Mechanism of Action
Benazepril and benazeprilat inhibit angiotensin-converting enzyme (ACE) in
human subjects and in animals. ACE is a peptidyl dipeptidase that catalyzes
the conversion of angiotensin I to the vasoconstrictor substance, angiotensin
II. Angiotensin II also stimulates aldosterone secretion by the adrenal cortex.
Inhibition of ACE results in decreased plasma angiotensin II, which leads to
decreased vasopressor activity and to decreased aldosterone secretion. The latter
decrease may result in a small increase of serum potassium. Hypertensive patients
treated with benazepril alone for up to 52 weeks had elevations of serum potassium
of up to 0.2 mEq/L. Similar patients treated with benazepril and hydrochlorothiazide
for up to 24 weeks had no consistent changes in their serum potassium (see PRECAUTIONS).
Removal of angiotensin II negative feedback on renin secretion leads to increased
plasma renin activity. In animal studies, benazepril had no inhibitory effect
on the vasopressor response to angiotensin II and did not interfere with the
hemodynamic effects of the autonomic neurotransmitters acetylcholine, epinephrine,
and norepinephrine.
ACE is identical to kininase, an enzyme that degrades bradykinin. Whether increased
levels of bradykinin, a potent vasodepressor peptide, play a role in the therapeutic
effects of Lotensin HCT remains to be elucidated.
While the mechanism through which benazepril lowers blood pressure is believed
to be primarily suppression of the renin-angiotensin-aldosterone system, benazepril
has an antihypertensive effect even in patients with low-renin hypertension.
Hydrochlorothiazide is a thiazide diuretic. Thiazides affect the renal tubular
mechanisms of electrolyte reabsorption, directly increasing excretion of sodium
and chloride in approximately equivalent amounts. Indirectly, the diuretic action
of hydrochlorothiazide reduces plasma volume, with consequent increases in plasma
renin activity, increases in aldosterone secretion, increases in urinary potassium
loss, and decreases in serum potassium. The renin-aldosterone link is mediated
by angiotensin, so coadministration of an ACE inhibitor tends to reverse the
potassium loss associated with these diuretics.
The mechanism of the antihypertensive effect of thiazides is unknown.
Pharmacokinetics and Metabolism
Following oral administration of Lotensin HCT, peak plasma concentrations of
benazepril are reached within 0.5-1.0 hours. As determined by urinary recovery,
the extent of absorption is at least 37%. The absorption of hydrochlorothiazide
is somewhat slower (1-2.5 hours) and somewhat more complete (50%-80%). In fasting
subjects, the rate and extent of absorption of benazepril and hydrochlorothiazide
from Lotensin HCT are not different, respectively, from the rate and extent
of absorption of benazepril and hydrochlorothiazide from immediate-release monotherapy
formulations.
The absorption of benazepril from Lotensin® tablets is not influenced
by the presence of food in the gastrointestinal tract, but possible effects
of food upon absorption of either component from Lotensin HCT tablets have not
been studied. The reported studies of food effects on hydrochlorothiazide absorption
have been inconclusive. The absorption of hydrochlorothiazide is increased by
agents that reduce gastrointestinal motility, but it is reported to be reduced
by 50% in patients with congestive heart failure.
Cleavage of the ester group (primarily in the liver) converts benazepril to
its active metabolite, benazeprilat. Peak plasma concentrations of benazeprilat
are reached 1-2 hours after drug intake in the fasting state and 2-4 hours after
drug intake in the nonfasting state. The serum protein binding of benazepril
is about 96.7% and that of benazeprilat about 95.3%, as measured by equilibrium
dialysis; on the basis of in vitro studies, the degree of protein binding should
be unaffected by age, hepatic dysfunction, or - over the concentration range
of 0.24-23.6 μmol/L - concentration.
Hydrochlorothiazide is not metabolized. Its apparent volume of distribution
is 3.6-7.8 L/kg, and its measured plasma protein binding is 67.9%. The drug
also accumulates in red blood cells, so that whole blood levels are 1.6-1.8
times those measured in plasma.
In studies of rats given 14C-benazepril, benazepril and its metabolites
crossed the blood-brain barrier only to an extremely low extent. Multiple doses
of benazepril did not result in accumulation in any tissue except the lung,
where, as with other ACE inhibitors in similar studies, there was a slight increase
in concentration due to slow elimination in that organ.
Some placental passage occurred when benazepril was administered to pregnant
rats. In humans, hydrochlorothiazide crosses the placenta freely, and levels
in umbilical-cord blood are similar to those in the maternal circulation.
Benazepril is almost completely metabolized to benazeprilat, which has much
greater ACE inhibitory activity than benazepril, and to the glucuronide conjugates
of benazepril and benazeprilat. Only trace amounts of an administered dose of
benazepril can be recovered unchanged in the urine; about 20% of the dose is
excreted as benazeprilat, 4% as benazepril glucuronide, and 8% as benazeprilat
glucuronide.
In patients with hepatic dysfunction due to cirrhosis, levels of benazeprilat
are essentially unaltered. Similarly, the pharmacokinetics of benazepril and
benazeprilat do not appear to be influenced by age.
The kinetics of benazepril are dose-proportional within the dosage range of
5-20 mg. Small deviations from dose proportionality were observed when the broader
range of 2-80 mg was studied, possibly due to the saturable binding of the compound
to ACE.
The effective half-life of accumulation of benazeprilat following multiple
dosing of benazepril hydrochloride is 10-11 hours. Thus, steady-state concentrations
of benazeprilat should be reached after 2 or 3 doses of benazepril hydrochloride
given once daily.
During chronic administration (28 days) of once-daily doses of benazepril between
5 mg and 20 mg, the kinetics did not change, and there was no significant accumulation.
Accumulation ratios based on AUC and urinary recovery of benazeprilat were 1.19
and 1.27, respectively.
When dialysis was started 2 hours after ingestion of 10 mg of benazepril, approximately
6% of benazeprilat was removed in 4 hours of dialysis. The parent compound,
benazepril, was not detected in the dialysate.
Benazepril and benazeprilat are cleared predominantly by renal excretion in
healthy subjects with normal renal function. Nonrenal (i.e., biliary) excretion
accounts for approximately 11%-12% of benazeprilat excretion in healthy subjects.
In patients with renal failure, biliary clearance may compensate to an extent
for deficient renal clearance.
The disposition of benazepril and benazeprilat in patients with mild-to-moderate
renal insufficiency (creatinine clearance > 30 mL/min) is similar to that
in patients with normal renal function. In patients with creatinine clearance
30 mL/min, peak benazeprilat levels and the initial (alpha phase) half-life
increase, and time to steady state may be delayed (see DOSAGE AND ADMINISTRATION).
Thiazide diuretics are eliminated by the kidney, with a terminal half-life
of 5-15 hours. In a study of patients with impaired renal function (mean creatinine
clearance of 19 mL/min), the half-life of hydrochlorothiazide elimination was
lengthened to 21 hours.
Pharmacodynamics
Single and multiple doses of 10 mg or more of benazepril cause inhibition of
plasma ACE activity by at least 80%-90% for at least 24 hours after dosing.
For up to 4 hours after a 10-mg dose, pressor responses to exogenous angiotensin
I were inhibited by 60%-90%.
Administration of benazepril to patients with mild-to-moderate hypertension
results in a reduction of both supine and standing blood pressure to about the
same extent, with no compensatory tachycardia. Symptomatic postural hypotension
is infrequent, although it can occur in patients who are salt and/or volume
depleted (see WARNINGS, Hypotension).
In single-dose studies, benazepril lowered blood pressure within 1 hour, with
peak reductions achieved 2-4 hours after dosing. The antihypertensive effect
of a single dose persisted for 24 hours. In multiple-dose studies, once-daily
doses of 20-80 mg decreased seated pressure (systolic/diastolic) 24 hours after
dosing by about 6-12/4-7 mmHg. The reductions at trough are about 50% of those
seen at peak.
Four dose-response studies of benazepril monotherapy using once-daily dosing
were conducted in 470 mild-to-moderate hypertensive patients not using diuretics.
The minimal effective once-daily dose of benazepril was 10 mg; further falls
in blood pressure, especially at morning trough, were seen with higher doses
in the studied dosing range (10-80 mg). In studies comparing the same daily
dose of benazepril given as a single morning dose or as a twice-daily dose,
blood pressure reductions at the time of morning trough blood levels were greater
with the divided regimen.
During chronic therapy with benazepril, the maximum reduction in blood pressure
with any given dose is generally achieved after 1-2 weeks. The antihypertensive
effects of benazepril have continued during therapy for at least 2 years. Abrupt
withdrawal of benazepril has not been associated with a rapid increase in blood
pressure.
In patients with mild-to-moderate hypertension, total daily doses of Lotensin
20-40 mg were similar in effectiveness to total daily doses of captopril 50-100
mg, hydrochlorothiazide 25-50 mg, nifedipine SR 40-80 mg, and propranolol 80-160
mg.
The antihypertensive effects of benazepril were not appreciably different in
patients receiving high- or low-sodium diets.
In hemodynamic studies in dogs, blood pressure reduction was accompanied by
a reduction in peripheral arterial resistance, with an increase in cardiac output
and renal blood flow and little or no change in heart rate. In normal human
volunteers, single doses of benazepril caused an increase in renal blood flow
but had no effect on glomerular filtration rate.
In clinical trials of benazepril/hydrochlorothiazide using benazepril doses
of 5-20 mg and hydrochlorothiazide doses of 6.25-25 mg, the antihypertensive
effects were sustained for at least 24 hours, and they increased with increasing
dose of either component. Although benazepril monotherapy is somewhat less effective
in blacks than in nonblacks, the efficacy of combination therapy appears to
be independent of race.
By blocking the renin-angiotensin-aldosterone axis, administration of benazepril
tends to reduce the potassium loss associated with the diuretic. In clinical
trials of Lotensin HCT, the average change in serum potassium was near zero
in subjects who received 5/6.25 mg or 20/12.5 mg, but the average subject who
received 10/12.5 mg or 20/25 mg experienced a mild reduction in serum potassium,
similar to that experienced by the average subject receiving the same dose of
hydrochlorothiazide monotherapy.
Last updated on RxList: 8/20/2008