CLINICAL PHARMACOLOGY
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, the onset of
diuresis occurs within 1 hour and the peak effect occurs during the first or
second hour and diuresis lasts about 6 to 8 hours. In healthy subjects given
single doses, the dose-response relationship for sodium excretion is linear
over the dose range of 2.5 mg to 20 mg. The increase in potassium excretion is
negligible after a single dose of up to 10 mg and only slight (5 mEq to 15 mEq)
after a single dose of 20 mg.
Edema
DEMADEX 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 DEMADEX in
these studies achieved significantly greater reductions in weight and edema
than did patients who received placebo.
Hypertension
In patients with essential hypertension, DEMADEX has been shown in controlled studies to lower blood
pressure when administered once a day at doses of 5 mg to 10 mg. The
antihypertensive effect is near maximal after 4 to 6 weeks of treatment, but it
may continue to increase for up to 12 weeks. Systolic and diastolic supine and
standing blood pressures are all reduced. There is no significant orthostatic
effect, and there is only a minimal peak-trough difference in blood pressure
reduction.
The antihypertensive effects of
DEMADEX are, like those of other diuretics, on the average greater in black
patients (a low-renin population) than in nonblack patients.
When DEMADEX 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 DEMADEX is suddenly stopped,
blood pressure returns to pretreatment levels over several days, without
overshoot.
DEMADEX 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
The bioavailability of DEMADEX
tablets is approximately 80%, with small inter-subject variation; the 90%
confidence interval is 75% to 89%. The drug is absorbed with little first-pass
metabolism, and the serum concentration reaches its peak (C max ) within 1 hour
after oral administration. Cmax and area under the serum concentration-time
curve (AUC) after oral administration are proportional to dose over the range
of 2.5 mg to 200 mg. Simultaneous food intake delays the time to Cmax by about
30 minutes, but overall bioavailability (AUC) and diuretic activity are
unchanged.
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
methyl-hydroxylation 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 that commonly
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 AUC 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 AUC for torsemide by 50%, but dose
adjustment of DEMADEX 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 AUC.
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.