CLINICAL PHARMACOLOGY
Sectral is a cardioselective, β-adrenoreceptor
blocking agent, which possesses mild intrinsic sympathomimetic activity (ISA)
in its therapeutically effective dose range.
Pharmacodynamics
β1 -cardioselectivity has been
demonstrated in experimental animal studies. In anesthetized dogs and cats, Sectral
is more potent in antagonizing isoproterenol-induced tachycardia (β1)
than in antagonizing isoproterenol-induced vasodilatation (β2).In
guinea pigs and cats, it is more potent in antagonizing this tachycardia than
in antagonizing isoproterenol- induced bronchodilatation (β2).
ISA of Sectral has been demonstrated in catecholamine-depleted rats by
tachycardia induced by intravenous administration of this agent. A
membrane-stabilizing effect has been detected in animals, but only with high
concentrations of Sectral.
Clinical studies have demonstrated β -blocking
activity at the recommended doses by: a) reduction in the resting heart rate
and decrease in exercise-induced tachycardia; b) reduction in cardiac output at
rest and after exercise; c) reduction of systolic and diastolic blood pressures
at rest and postexercise; d) inhibition of isoproterenol-induced tachycardia.
The β1-selectivity of Sectral has also
been demonstrated on the basis of the following vascular and bronchial effects:
Vascular Effects
Sectral has less antagonistic effects on peripheral
vascular β2-receptors at rest and after epinephrine stimulation
than nonselective β-antagonists.
Bronchial Effects
In single-dose studies in asthmatics examining effects of
various beta-blockers on pulmonary function, low doses of acebutolol produce
less evidence of bronchoconstriction and less reduction of beta2
agonist, bronchodilating effects, than nonselective agents like propranolol but
more than atenolol.
ISA has been observed with Sectral in man, as shown by a
slightly smaller (about 3 beats per minute) decrease in resting heart rate when
compared to equivalent β-blocking doses of propranolol, metoprolol or
atenolol. Chronic therapy with Sectral induced no significant alteration in the
blood lipid profile.
Sectral has been shown to delay AV conduction time and to
increase the refractoriness of the AV node without significantly affecting
sinus node recovery time, atrial refractory period, or the HV conduction time.
The membrane-stabilizing effect of Sectral is not manifest at the doses used
clinically.
Significant reductions in resting and exercise heart
rates and systolic blood pressures have been observed 1.5 hours after Sectral
administration with maximal effects occurring between 3 and 8 hours postdosing
in normal volunteers. Sectral has demonstrated a significant effect on
exercise-induced tachycardia 24 to 30 hours after drug administration.
There are significant correlations between plasma levels
of acebutolol and both the reduction in resting heart rate and the percent of
β-blockade of exercise-induced tachycardia.
The antihypertensive effect of Sectral has been shown in
double-blind controlled studies to be superior to placebo and similar to
propranolol and hydrochlorothiazide. In addition, patients responding to Sectral
administered twice daily had a similar response whether the dosage regimen was
changed to once daily administration or continued on a b.i.d. regimen. Most
patients responded to 400 to 800 mg per day in divided doses.
The antiarrhythmic effect of Sectral was compared with
placebo, propranolol, and quinidine. Compared with placebo, Sectral
significantly reduced mean total ventricular ectopic beats (VEB), paired VEB, multiform
VEB, R-on-T beats, and ventricular tachycardia (VT). Both Sectral and
propranolol significantly reduced mean total and paired VEB and VT. Sectral and
quinidine significantly reduced resting total and complex VEB; the
antiarrhythmic efficacy of Sectral was also observed during exercise.
Pharmacokinetics And Metabolism
Sectral is well absorbed from the GI tract. It is subject
to extensive first-pass hepatic biotransformation, with an absolute
bioavailability of approximately 40% for the parent compound. The major
metabolite, an N-acetyl derivative (diacetolol), is pharmacologically active.
This metabolite is equipotent to Sectral and in cats is more cardioselective
than Sectral; therefore, this first-pass phenomenon does not attenuate the
therapeutic effect of Sectral. Food intake does not have a significant effect
on the area under the plasma concentration-time curve (AUC) of Sectral although
the rate of absorption and peak concentration decreased slightly.
The plasma elimination half-life of Sectral is
approximately 3 to 4 hours, while that of its metabolite, diacetolol, is 8 to
13 hours. The time to reach peak concentration for Sectral is 2.5 hours and for
diacetolol, after oral administration of Sectral, 3.5 hours.
Within the single oral dose range of 200 to 400 mg, the
kinetics are dose proportional. However, this linearity is not seen at higher
doses, probably due to saturation of hepatic biotransformation sites. In addition,
after multiple dosing the lack of linearity is also seen by AUC increases of
approximately 100% as compared to single oral dosing. Elimination via renal
excretion is approximately 30% to 40% and by nonrenal mechanisms 50% to 60%,
which includes excretion into the bile and direct passage through the
intestinal wall.
Sectral has a low binding affinity for plasma proteins
(about 26%). Sectral and its metabolite, diacetolol, are relatively hydrophilic
and, therefore, only minimal quantities have been detected in the cerebrospinal
fluid (CSF).
Drug interaction studies with tolbutamide and warfarin
indicated no influence on the therapeutic effects of these compounds. Digoxin
and hydrochlorothiazide plasma levels were not affected by concomitant Sectral
administration. The kinetics of Sectral were not significantly altered by
concomitant administration of hydrochlorothiazide, hydralazine, sulfinpyrazone,
or oral contraceptives.
In patients with renal impairment, there is no effect on
the elimination half-life of Sectral, but there is decreased elimination of the
metabolite, diacetolol, resulting in a two- to three-fold increase in its
halflife. For this reason, the drug should be administered with caution in
patients with renal insufficiency (see PRECAUTIONS). Sectral and its
major metabolite are dialyzable.
Sectral crosses the placental barrier and is secreted in
breast milk.
In geriatric patients, the bioavailability of Sectral and
its metabolite is increased, approximately twofold, probably due to decreases
in the first-pass metabolism and renal function in the elderly.