CLINICAL PHARMACOLOGY
Mechanism Of Action
Enalapril, after hydrolysis to enalaprilat, inhibits
angiotensin-converting enzyme (ACE) in human subjects and 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. The beneficial effects of
enalapril in hypertension and heart failure appear to result primarily from
suppression of the renin-angiotensin-aldosterone system. Inhibition of ACE
results in decreased plasma angiotensin II, which leads to decreased
vasopressor activity and to decreased aldosterone secretion. Although the
latter decrease is small, it results in small increases of serum potassium. In
hypertensive patients treated with VASOTEC alone for up to 48 weeks, mean
increases in serum potassium of approximately 0.2 mEq/L were observed. In
patients treated with VASOTEC plus a thiazide diuretic, there was essentially
no change in serum potassium (see PRECAUTIONS). Removal of angiotensin
II negative feedback on renin secretion leads to increased plasma renin
activity.
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 VASOTEC remains to be
elucidated.
While the mechanism through which VASOTEC lowers blood
pressure is believed to be primarily suppression of the renin-angiotensin-aldosterone
system, VASOTEC is antihypertensive even in patients with low-renin
hypertension. Although VASOTEC was antihypertensive in all races studied, black
hypertensive patients (usually a low-renin hypertensive population) had a
smaller average response to enalapril monotherapy than non-black patients.
Pharmacokinetics And Metabolism
Following oral administration of VASOTEC, peak serum
concentrations of enalapril occur within about one hour. Based on urinary
recovery, the extent of absorption of enalapril is approximately 60 percent.
Enalapril absorption is not influenced by the presence of food in the
gastrointestinal tract. Following absorption, enalapril is hydrolyzed to
enalaprilat, which is a more potent angiotensin converting enzyme inhibitor
than enalapril; enalaprilat is poorly absorbed when administered orally. Peak
serum concentrations of enalaprilat occur three to four hours after an oral
dose of enalapril maleate. Excretion of VASOTEC is primarily renal.
Approximately 94 percent of the dose is recovered in the urine and feces as
enalaprilat or enalapril. The principal components in urine are enalaprilat,
accounting for about 40 percent of the dose, and intact enalapril. There is no
evidence of metabolites of enalapril, other than enalaprilat.
The serum concentration profile of enalaprilat exhibits a
prolonged terminal phase, apparently representing a small fraction of the
administered dose that has been bound to ACE. The amount bound does not
increase with dose, indicating a saturable site of binding. The effective
half-life for accumulation of enalaprilat following multiple doses of enalapril
maleate is 11 hours. The disposition of enalapril and enalaprilat in patients
with renal insufficiency is similar to that in patients with normal renal
function until the glomerular filtration rate is 30 mL/min or less. With
glomerular filtration rate ≤ 30 mL/min, peak and trough enalaprilat levels
increase, time to peak concentration increases and time to steady state may be
delayed. The effective half-life of enalaprilat following multiple doses of
enalapril maleate is prolonged at this level of renal insufficiency (see DOSAGE
AND ADMINISTRATION). Enalaprilat is dialyzable at the rate of 62 mL/min.
Studies in dogs indicate that enalapril crosses the
blood-brain barrier poorly, if at all; enalaprilat does not enter the brain.
Multiple doses of enalapril maleate in rats do not result in accumulation in
any tissues. Milk of lactating rats contains radioactivity following
administration of 14C-enalapril maleate. Radioactivity was found to
cross the placenta following administration of labeled drug to pregnant
hamsters.
Pharmacodynamics And Clinical Effects
Hypertension
Administration of VASOTEC to patients with hypertension
of severity ranging from mild to severe results in a reduction of both supine
and standing blood pressure usually with no orthostatic component. Symptomatic
postural hypotension is therefore infrequent, although it might be anticipated
in volume-depleted patients (see WARNINGS).
In most patients studied, after oral administration of a
single dose of enalapril, onset of antihypertensive activity was seen at one
hour with peak reduction of blood pressure achieved by four to six hours.
At recommended doses, antihypertensive effects have been
maintained for at least 24 hours. In some patients the effects may diminish
toward the end of the dosing interval (see DOSAGE AND ADMINISTRATION).
In some patients achievement of optimal blood pressure
reduction may require several weeks of therapy.
The antihypertensive effects of VASOTEC have continued
during long-term therapy. Abrupt withdrawal of VASOTEC has not been associated
with a rapid increase in blood pressure.
In hemodynamic studies in patients with essential hypertension,
blood pressure reduction was accompanied by a reduction in peripheral arterial
resistance with an increase in cardiac output and little or no change in heart
rate. Following administration of VASOTEC, there is an increase in renal blood
flow; glomerular filtration rate is usually unchanged. The effects appear to be
similar in patients with renovascular hypertension.
When given together with thiazide-type diuretics, the
blood pressure lowering effects of VASOTEC are approximately additive.
In a clinical pharmacology study, indomethacin or
sulindac was administered to hypertensive patients receiving VASOTEC. In this
study there was no evidence of a blunting of the antihypertensive action of
VASOTEC (see DRUG INTERACTIONS).
Heart Failure
In trials in patients treated with digitalis and
diuretics, treatment with enalapril resulted in decreased systemic vascular
resistance, blood pressure, pulmonary capillary wedge pressure and heart size,
and increased cardiac output and exercise tolerance. Heart rate was unchanged
or slightly reduced, and mean ejection fraction was unchanged or increased.
There was a beneficial effect on severity of heart failure as measured by the
New York Heart Association (NYHA) classification and on symptoms of dyspnea and
fatigue. Hemodynamic effects were observed after the first dose and appeared to
be maintained in uncontrolled studies lasting as long as four months. Effects
on exercise tolerance, heart size, and severity and symptoms of heart failure were
observed in placebo-controlled studies lasting from eight weeks to over one
year.
Heart Failure, Mortality Trials
In a multicenter, placebo-controlled clinical trial,
2,569 patients with all degrees of symptomatic heart failure and ejection
fraction ≤ 35 percent were randomized to placebo or enalapril and followed
for up to 55 months (SOLVD-Treatment). Use of enalapril was associated with an
11 percent reduction in all-cause mortality and a 30 percent reduction in
hospitalization for heart failure. Diseases that excluded patients from
enrollment in the study included severe stable angina (>2 attacks/day),
hemodynamically significant valvular or outflow tract obstruction, renal
failure (creatinine >2.5 mg/dL), cerebrovascular disease (e.g., significant
carotid artery disease), advanced pulmonary disease, malignancies, active
myocarditis and constrictive pericarditis. The mortality benefit associated
with enalapril does not appear to depend upon digitalis being present.
A second multicenter trial used the SOLVD protocol for
study of asymptomatic or minimally symptomatic patients. SOLVD-Prevention
patients, who had left ventricular ejection fraction ≤ 35% and no history
of symptomatic heart failure, were randomized to placebo (n=2117) or enalapril
(n=2111) and followed for up to 5 years. The majority of patients in the
SOLVD-Prevention trial had a history of ischemic heart disease. A history of
myocardial infarction was present in 80 percent of patients, current angina
pectoris in 34 percent, and a history of hypertension in 37 percent. No
statistically significant mortality effect was demonstrated in this population.
Enalapril-treated subjects had 32% fewer first hospitalizations for heart
failure, and 32% fewer total heart failure hospitalizations. Compared to
placebo, 32 percent fewer patients receiving enalapril developed symptoms of
overt heart failure. Hospitalizations for cardiovascular reasons were also
reduced. There was an insignificant reduction in hospitalizations for any cause
in the enalapril treatment group (for enalapril vs. placebo, respectively, 1166
vs. 1201 first hospitalizations, 2649 vs. 2840 total hospitalizations),
although the study was not powered to look for such an effect.
The SOLVD-Prevention trial was not designed to determine
whether treatment of asymptomatic patients with low ejection fraction would be
superior, with respect to preventing hospitalization, to closer follow-up and
use of enalapril at the earliest sign of heart failure. However, under the conditions
of follow-up in the SOLVD-Prevention trial (every 4 months at the study clinic;
personal physician as needed), 68% of patients on placebo who were hospitalized
for heart failure had no prior symptoms recorded which would have signaled
initiation of treatment.
The SOLVD-Prevention trial was also not designed to show
whether enalapril modified the progression of underlying heart disease.
In another multicenter, placebo-controlled trial
(CONSENSUS) limited to patients with NYHA Class IV congestive heart failure and
radiographic evidence of cardiomegaly, use of enalapril was associated with
improved survival. The results are shown in the following table.
|
SURVIVAL (%) |
Six Months |
One Year |
VASOTEC (n=127) |
74 |
64 |
Placebo (n=126) |
56 |
48 |
In both CONSENSUS and SOLVD-Treatment trials, patients were also usually receiving digitalis,
diuretics or both.
Clinical Pharmacology In Pediatric Patients
A multiple dose pharmacokinetic
study was conducted in 40 hypertensive male and female pediatric patients aged
2 months to ≤ 16 years following daily oral administration of 0.07 to 0.14
mg/kg enalapril maleate. At steady state, the mean effective half-life for
accumulation of enalaprilat was 14 hours, and the mean urinary recovery of
total enalapril and enalaprilat in 24 hours was 68% of the administered dose.
Conversion of enalapril to enalaprilat was in the range of 63-76%. The overall
results of this study indicate that the pharmacokinetics of enalapril in
hypertensive children aged 2 months to ≤ 16 years are consistent across
the studied age groups and consistent with pharmacokinetic historic data in
healthy adults.
In a clinical study involving
110 hypertensive pediatric patients 6 to 16 years of age, patients who weighed
<50 kg received either 0.625, 2.5 or 20 mg of enalapril daily and patients
who weighed ≥ 50 kg received either 1.25, 5, or 40 mg of enalapril daily.
Enalapril administered once daily lowered trough blood pressure in a
dose-dependent manner. The dose-dependent antihypertensive efficacy of
enalapril was consistent across all subgroups (age, Tanner stage, gender, race).
However, the lowest doses studied, 0.625 mg and 1.25 mg, corresponding to an
average of 0.02 mg/kg once daily, did not appear to offer consistent
antihypertensive efficacy. In this study, VASOTEC was generally well tolerated.
In the above pediatric studies,
enalapril maleate was given as tablets of VASOTEC and for those children and
infants who were unable to swallow tablets or who required a lower dose than is
available in tablet form, enalapril was administered in a suspension
formulation (see DOSAGE AND ADMINISTRATION, Preparation Of Suspension).