pK_a determination of angiotensin II receptor antagonists (ARA II) by spectrofluorimetry

The acid-base equilibrium constants of a new family of antihypertensive drugs, the angiotensin II receptor antagonists (ARA II), Losartan, Irbesartan, Valsartan, Candesartan cilexetil, its metabolite Candesartan M1 and Telmisartan were determined by spectrofluorimetry. Relative fluorescent intensity (I_F,rel)-pH data were treated by graphical (derivatives and curve-fitting) and numerical methods (LETAGROP SPEFO). The resultant pK_a values at an ionic strength of 0.5 M were (3.15 ± 0.07) for Losartan, (4.70 ± 0.06) for Irbesartan, (4.90 ± 0.09) for Valsartan, (6.0 ± 0.1) for Candesartan cilexetil, (3.9 ± 0.1) for Candesartan M1, and (4.45 ± 0.09) for Telmisartan.