Atenolol is a beta1-selective
(cardioselective) beta-adrenergic receptor blocking agent without membrane
stabilizing or intrinsic sympathomimetic (partial agonist) activities. This
preferential effect is not absolute, however, and at higher doses, atenolol
inhibits beta2-adrenoreceptors, chiefly located in the
bronchial and vascular musculature.
Pharmacokinetics and Metabolism
In man, absorption of an oral dose is rapid and consistent
but incomplete. Approximately 50% of an oral dose is absorbed from the
gastrointestinal tract, the remainder being excreted unchanged in the feces.
Peak blood levels are reached between two (2) and four (4) hours after
ingestion. Unlike propranolol or metoprolol, but like nadolol, atenolol
undergoes little or no metabolism by the liver, and the absorbed portion is
eliminated primarily by renal excretion. Over 85% of an intravenous dose is
excreted in urine within 24 hours compared with approximately 50% for an oral
dose. Atenolol also differs from propranolol in that only a small amount (6% to
16%) is bound to proteins in the plasma. This kinetic profile results in
relatively consistent plasma drug levels with about a fourfold interpatient
variation.
The elimination half-life of oral atenolol is approximately 6 to 7 hours, and
there is no alteration of the kinetic profile of the drug by chronic
administration. Following intravenous administration, peak plasma levels are
reached within 5 minutes. Declines from peak levels are rapid (5- to 10-fold)
during the first 7 hours; thereafter, plasma levels decay with a half-life
similar to that of orally administered drug. Following oral doses of 50 mg or
100 mg, both beta-blocking and antihypertensive effects persist for at least 24
hours. When renal function is impaired, elimination of atenolol is closely
related to the glomerular filtration rate; significant accumulation occurs when
the creatinine clearance falls below 35 mL/min/1.73 m2.
(See DOSAGE AND ADMINISTRATION.)
Pharmacodynamics
In standard animal or human pharmacological tests,
beta-adrenoreceptor blocking activity of atenolol has been demonstrated by: (1)
reduction in resting and exercise heart rate and cardiac output, (2) reduction
of systolic and diastolic blood pressure at rest and on exercise, (3) inhibition
of isoproterenol induced tachycardia, and (4) reduction in reflex orthostatic
tachycardia.
A significant beta-blocking effect of atenolol, as measured by reduction of
exercise tachycardia, is apparent within one hour following oral administration
of a single dose. This effect is maximal at about 2 to 4 hours, and persists for
at least 24 hours. Maximum reduction in exercise tachycardia occurs within 5
minutes of an intravenous dose. For both orally and intravenously administered
drug, the duration of action is dose related and also bears a linear
relationship to the logarithm of plasma atenolol concentration. The effect on
exercise tachycardia of a single 10 mg intravenous dose is largely dissipated by
12 hours, whereas beta-blocking activity of single oral doses of 50 mg and 100
mg is still evident beyond 24 hours following administration. However, as has
been shown for all beta-blocking agents, the antihypertensive effect does not
appear to be related to plasma level.
In normal subjects, the beta1 selectivity of atenolol
has been shown by its reduced ability to reverse the beta2-mediated vasodilating effect of isoproterenol as compared to
equivalent beta-blocking doses of propranolol. In asthmatic patients, a dose of
atenolol producing a greater effect on resting heart rate than propranolol
resulted in much less increase in airway resistance. In a placebo controlled
comparison of approximately equipotent oral doses of several beta-blockers,
atenolol produced a significantly smaller decrease of FEV1 than nonselective beta-blockers such as propranolol and,
unlike those agents, did not inhibit bronchodilation in response to
isoproterenol.
Consistent with its negative chronotropic effect due to beta-blockade of the
SA node, atenolol increases sinus cycle length and sinus node recovery time.
Conduction in the AV node is also prolonged. Atenolol is devoid of membrane
stabilizing activity, and increasing the dose well beyond that producing
beta-blockade does not further depress myocardial contractility. Several studies
have demonstrated a moderate (approximately 10%) increase in stroke volume at
rest and during exercise.
In controlled clinical trials, atenolol, given as a single daily oral dose,
was an effective antihypertensive agent providing 24-hour reduction of blood
pressure. Atenolol has been studied in combination with thiazide-type diuretics,
and the blood pressure effects of the combination are approximately additive.
Atenolol is also compatible with methyldopa, hydralazine, and prazosin, each
combination resulting in a larger fall in blood pressure than with the single
agents. The dose range of atenolol is narrow and increasing the dose beyond 100
mg once daily is not associated with increased antihypertensive effect. The
mechanisms of the antihypertensive effects of beta-blocking agents have not been
established. Several possible mechanisms have been proposed and include: (1)
competitive antagonism of catecholamines at peripheral (especially cardiac)
adrenergic neuron sites, leading to decreased cardiac output, (2) a central
effect leading to reduced sympathetic outflow to the periphery, and (3)
suppression of renin activity. The results from long-term studies have not shown
any diminution of the antihypertensive efficacy of atenolol with prolonged
use.
By blocking the positive chronotropic and inotropic effects of catecholamines
and by decreasing blood pressure, atenolol generally reduces the oxygen
requirements of the heart at any given level of effort, making it useful for
many patients in the long-term management of angina pectoris. On the other hand,
atenolol can increase oxygen requirements by increasing left ventricular fiber
length and end diastolic pressure, particularly in patients with heart
failure.
In a multicenter clinical trial (ISIS-1) conducted in 16,027 patients with
suspected myocardial infarction, patients presenting within 12 hours (mean = 5
hours) after the onset of pain were randomized to either conventional therapy
plus atenolol (n = 8,037), or conventional therapy alone (n = 7,990). Patients
with a heart rate of less than 50 bpm or systolic blood pressure less than 100 mm Hg, or
with other contraindications to beta-blockade were excluded. Thirty-eight
percent of each group were treated within 4 hours of onset of pain. The mean
time from onset of pain to entry was 5 ± 2.7 hours in both groups. Patients in
the atenolol group were to receive atenolol I.V. Injection 5 to 10 mg given over
5 minutes plus atenolol tablets 50 mg every 12 hours orally on the first study
day (the first oral dose administered about 15 minutes after the IV dose)
followed by either atenolol tablets 100 mg once daily or atenolol tablets 50 mg
twice daily on days 2 to 7. The groups were similar in demographic and medical
history characteristics and in electrocardiographic evidence of myocardial
infarction, bundle branch block, and first degree atrioventricular block at
entry.
During the treatment period (days 0 to 7), the vascular mortality rates were
3.89% in the atenolol group (313 deaths) and 4.57% in the control group (365
deaths). This absolute difference in rates, 0.68%, is statistically significant
at the P less than 0.05 level. The absolute difference translates into a proportional
reduction of 15% (3.89 to 4.57/4.57 = -0.15). The 95% confidence limits are 1%
to 27%. Most of the difference was attributed to mortality in days 0 to 1
(atenolol - 121 deaths; control - 171 deaths).
Despite the large size of the ISIS-1 trial, it is not possible to identify
clearly subgroups of patients most likely or least likely to benefit from early
treatment with atenolol. Good clinical judgment suggests, however, that patients
who are dependent on sympathetic stimulation for maintenance of adequate cardiac
output and blood pressure are not good candidates for beta-blockade. Indeed, the
trial protocol reflected that judgment by excluding patients with blood pressure
consistently below 100 mm Hg systolic. The overall results of the study are
compatible with the possibility that patients with borderline blood pressure
(less than 120 mm Hg systolic), especially if over 60 years of age, are less
likely to benefit.
The mechanism through which atenolol improves survival in patients with
definite or suspected acute myocardial infarction is unknown, as is the case for
other beta-blockers in the postinfarction setting. Atenolol, in addition to its
effects on survival, has shown other clinical benefits including reduced
frequency of ventricular premature beats, reduced chest pain, and reduced enzyme
elevation
Atenolol Geriatric Pharmacology:
In general, elderly patients present higher atenolol plasma levels with total
clearance values about 50% lower than younger subjects. The half-life is
markedly longer in the elderly compared to younger subjects. The reduction in
atenolol clearance follows the general trend that the elimination of renally
excreted drugs is decreased with increasing age.