Phenoxybenzamine

Phenoxybenzamine is a drug with the molecular formula C18H22ClNO and a molar mass of 303.83 g-mol-1, which belongs to the group of alpha blockers (α sympatholytics; α receptor antagonists). Phenoxybenzamine has a peripheral vasodilating effect and is used as an antihypertensive agent in pheochromocytoma. Phenoxybenzamine is the only substance that inhibits_α1 and_{α2 receptors} in therapeutic doses (Graefe et al. KH 2016).

Phenoxybenzamine exerts its effect in the arterial vessels by irreversible and non-selective blockade of α1-adrenoceptors. This causes the peripheral arterial vessels to dilate, vascular resistance decreases and afterload and blood pressure decrease. These effects are enhanced by a central attenuating effect due to a blockade of postsynaptic α1-adrenoceptors and presynaptic_{α2-adrenoceptors}via a reduction of sympathetic activity. The consequences include vasodilatation and increased gastrointestinal activity.

The positive effect on bladder voiding disorders as a result of an increased α-adrenergic tone of the sphincter also results from the alpha-receptor blockade. As a result, the outlet resistance of the bladder is reduced and urination is normalised.

Urinary bladder emptying disorders: In the case of neurogenic bladder emptying disorders, phenoxybenzamine is taken by adults for a short period of time at an initial dose of 10 mg/day. The dosage can be slowly increased by 10 mg every 4-7 days. The maximum daily dose is 60 mg daily, divided into two to three single doses.

Preoperative control of blood pressure in pheochromocytoma: Phenoxybenzamine therapy is started one to three weeks before the planned procedure with an initial dose of 10 mg/day. The daily dose can be increased up to 100 mg/day.

Side effects can occur especially at the beginning of the treatment. The most serious side effect characteristic of alpha-receptor blockers is orthostatic hypotension. Side effects by frequency:

Very common:

• dizziness, light-headedness
• orthostatic hypotension
• Miosis
• Reflex Tachycardia
• Swelling of the nasal mucosa.

Frequently:

• pronounced hypotension.

1. Graefe KH et al. Autonomic Nervous System. In: Graefe KH et al. (Eds) Pharmacology and Toxicology. Georg Thieme Publisher Stuttgart S.97