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

Abstract 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 p K 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.