Mechanistic Differences between Torsemide and Furosemide

Key Points Oral torsemide was not superior to furosemide in measures of renal tubular delivery or duration of action. A dose equivalence of approximately 40 mg oral furosemide:10 mg oral torsemide resulted in similar natriuresis. The two-fold higher doses of torsemide did not improve fluid status due to the kidney’s compensation. 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) randomized trial. Methods We conducted a multicenter mechanistic substudy of patients with heart failure randomized to oral furosemide or torsemide (TRANSFORM-Mechanism trial). At baseline and 30 days, participants underwent detailed assessments of pharmacokinetic and pharmacodynamic parameters. Results The TRANSFORM-Mechanism trial enrolled 88 participants. Kidney bioavailability, or the proportion of dose delivered to the tubular site of action, was significantly less with torsemide compared with furosemide (median, 17.1% [interquartile range, 12.3%–23.5%] versus 24.8% [16.6%–34.1%], P < 0.001). Furosemide had a longer duration of kidney drug delivery and natriuresis ( P ≤ 0.004 for both). Prescribed doses of furosemide and torsemide in the TRANSFORM-Mechanism trial were similar to the TRANSFORM trial, with clinicians 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 approximately 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 versus furosemide. Conclusions We observed no meaningful pharmacokinetic/pharmacodynamic advantages for torsemide versus furosemide. The greater natriuresis from higher diuretic doses in the torsemide group was offset by greater neurohormonal activation and kidney dysfunction. Clinical Trial registry name and registration number: TRANSFORM-HF: ToRsemide compArisoN With furoSemide FORManagement of Heart Failure (TRANSFORM-HF), NCT03296813; Torsemide Comparison With Furosemide for Management of Patients With Stable Heart Failure (TFO), NCT05093621.