The therapeutic effects of diltiazem hydrochloride are believed
to be related to its ability to inhibit the influx of calcium ions during
membrane depolarization of cardiac and vascular smooth muscle.
Mechanisms Of Action: Hypertension: Diltiazem
produces its antihypertensive effect primarily by relaxation of vascular smooth
muscle and the resultant decrease in peripheral vascular resistance. The
magnitude of blood pressure reduction is related to the degree of hypertension;
thus hypertensive individuals experience an antihypertensive effect, whereas
there is only a modest fall in blood pressure in normotensives.
Angina: Diltiazem has been shown to produce increases in exercise tolerance,
probably due to its ability to reduce myocardial oxygen demand. This is
accomplished via reductions in heart rate and systemic blood pressure at
submaximal and maximal work loads. Diltiazem has been shown to be a potent
dilator of coronary arteries, both epicardial and subendocardial. Spontaneous
and ergonovine-induced coronary artery spasm are inhibited by diltiazem.
In animal models, diltiazem interferes with the slow inward (depolarizing)
current in excitable tissue. It causes excitation-contraction uncoupling in
various myocardial tissues without changes in the configuration of the action
potential. Diltiazem produces relaxation of coronary vascular smooth muscle and
dilation of both large and small coronary arteries at drug levels which cause
little or no negative inotropic effect. The resultant increases in coronary
blood flow (epicardial and subendocardial) occur in ischemic and nonischemic
models and are accompanied by dose-dependent decreases in systemic blood
pressure and decreases in peripheral resistance.
Hemodynamic And Electrophysiologic Effects: Like
other calcium channel antagonists, diltiazem decreases sinoatrial and
atrioventricular conduction in isolated tissues and has a negative inotropic
effect in isolated preparations. In the intact animal, prolongation of the AH
interval can be seen at higher doses.
In man, diltiazem prevents spontaneous and ergonovine-provoked coronary
artery spasm. It causes a decrease in peripheral vascular resistance and a
modest fall in blood pressure in normotensive individuals and, in exercise
tolerance studies in patients with ischemic heart disease, reduces the heart
rate-blood pressure product for any given work load. Studies to date, primarily
in patients with good ventricular function, have not revealed evidence of a
negative inotropic effect: cardiac output, ejection fraction, and left
ventricular end diastolic pressure have not been affected. Such data have no
predictive value with respect to effects in patients with poor ventricular
function, and increased heart failure has been reported in patients with
preexisting impairment of ventricular function. There are as yet few data on the
interaction of diltiazem and beta-blockers in patients with poor ventricular
function. Resting heart rate is usually slightly reduced by diltiazem.
In hypertensive patients, diltiazem hydrochloride extended-release produces
antihypertensive effects both in the supine and standing positions. In a
double-blind, parallel, dose-response study utilizing doses ranging from 90 to
540 mg once daily, diltiazem lowered supine diastolic blood pressure in an
apparent linear manner over the entire dose range studied. The changes in
diastolic blood pressure, measured at trough, for placebo, 90 mg, 180 mg, 360
mg, and 540 mg were -2.9, -4.5, -6.1, -9.5, and -10.5 mm Hg, respectively.
Postural hypotension is infrequently noted upon suddenly assuming an upright
position. No reflex tachycardia is associated with the chronic antihypertensive
effects. Diltiazem decreases vascular resistance, increases cardiac output (by
increasing stroke volume), and produces a slight decrease or no change in heart
rate. During dynamic exercise, increases in diastolic pressure are inhibited,
while maximum achievable systolic pressure is usually reduced. Chronic therapy
with diltiazem produces no change or an increase in plasma catecholamines. No
increased activity of the renin-angiotensin-aldosterone axis has been observed.
Diltiazem reduces the renal and peripheral effects of angiotensin II.
Hypertensive animal models respond to diltiazem with reductions in blood
pressure and increased urinary output and natriuresis without a change in
urinary sodium/potassium ratio.
In a double-blind, parallel dose-response study of doses from 60 mg to 480 mg
once daily, diltiazem increased time to termination of exercise in a linear
manner over the entire dose range studied. The improvement in time to
termination of exercise utilizing a Bruce exercise protocol, measured at trough,
for placebo, 60 mg, 120 mg, 240 mg, 360 mg, and 480 mg was 29, 40, 56, 51, 69
and 68 seconds, respectively. As doses of diltiazem were increased, overall
angina frequency was decreased. Diltiazem, 180 mg once daily, or placebo was
administered in a double-blind study to patients receiving concomitant treatment
with long-acting nitrates and/or beta-blockers. A significant increase in time
to termination of exercise and a significant decrease in overall angina
frequency was observed. In this trial the overall frequency of adverse events in
the diltiazem treatment group was the same as the placebo group.
Intravenous diltiazem hydrochloride in doses of 20 mg prolongs AH conduction
time and AV node functional and effective refractory periods by approximately
20%. In a study involving single oral doses of 300 mg of diltiazem hydrochloride
in six normal volunteers, the average maximum PR prolongation was 14% with no
instances of greater than first-degree AV block. Diltiazem-associated
prolongation of the AH interval is not more pronounced in patients with
first-degree heart block. In patients with sick sinus syndrome, diltiazem
significantly prolongs sinus cycle length (up to 50% in some cases).
Chronic oral administration of diltiazem hydrochloride to patients in doses
of up to 540 mg/day has resulted in small increases in PR interval, and on
occasion produces abnormal prolongation. (See WARNINGS.)
Pharmacokinetics And Metabolism: Diltiazem is well
absorbed from the gastrointestinal tract and is subject to an extensive
first-pass effect, giving an absolute bioavailability (compared to intravenous
administration) of about 40%. Diltiazem undergoes extensive metabolism in which
only 2% to 4% of the unchanged drug appears in the urine. Drugs which induce or
inhibit hepatic microsomal enzymes may alter diltiazem disposition.
Total radioactivity measurement following short IV administration in healthy
volunteers suggests the presence of other unidentified metabolites, which attain
higher concentrations than those of diltiazem and are more slowly eliminated;
half-life of total radioactivity is about 20 hours compared to 2 to 5 hours for
diltiazem.
In vitro binding studies show diltiazem is 70% to 80% bound to plasma
proteins. Competitive in vitro ligand binding studies have also shown diltiazem
binding is not altered by therapeutic concentrations of digoxin,
hydrochlorothiazide, phenylbutazone, propranolol, salicylic acid, or warfarin.
The plasma elimination half-life following single or multiple drug
administration is approximately 3.0 to 4.5 hours. Desacetyl diltiazem is also
present in the plasma at levels of 10% to 20% of the parent drug and is 25% to
50% as potent as a coronary vasodilator as diltiazem. Minimum therapeutic plasma
diltiazem concentrations appear to be in the range of 50 to 200 ng/mL. There is
a departure from linearity when dose strengths are increased; the half-life is
slightly increased with dose. A study that compared patients with normal hepatic
function to patients with cirrhosis found an increase in half-life and a 69%
increase in bioavailability in the hepatically impaired patients. A single study
in patients with severely impaired renal function showed no difference in the
pharmacokinetic profile of diltiazem compared to patients with normal renal
function.
Diltiazem Hydrochloride Extended-Release Capsules:
When compared to a regimen of diltiazem hydrochloride tablets at
steady-state, more than 95% of drug is absorbed from the diltiazem hydrochloride
extended-release capsules formulation. A single 360-mg dose of the capsule
results in detectable plasma levels within 2 hours and peak plasma levels
between 10 and 14 hours; absorption occurs throughout the dosing interval. When
diltiazem hydrochloride extended-release capsules were coadministered with a
high fat content breakfast, the extent of diltiazem absorption was not affected.
Dose-dumping does not occur. The apparent elimination half-life after single or
multiple dosing is 5 to 8 hours. A departure from linearity similar to that seen
with diltiazem hydrochloride tablets and diltiazem hydrochloride
sustained-release capsules is observed. As the dose of diltiazem hydrochloride
extended-release capsules is increased from a daily dose of 120 mg to 240 mg,
there is an increase in the area-under-the-curve of 2.7 times. When the dose is
increased from 240 mg to 360 mg there is an increase in the area-under-the-curve
of 1.6 times.