Mechanism of Action
Amlodipine is a dihydropyridine calcium antagonist (calcium ion antagonist
or slow-channel blocker) that inhibits the transmembrane influx of calcium ions
into vascular smooth muscle and cardiac muscle. Experimental data suggest that
amlodipine binds to both dihydropyridine and nondihydropyridine binding sites.
The contractile processes of cardiac muscle and vascular smooth muscle are dependent
upon the movement of extracellular calcium ions into these cells through specific
ion channels. Amlodipine inhibits calcium ion influx across cell membranes selectively,
with a greater effect on vascular smooth muscle cells than on cardiac muscle
cells. Negative inotropic effects can be detected in vitro but such effects
have not been seen in intact animals at therapeutic doses. Serum calcium concentration
is not affected by amlodipine. Within the physiologic pH range, amlodipine is
an ionized compound (pKa=8.6), and its kinetic interaction with the calcium
channel receptor is characterized by a gradual rate of association and dissociation
with the receptor binding site, resulting in a gradual onset of effect.
Amlodipine is a peripheral arterial vasodilator that acts directly on vascular
smooth muscle to cause a reduction in peripheral vascular resistance and reduction
in blood pressure.
The precise mechanisms by which amlodipine relieves angina have not been fully
delineated, but are thought to include the following:
Exertional Angina: In patients with exertional angina, NORVASC reduces
the total peripheral resistance (afterload) against which the heart works and
reduces the rate pressure product, and thus myocardial oxygen demand, at any
given level of exercise.
Vasospastic Angina: NORVASC has been demonstrated to block constriction
and restore blood flow in coronary arteries and arterioles in response to calcium,
potassium epinephrine, serotonin, and thromboxane A2 analog in experimental
animal models and in human coronary vessels in vitro. This inhibition
of coronary spasm is responsible for the effectiveness of NORVASC in vasospastic
(Prinzmetal's or variant) angina.
Pharmacokinetics and Metabolism
After oral administration of therapeutic doses of NORVASC, absorption produces
peak plasma concentrations between 6 and 12 hours. Absolute bioavailability
has been estimated to be between 64 and 90%. The bioavailability of NORVASC
is not altered by the presence of food.
Amlodipine is extensively (about 90%) converted to inactive metabolites via
hepatic metabolism with 10% of the parent compound and 60% of the metabolites
excreted in the urine. Ex vivo studies have shown that approximately
93% of the circulating drug is bound to plasma proteins in hypertensive patients.
Elimination from the plasma is biphasic with a terminal elimination half-life
of about 30-50 hours. Steady-state plasma levels of amlodipine are reached after
7 to 8 days of consecutive daily dosing.
The pharmacokinetics of amlodipine are not significantly influenced by renal
impairment. Patients with renal failure may therefore receive the usual initial
dose.
Elderly patients and patients with hepatic insufficiency have decreased clearance
of amlodipine with a resulting increase in AUC of approximately 40-60%, and
a lower initial dose may be required. A similar increase in AUC was observed
in patients with moderate to severe heart failure.
Pediatric Patients
Sixty-two hypertensive patients aged 6 to 17 years received doses of NORVASC
between 1.25 mg and 20 mg. Weight-adjusted clearance and volume of distribution
were similar to values in adults.
Pharmacodynamics
Hemodynamics: Following administration of therapeutic doses to
patients with hypertension, NORVASC produces vasodilation resulting in a reduction
of supine and standing blood pressures. These decreases in blood pressure are
not accompanied by a significant change in heart rate or plasma catecholamine
levels with chronic dosing. Although the acute intravenous administration of
amlodipine decreases arterial blood pressure and increases heart rate in hemodynamic
studies of patients with chronic stable angina, chronic oral administration
of amlodipine in clinical trials did not lead to clinically significant changes
in heart rate or blood pressures in normotensive patients with angina.
With chronic once daily oral administration, antihypertensive effectiveness
is maintained for at least 24 hours. Plasma concentrations correlate with effect
in both young and elderly patients. The magnitude of reduction in blood pressure
with NORVASC is also correlated with the height of pretreatment elevation; thus,
individuals with moderate hypertension (diastolic pressure 105-114 mmHg) had
about a 50% greater response than patients with mild hypertension (diastolic
pressure 90-104 mmHg). Normotensive subjects experienced no clinically significant
change in blood pressures (+1/-2 mmHg).
In hypertensive patients with normal renal function, therapeutic doses of NORVASC
resulted in a decrease in renal vascular resistance and an increase in glomerular
filtration rate and effective renal plasma flow without change in filtration
fraction or proteinuria.
As with other calcium channel blockers, hemodynamic measurements of cardiac
function at rest and during exercise (or pacing) in patients with normal ventricular
function treated with NORVASC have generally demonstrated a small increase in
cardiac index without significant influence on dP/dt or on left ventricular
end diastolic pressure or volume. In hemodynamic studies, NORVASC has not been
associated with a negative inotropic effect when administered in the therapeutic
dose range to intact animals and man, even when co-administered with beta-blockers
to man. Similar findings, however, have been observed in normals or well-compensated
patients with heart failure with agents possessing significant negative inotropic
effects.
Electrophysiologic Effects: NORVASC does not change sinoatrial
nodal function or atrioventricular conduction in intact animals or man. In patients
with chronic stable angina, intravenous administration of 10 mg did not significantly
alter A-H and H-V conduction and sinus node recovery time after pacing. Similar
results were obtained in patients receiving NORVASC and concomitant beta blockers.
In clinical studies in which NORVASC was administered in combination with beta-blockers
to patients with either hypertension or angina, no adverse effects on electrocardiographic
parameters were observed. In clinical trials with angina patients alone, NORVASC
therapy did not alter electrocardiographic intervals or produce higher degrees
of AV blocks.
Clinical Studies
Effects in Hypertension
Adult Patients: The antihypertensive efficacy of NORVASC has
been demonstrated in a total of 15 double-blind, placebo-controlled, randomized
studies involving 800 patients on NORVASC and 538 on placebo. Once daily administration
produced statistically significant placebo-corrected reductions in supine and
standing blood pressures at 24 hours postdose, averaging about 12/6 mmHg in
the standing position and 13/7 mmHg in the supine position in patients with
mild to moderate hypertension. Maintenance of the blood pressure effect over
the 24-hour dosing interval was observed, with little difference in peak and
trough effect. Tolerance was not demonstrated in patients studied for up to
1 year. The 3 parallel, fixed dose, dose response studies showed that the reduction
in supine and standing blood pressures was dose-related within the recommended
dosing range. Effects on diastolic pressure were similar in young and older
patients. The effect on systolic pressure was greater in older patients, perhaps
because of greater baseline systolic pressure. Effects were similar in black
patients and in white patients.
Pediatric Patients: Two-hundred sixty-eight hypertensive patients
aged 6 to 17 years were randomized first to NORVASC 2.5 or 5 mg once daily for
4 weeks and then randomized again to the same dose or to placebo for another
4 weeks. Patients receiving 5 mg at the end of 8 weeks had lower blood pressure
than those secondarily randomized to placebo. The magnitude of the treatment
effect is difficult to interpret, but it is probably less than 5 mmHg systolic
on the 5-mg dose. Adverse events were similar to those seen in adults.
Effects in Chronic Stable Angina: The effectiveness of 5-10 mg/day
of NORVASC in exercise-induced angina has been evaluated in 8 placebo-controlled,
double-blind clinical trials of up to 6 weeks duration involving 1038 patients
(684 NORVASC, 354 placebo) with chronic stable angina. In 5 of the 8 studies
significant increases in exercise time (bicycle or treadmill) were seen with
the 10 mg dose. Increases in symptom-limited exercise time averaged 12.8% (63
sec) for NORVASC 10 mg, and averaged 7.9% (38 sec) for NORVASC 5 mg. NORVASC
10 mg also increased time to 1 mm ST segment deviation in several studies and
decreased angina attack rate. The sustained efficacy of NORVASC in angina patients
has been demonstrated over long-term dosing. In patients with angina there were
no clinically significant reductions in blood pressures (4/1 mmHg) or changes
in heart rate (+0.3 bpm).
Effects in Vasospastic Angina: In a double-blind, placebo-controlled
clinical trial of 4 weeks duration in 50 patients, NORVASC therapy decreased
attacks by approximately 4/week compared with a placebo decrease of approximately
1/week (p<0.01). Two of 23 NORVASC and 7 of 27 placebo patients discontinued
from the study due to lack of clinical improvement.
Effects in Documented Coronary Artery Disease: In PREVENT, 825
patients with angiographically documented coronary artery disease were randomized
to NORVASC (5-10 mg once daily) or placebo and followed for 3 years. Although
the study did not show significance on the primary objective of change in coronary
luminal diameter as assessed by quantitative coronary angiography, the data
suggested a favorable outcome with respect to fewer hospitalizations for angina
and revascularization procedures in patients with CAD.
CAMELOT enrolled 1318 patients with CAD recently documented by angiography,
without left main coronary disease and without heart failure or an ejection
fraction <40%. Patients (76% males, 89% Caucasian, 93% enrolled at US sites,
89% with a history of angina, 52% without PCI, 4% with PCI and no stent, and
44% with a stent) were randomized to double-blind treatment with either NORVASC
(5 - 10 mg once daily) or placebo in addition to standard care that included
aspirin (89%), statins (83%), beta-blockers (74%), nitroglycerin (50%), anti-coagulants
(40%), and diuretics (32%), but excluded other calcium channel blockers. The
mean duration of follow-up was 19 months. The primary endpoint was the time
to first occurrence of one of the following events: hospitalization for angina
pectoris, coronary revascularization, myocardial infarction, cardiovascular
death, resuscitated cardiac arrest, hospitalization for heart failure, stroke/TIA,
or peripheral vascular disease. A total of 110 (16.6%) and 151 (23.1%) first
events occurred in the NORVASC and placebo groups respectively for a hazard
ratio of 0.691 (95% CI: 0.540-0.884, p= 0.003). The primary endpoint is summarized
in Figure 1 below. The outcome of this study was largely derived from the prevention
of hospitalizations for angina and the prevention of revascularization procedures
(see Table 1). Effects in various subgroups are shown in Figure 2.
In an angiographic substudy (n=274) conducted within CAMELOT, there was no
significant difference between amlodipine and placebo on the change of atheroma
volume in the coronary arteryas assessed by intravascular ultrasound.
Figure 1: Kaplan-Meier analysis of composite clinical outcomes
for NORVASC versus placebo
Figure 2: Effects on primary endpoint of NORVASC versus
placebo across sub-groups
Table 1 below summarizes the significant clinical outcomes from the composites
of the primary endpoint. The other components of the primary endpoint including
cardiovascular death, resuscitated cardiac arrest, myocardial infarction, hospitalization
for heart failure, stroke/TIA, or peripheral vascular disease did not demonstrate
a significant difference between NORVASC and placebo.
Table 1. Incidence of Significant Clinical Outcomes for CAMELOT
Clinical Outcomes
N (%) |
NORVASC
(N=663) |
Placebo
(N=655) |
Risk
Reduction
(p-value) |
| Composite CV |
110 |
151 |
31% |
| Endpoint |
(16.6) |
(23.1) |
(0.003) |
| Hospitalization for |
51 |
84 |
42% |
| Angina* |
(7.7) |
(12.8) |
(0.002) |
| Coronary |
78 |
103 |
27% |
| Revascularization* |
(11.8) |
(15.7) |
(0.033) |
| *Total patients with these events |
Studies in Patients with Congestive Heart Failure: NORVASC has
been compared to placebo in four 8-12 week studies of patients with NYHA class
II/III heart failure, involving a total of 697 patients. In these studies, there
was no evidence of worsened heart failure based on measures of exercise tolerance,
NYHA classification, symptoms, or left ventricular ejection fraction. In a long-term
(follow-up at least 6 months, mean 13.8 months) placebo-controlled mortality/morbidity
study of NORVASC 5-10 mg in 1153 patients with NYHA classes III (n=931) or IV
(n=222) heart failure on stable doses of diuretics, digoxin, and ACE inhibitors,
NORVASC had no effect on the primary endpoint of the study which was the combined
endpoint of all-cause mortality and cardiac morbidity (as defined by life-threatening
arrhythmia, acute myocardial infarction, or hospitalization for worsened heart
failure), or on NYHA classification, or symptoms of heart failure. Total combined
all-cause mortality and cardiac morbidity events were 222/571 (39%) for patients
on NORVASC and 246/583 (42%) for patients on placebo; the cardiac morbid events
represented about 25% of the endpoints in the study.
Another study (PRAISE-2) randomized patients with NYHA class III (80%) or IV
(20%) heart failure without clinical symptoms or objective evidence of underlying
ischemic disease, on stable doses of ACE inhibitor (99%), digitalis (99%) and
diuretics (99%), to placebo (n=827) or NORVASC (n=827) and followed them for
a mean of 33 months. There was no statistically significant difference between
NORVASC and placebo in the primary endpoint of all cause mortality (95% confidence
limits from 8% reduction to 29% increase on NORVASC). With NORVASC there were
more reports of pulmonary edema.
Last updated on RxList: 5/1/2007