Alpha1-specific Adrenergic Antagonist

Overview

Alpha1-specific adrenergic antagonists are compounds that antagonize with high specificity the alpha1 subtype of adrenergic receptors. In general, these drugs have the suffix "-zosin" with common examples being Terazosin, Prazosin, Doxazosin. They have strong effects on the vasculature, resulting in vasodilation, reduced peripheral resistance and ultimately reduced blood pressure that often yields a reflex tachycardia. However, their most common clinical usage is for treatment of the urological symptoms of benign prostatic hyperplasia (BPH).

Mechanism of Action

Alpha1-specific adrenergic agonists reversibly bind alpha1 receptors and block downstream receptor signaling.

Pharmacological Effects

Specific blockade of alpha1 receptors results in inhibition of baseline sympathetic vasoconstrictive tone on the vasculature, yielding systemic vasodilation and in turn a fall in systemic vascular resistance. In most patients this results in reduced systemic arterial pressure. Because short-term regulation of blood pressure is largely mediated by rapid modulation of alpha1 receptors, these drugs often render patients prone to postural hypotension.
Unlike non-specific alpha antagonists, these drugs do not block presynaptic alpha2 receptors and thus alpha2-mediated inhibition of reflex sympathetic output to the heart is intact. Consequently, reflex tachycardia is relatively mild, thus avoiding the arrhythmic adverse effects of non-specific alpha antagonists

Baseline alpha1 tone in the smooth muscle of the bladder neck and the prostate appears is responsible for some mechanical pressure on the urethra traveling through these regions. Consequently, blockade of alpha1 receptors appears to yield some urethral dilation.

Therapeutic Uses

The current primary clinical use of these drugs is the alleviation of the obstructive urinary symptoms seen in BPH. It is important to point out that these drugs do not alter the fundamental hyperplastic pathology of BPH or its natural course. They simply result in some symptomatic relief by yielding reduced mechanical pressure on the urethra.

Alpha1 blockers are infrequently used as monotherapy for hypertension although their anti-hypetensive effect is frequently a therapeutic bonus in patients with comorbid BPH and hypertension. However, their capacity to reduce blood pressure combined with their inhibition of short-term blood pressure regulatory mechanisms combines to yield postural hypotension in many patients.

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