Study of the elimination kinetics of torasemide, a novel loop diuretic, in renal insufficiency

Role of electrolyte concentrations and renin-angiotensin-aldosterone activation in the staging of canine heart disease

Background

Refractory congestive heart failure (CHF) and associated diuretic resistance are not well defined.

Objectives

To characterize renal function, electrolyte concentrations, indices of diuretic efficacy, and renin‐angiotensin‐aldosterone system (RAAS) activation in dogs with naturally occurring heart disease (HD) in American College of Veterinary Internal Medicine stages B1, B2, C, and D and to determine their usefulness in defining HD stages.

Animals

Group 1:149 dogs with HD stages B1, B2, C, and D. Group 2:22 dogs with HD stages C and D.

Methods

Group 1: Renal parameters, serum and urine electrolyte and diuretic concentrations, and urine aldosterone concentrations were measured. Medication dosages and measured variables were compared among stages. Correlation of furosemide dosages to serum concentrations was explored. Group 2: Angiotensin‐converting enzyme activity and RAAS components were measured and compared among CHF stages.

Results

Serum chloride concentration was the best differentiator of HD stage. Furosemide PO dosages (≤6 mg/kg/day) were weakly correlated with serum furosemide concentrations, whereas higher dosages were not significantly correlated. Angiotensin‐converting enzyme inhibitor dosage and RAAS inhibition were greater in stage D, compared to stage C dogs.

Conclusions and Clinical Importance

Hypochloremia is a useful marker for stage D HD in dogs. Poor furosemide dosage correlation to serum concentration may indicate variable and poor absorption, especially at higher dosages, advanced disease, or both. A small number of stage D dogs met proposed criteria for diuretic resistance. Greater RAAS inhibition in stage D versus stage C indicates effectiveness of RAAS‐suppressive treatments in this group of dogs with refractory CHF.

Pharmacokinetics of torasemide and its metabolites in healthy controls and in chronic renal failure

The pharmacokinetics of 20 mg torasemide i.v. has been studied in 7 healthy controls and 9 patients with varying degrees of renal impairment. Torasemide had a t1/2 of about 4h which was independent of kidney function, as the nonrenal clearance of torasemide was 3-times greater than its renal clearance. The active metabolite M 1 and the main metabolite M 5 were accumulated in chronic renal failure. In contrast to liver function, therefore, kidney failure does not have an important effect on the pharmacokinetics of torasemide.

Saluretic effect of the loop diuretic torasemide in chronic renal failure. Interdependence of electrolyte excretion

The effects of torasemide have been studied in 7 healthy controls and 9 patients with stable chronic renal failure of various degrees. After a control period of 3 days torasemide 20 mg i.v. caused a dramatic increase in diuresis and electrolyte excretion without affecting the glomerular filtration rate. The duration of action of torasemide, tau, averaged 6 h and was independent of the creatinine clearance, CLCR. When related to tau the drug-induced excretion of Cl-, Na+, K+, Ca2+ and Mg2+ showed strong linear dependence on CLCR. Both the kaliuresis and the calciuresis during tau were tightly correlated with the natriuresis over the broad range of CLCR. Similarly, the excretion of Mg2+ was dependent on the kaliuresis. The torasemide-induced kaliuresis amounted to 12% of natriuresis, as after furosemide. The kaliuretic effect of loop diuretics is smaller than that of the thiazides. After tau, e.g. over a 24 h period, kaliuresis was not correlated with natriuresis. The magnitude of the rebound effect was diminished with increasing renal impairment.

The influence of haemodialysis and haemofiltration on the clearance of torasemide in renal failure

The concentration of Torasemide in plasma, dialysate and ultrafiltrate were determined during one haemofiltration and three dialyses. Results show that Torasemide is not significantly eliminated from the blood by these technics.