Clinical Pharmacology for Soaanz
Mechanism Of Action
Micropuncture studies in animals have shown that torsemide acts from within the lumen of the thick ascending portion of the loop of Henle, where it inhibits the Na+/K+/2Cl--carrier system. Clinical pharmacology studies have confirmed this site of action in humans, and effects in other segments of the nephron have not been demonstrated. Diuretic activity thus correlates better with the rate of drug excretion in the urine than with the concentration in the blood.
Torsemide increases the urinary excretion of sodium, chloride, and water, but it does not significantly alter glomerular filtration rate, renal plasma flow, or acid-base balance.
Pharmacodynamics
With oral dosing of SOAANZ, the onset of diuresis occurs within 1 hour and the peak effect occurs during the first four hours and diuresis lasts about 6 to 8 hours.The mean increase in potassium excretion over 23 hours following a single oral dose of SOAANZ 20 mg in healthy adults was 12.7 mEq.
Edema
Torsemide has been studied in controlled trials in patients with New York Heart Association Class II to Class IV heart failure. Patients who received 10 mg to 20 mg of daily torsemide in these studies achieved significantly greater reductions in weight and edema than did patients who received placebo.
When torsemide is first administered, daily urinary sodium excretion increases for at least a week. With chronic administration, however, daily sodium loss comes into balance with dietary sodium intake. If the administration of torsemide is suddenly stopped, blood pressure returns to pretreatment levels over several days, without overshoot.
Torsemide has been administered together with β-adrenergic blocking agents, ACE inhibitors, and calcium-channel blockers. Adverse drug interactions have not been observed, and special dosage adjustment has not been necessary.
Pharmacokinetics
Absorption
Torsemide reaches peak plasma concentration (Cmax ) within 2.5 hours after oral administration of SOAANZ. Cmax and area under the serum concentration-time curve (AUCmax ) of torsemide after oral administration are proportional to dose over the range of 2.5 mg to 200 mg.
Effect Of Food
Administration of SOAANZ with a high fat, high calorie meal in healthy adults delays the time to Cmax by about 45 minutes, increases Cmax by 100% and increases area under the plasma concentration over time curve (AUCmax ) by 48%. No clinically significant change in overall diuretic activity was observed.
Distribution
The volume of distribution of torsemide is 12 to 15 liters in normal adults or in patients with mild to moderate renal failure or congestive heart failure. In patients with hepatic cirrhosis, the volume of distribution is approximately doubled. Torsemide is extensively bound to plasma protein (>99%).
Metabolism
Torsemide is metabolized by the hepatic cytochrome CYP2C9 and, to a minor extent, CYP2C8 and CYP2C18. Three main metabolites have been identified in humans. Metabolite M1 is formed by methylhydroxylation of torsemide, metabolite M3 is formed by ring hydroxylation of torsemide, and metabolite M5 is formed by oxidation of M1. The major metabolite in humans is the carboxylic acid derivative M5, which is biologically inactive. Metabolites M1 and M3 possess some pharmacological activity; however, their systemic exposures are much lower when compared to torsemide.
Elimination
In normal subjects the elimination half-life of torsemide is approximately 3.5 hours. Torsemide is cleared from the circulation by both hepatic metabolism (approximately 80% of total clearance) and excretion into the urine (approximately 20% of total clearance in patients with normal renal function).
Because torsemide is extensively bound to plasma protein (>99%), very little enters tubular urine via glomerular filtration. Most renal clearance of torsemide occurs via active secretion of the drug by the proximal tubules into tubular urine.
After a single oral dose, the amounts recovered in urine were: torsemide 21%, metabolite M1 12%, metabolite M3 2%, and metabolite M5 34%.
Renal Impairment
In patients with renal failure, renal clearance of torsemide is markedly decreased but total plasma clearance is not significantly altered. A smaller fraction of the administered dose is delivered to the intraluminal site of action, and the natriuretic action of any given dose of diuretic is reduced.
Hepatic Impairment
In patients with hepatic cirrhosis, the volume of distribution, plasma half-life, and renal clearance are all increased, but total clearance is unchanged.
Geriatric Patients
The renal clearance of torsemide is lower in elderly subjects as compared to younger adults, which is related to the decline in renal function thatcommonly occurs with aging. However, total plasma clearance and elimination half-life remain unchanged.
Heart Failure
In patients with decompensated congestive heart failure, hepatic and renal clearance are both reduced, probably because of hepatic congestion and decreased renal plasma flow, respectively. The total clearance of torsemide is approximately 50% of that seen in healthy volunteers, and the plasma half-life and AUCmax are correspondingly increased. Because of reduced renal clearance, a smaller fraction of any given dose is delivered to the intraluminal site of action, so at any given dose there is less natriuresis in patients with heart failure than in normal subjects.
Drug Interactions
Digoxin
Coadministration of digoxin is reported to increase the AUCmax for torsemide by 50%, but dose adjustment of SOAANZ is not necessary. Torsemide does not affect the pharmacokinetics of digoxin.
Spironolactone
In healthy subjects, coadministration of torsemide was associated with significant reduction in the renal clearance of spironolactone, with corresponding increases in the AUCmax. However, the pharmacokinetic profile and diuretic activity of torsemide are not altered by spironolactone.
Torsemide does not affect the protein binding of glyburide or warfarin.
Cimetidine
The pharmacokinetic profile and diuretic activity of torsemide are not altered by cimetidine.