Phenoxybenzamine

Mechanism of action:

It gets bind to α-adrenergic receptors causing the irreversible blockade of these receptors.

It blocks reuptake of norepinephrine by the presynaptic adrenergic nerve terminals.

In addition to α-adrenergic receptors blockade, it also causes blockade of histamine (H1), acetylcholine and serotonin receptors.

Actions:
It inhibits vasoconstriction caused by catecholamine resulting in decrease of total peripheral resistance and blood pressure.

It decreases blood pressure in upright position or as a result of reduced blood volume. In these situations sympathetic tone is elevated.

It causes an elevation of cardiac output as a result of reflex effects and by some blockade of presynaptic α2 receptors present in cardiac sympathetic nerves.

Pharmacokinetics:
It is well absorbed orally. It has plasma half life of about 12 hours.

Therapeutic uses:
They are used for regulating hypertension in Pheochromocytoma.

It is also used to relieve spasm of the vessels in Raynaud’s phenomenon.

It is used to alleviate urinary blockage in patients of spinal cord injury and prostatic hypertrophy (by partial reversal of contraction of the smooth muscles)

It is also used to regulate autonomic hyperreflexia caused by spinal cord transaction.

Adverse effects:
It may cause fatigue, sedation, miosis, nasal stuffiness, nausea and vomiting local tissue irritation by injection and hyperventilation.

Dosage:
It is given orally in the dose of 10-20 mg /day; The dose may be increased up to 200 mg, gradually.
It is also given by IV infusion in the dose of 1 mg/kg diluted in 5% dextrose or 0.9% saline.