Unihist Mechanism of Action

Pharmacotherapeutic group: Antihistamine (H₁-Receptor)/Decongestant.
Pharmacology: Antihistamine (H₁-receptor): Antihistamine used in the treatment of allergy act by competing with histamine for H₁-receptor sites on effector cells. They thereby prevent, but do not reverse, responses mediated by histamine alone. The anticholinergic actions of most antihistamines provide a drying effect on the nasal and oral mucosa.
Decongestant: Sympathomimetic amines act on alpha-adrenergic receptors in the mucosa of the respiratory tract to produce vasoconstriction, which temporarily reduces the swelling associated with inflammation of the mucous membranes lining the nasal passages.
Pharmacokinetics: Absorption: Phenylephrine has reduced bioavailability (about 38%) from gastrointestinal tract because of first pass metabolism by monoamine oxidase in the stomach and liver.
Biotransformation: Antihistamines - Hepatic (cytochrome P-450 system); some renal.
Sympathomimetic amines - Phenylephrine: Extensive in the intestinal wall and in the liver. Sulfate conjugates are formed largely in the intestinal wall. Also, undergoes oxidative deamination by monoamine oxidase.
Half-life: Brompheniramine - 25 hours; Phenylephrine - 2.1 to 3.4 hours.
Onset of action: Most first generation antihistamines: 15 to 60 minutes.
Time to peak concentration: Brompheniramine - 2 to 5 hours; Phenylephrine - 0.75 to 2 hours (to achieve peak levels ranging from 0.9 to 298 ng/mL, respectively).
Time to peak effect: Brompheniramine - 3 to 9 hours
Elimination: Antihistamines - Renal. Most of the antihistamines studied are excreted as metabolites within 24 hours; Phenylephrine: 2.6% of the administered oral dose is excreted unchanged; 80 to 86% of unchanged Phenylephrine and metabolites are recovered in 48 hours after oral administration.