Mechanistic Differences between Torsemide and Furosemide

Background: Torsemide is proposed to have clinically important pharmacokinetic and pharmacodynamic advantages over furosemide. However, clinical outcomes did not differ in the Torsemide Comparison with Furosemide for Management of Heart Failure (TRANSFORM-HF) randomized trial. Methods: We conducted a multicenter mechanistic substudy of patients with heart failure randomized to oral furosemide or torsemide (TRANSFORM-Mechanism). At baseline and 30 days, participants underwent detailed assessments of pharmacokinetic and pharmacodynamic parameters. Results: TRANSFORM-Mechanism enrolled 88 participants. Kidney bioavailability, or the proportion of dose delivered to the tubular site of action, was significantly less with torsemide compared to furosemide [median 17.1%, (IQR 12.3, 23.5%) vs. 24.8% (16.6, 34.1%), p < 0.001]. Furosemide had a longer duration of kidney drug delivery and duration of natriuresis (p≤0.004 for both). Prescribed doses of furosemide and torsemide in TRANSFORM-Mechanism were similar to TRANSFORM-HF, with providers on average using a 2:1 dose equivalence conversion between drugs. However, these doses resulted in a substantially greater natriuresis with torsemide (p<0.001). A dose equivalence of ∼4:1 resulted in similar natriuresis. Higher diuretic doses in the torsemide group resulted in mild perturbations in kidney function and significant increases in renin, aldosterone, and norepinephrine (p<0.05 for all). Plasma volume (p=0.52) and body weight (p=0.89) did not improve with torsemide vs. furosemide. Conclusions: We observed no meaningful pharmacokinetic/pharmacodynamic advantages for torsemide vs. furosemide. The greater natriuresis from higher diuretic doses in the torsemide group was offset by greater neurohormonal activation and kidney dysfunction.