Chemical Structure/Formula:
C9H13NO2
Mechanism of Action:
Phenylephrine primarily targets alpha-adrenergic receptors, causing vasoconstriction.
Unlike pseudoephedrine, it has minimal activity on the central nervous system.
Structure-Activity Relationship (SAR) of Phenylephrine
The structure-activity relationship (SAR) of a drug refers to the relationship between its chemical structure and its pharmacological activity. For phenylephrine, its SAR can be understood by examining its structural components:
1.Phenyl Ring:
The phenyl ring (a benzene ring) is essential for the activity. Substitution on this ring can alter the drug's affinity and selectivity for adrenergic receptors.
2.Hydroxyl Groups:
The presence of hydroxyl groups (-OH) on the benzene ring is crucial for agonistic activity on the alpha-adrenergic receptor. Specifically, the -OH group at the para position (4th position) is critical for the mimicking of natural catecholamines like norepinephrine.
3.Ethylamine Side Chain:
The ethylamine side chain (CH2CH2NH2) attached to the benzene ring is necessary for receptor binding. Modifications here can significantly affect both the potency and selectivity of the compound.
4.Lack of Beta-Hydroxyl Group:
Unlike epinephrine and norepinephrine, phenylephrine does not have a beta-hydroxyl group. This absence makes phenylephrine selective for alpha receptors, mainly alpha-1, and less active on beta receptors.
Synthesis of Phenylephrine
The synthesis of Phenylephrine can be outlined in a series of chemical reactions as follows:
Starting from m-Hydroxymandelic Acid:
m−Hydroxymandelic acid → m-Hydroxyphenylethanol
The reduction can be done using a reducing agent like lithium aluminum hydride (LiAlH4).
Amination:
m-Hydroxyphenylethanol → Phenylephrine
The reductive amination typically involves reacting the m-hydroxyphenylethanol with an amine (like ethylamine) in the presence of a reducing agent such as hydrogen gas with a catalyst or sodium borohydride (NaBH4).
Uses:
It is often used as a decongestant in cold medications and also in eye drops to dilate the pupil (mydriasis).
Side Effects:
Side effects may include headache, reflex bradycardia, excitability, and restlessness.