is probably exerted by EXP-3174. It is concluded that AT 1 -receptor blockers differ in their ability to inhibit angiotensin II-mediated vascular contraction, and that the antagonistic characteristics are similar in vessel preparations of different origins and with different degrees of AT 1 -receptor reserve.