- Phenylephrine is a selective α<sub>1</sub>-adrenergic receptor agonist commonly used as a decongestant, vasopressor, and mydriatic agent.
- It works by stimulating α<sub>1</sub>-receptors, leading to vasoconstriction, which reduces nasal congestion and increases blood pressure.
Chemical Structure & Formula
- Chemical Formula: C₉H₁₃NO₂
- Structure:
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Mechanism of Action (Detailed)
- Selective α1 agonist → Causes vasoconstriction.
- Increases blood pressure without affecting β receptors significantly.
- No direct effect on the heart but may cause reflex bradycardia.
Structure-Activity Relationship (SAR)
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Aromatic Ring Substitution:
- Only one hydroxyl group (at meta position): makes it selective for α1
- Lack of 3’,5’-dihydroxy (as in norepinephrine) reduces β-activity and CNS penetration.
-
Hydroxyl Group at β-carbon (ethanolamine chain):
- Essential for direct agonist activity, forms H-bonds with adrenergic receptors.
-
Amine Substitution:
- Primary or secondary amine: small groups (like methyl) favor α-activity.
- Bulky N-substituents shift selectivity toward β-receptors, but phenylephrine has minimal substitution → favors α1.
-
Stereochemistry:
- The (R)-enantiomer is more potent.
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Synthesis of Phenylephrine
Side Effects of Phenylephrine
- Hypertension
- Reflex bradycardia
- Headache
- Rebound congestion (if used as a nasal spray)
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Clinical Uses of Phenylephrine
- Hypotension treatment (e.g., during anesthesia).
- Nasal decongestant (vasoconstricts nasal mucosa).
- Mydriatic agent (used in eye exams to dilate pupils).
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