The loop diuretic torasemide in chronic renal failure. Pharmacokinetics and pharmacodynamics

The pharmacodynamic effect of a diuretic agent is essentially dependent on its renal elimination characteristics. The influence of renal function on the pharmacodynamic and pharmacokinetic characteristics of a diuretic should therefore be considered. The results of a study with torasemide given intravenously in healthy subjects and in patients with stable chronic renal failure of various degrees are reported and discussed. After a single dose of torasemide 20mg, a marked diuresis was observed and electrolyte excretion was increased, whereas the glomerular filtration rate was unchanged. Throughout the duration of action (tau) of torasemide, drug-induced excretion of Cl-, Na+, K+, Ca++ and Mg++ was linearly related to the creatinine clearance (CLcr), and excretion of K+ and Ca++ were closely related to that of Na+ over the entire range of CLcr. Similarly, excretion of Mg++ was related to that of K+. As occurs with furosemide (frusemide), torasemide induced a kaliuresis which amounted to 12% of natriuresis. This kaliuretic effect of loop diuretics is less than that of thiazides. Beyond tau, e.g. over a 24-hour period, kaliuresis was no longer correlated with natriuresis. The extent to which a rebound effect occurred was diminished with increasing renal impairment. tau averaged 6 hours and was independent of CLcr. The mean half-life (t1/2) of torasemide was approximately 5 hours and was independent of renal function, since renal clearance accounted for only around 25% of total body clearance. In contrast to the parent drug, however, the active minor metabolite M1 and the inactive main metabolite, M5, were found to accumulate in patients with chronic renal failure.