Phenylephrine

Absorption

Phenylephrine is 38% orally bioavailable. Clinically significant systemic absorption of ophthalmic formulations is possible, especially at higher strengths and when the cornea is damaged.

Toxicity

Patients experiencing and overdose may present with headache, hypertension, reflex bradycardia, tingling limbs, cardiac arrhythmias, and a feeling of fullness in the head. Overdose may be treated by supportive care and discontinuing phenylephrine, chronotropic medications, and vasodilators. Subcutaneous phentolamine may be used to treat tissue extravasation.

Description

This synthetic drug has both chemical and pharmacological similarities to norepinephrine. A characteristic quality of phenylephrine is the distinctly expressed selectivity to α- adrenoreceptors, especially α1-adrenoreceptors. Although phenylephrine increases the contractibility of blood vessels, in practical terms it is not considered a cardiostimulant.

The Uses of Phenylephrine

L-Phenylephrine is an adrenergic α1A receptor agonist (Ki = 1.4 μM) that demonstrates selectivity against the α1B and α1C receptor subtypes (Kis = 23.9 and 47.8 μM, respectively). By stimulating adrenergic α1 receptors, L-phenylephrine can induce aortic smooth muscle contractions, although reported relative affinity and potency values in rabbit are 5-fold weaker compared to that of L-norepinephrine. This compound is frequently used to precontract smooth muscle in preparations designed to study the properties of various vasodilator agents. Because L-phenylephrine acts on adrenergic α1 receptors in the arterioles of the nasal mucosa to produce constriction, it has been examined clinically as an oral decongestant.

Background

Phenylephrine is an alpha-1 adrenergic receptor agonist used to treat hypotension, dilate the pupil, and induce local vasoconstriction. The action of phenylephrine, or neo-synephrine, was first described in literature in the 1930s.
Phenylephrine was granted FDA approval in 1939.

Indications

Phenylephrine is available in various drug formulations, which have different indications. Phenylephrine injections are indicated to treat hypotension caused by shock or anesthesia. The ophthalmic formulation is indicated to induce mydriasis and conjunctival vasoconstriction. The intranasal formulation is used to treat congestion, and a topical formulation is used to treat hemorrhoids. Off-label uses include priapism and induction of local vasoconstriction.

What are the applications of Application

L-phenylephrine is an α1 adrenergic receptor agonist

Pharmacokinetics

Phenylephrine is an alpha-1 adrenergic agonist that raises blood pressure, dilates the pupils, and causes local vasoconstriction. Ophthalmic formulations of phenylephrine act for 3-8 hours while intravenous solutions have an effective half life of 5 minutes and an elimination half life of 2.5 hours. Patients taking ophthalmic formulations of phenylephrine should be counselled about the risk of arrhythmia, hypertension, and rebound miosis. Patients taking an intravenous formulation should be counselled regarding the risk of bradycardia, allergic reactions, extravasation causing necrosis or tissue sloughing, and the concomitant use of oxytocic drugs.

Metabolism

Phenylephrine is mainly metabolized by monoamine oxidase A, monoamine oxidase B, and SULT1A3. The major metabolite is the inactive meta-hydroxymandelic acid, followed by sulfate conjugates. Phenylephrine can also be metabolized to phenylephrine glucuronide.