Phentolamine

 ( Alpha-1 Blockers ) 

Phentolamine, sold under the brand name Regitine among others, is a reversible

nonselective α-adrenergic antagonist.

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Mechanism Its primary action is vasodilation due to α₁ blockade.

Non-selective α-blockers can cause a much more pronounced reflex tachycardia than the selective α₁ blockers. Like the selective α₁ blockers, phentolamine causes a relaxation of systemic vasculature, leading to hypotension. This hypotension is sensed by the Baroreflex, which results in increased sympathetic nerve firing on the heart, releasing norepinephrine. In response, the β₁ adrenergic receptors on the heart increase their Heart rate, contractility, and Dromotropic, which help to offset the decrease in systemic blood pressure. Unlike the α₁ selective blockers, phentolamine also inhibits the α₂ receptors, which function predominantly as Chemical synapse negative feedback for norepinephrine release. By abolishing this negative feedback phentolamine leads to even less regulated norepinephrine release, which results in a more drastic increase in heart rate.

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Uses The primary application for phentolamine is for the control of hypertensive emergencies, most notably due to pheochromocytoma.

It also has usefulness in the treatment of cocaine-induced cardiovascular complications, where one would generally avoid Beta blocker (e.g. metoprolol), as they can cause unopposed α-adrenergic mediated coronary vasoconstriction, worsening myocardial ischemia and hypertension. Phentolamine is not a first-line agent for this indication. Phentolamine should only be given to patients who do not fully respond to , nitroglycerin, and calcium channel blockers.

When given by injection, it causes blood vessels to vasodilation, thereby increasing blood flow. When injected into the penis (intracavernosal), it increases blood flow to the penis, which results in an erection.

It may be stored in to counteract severe peripheral vasoconstriction secondary to extravasation of peripherally placed vasopressor infusions, typically of norepinephrine. Epinephrine infusions are less vasoconstrictive than norepinephrine as they primarily stimulate β receptor more than α receptors, but the effect remains dose-dependent.

Phentolamine also has diagnostic and therapeutic roles in complex regional pain syndrome (CRPS).

Phentolamine is marketed in the dental field as a local anesthetic reversal agent. Branded as OraVerse, it is a phentolamine mesylate injection designed to reverse the local vasoconstrictor properties used in many local anesthetics to prolong anesthesia.

Phentolamine is also used ophthalmically under the brand name Ryzumvi to reverse the effects of pharmacologically-induced mydriasis.

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Chemistry Phentolamine can be synthesized by alkylation of 3-(4-methylanilino)phenol using 2-chloromethylimidazoline:

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Adverse effects

• Orthostatic hypotension.

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