Tricyclic ring system or phenothiazines

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Chapter: Medicinal Chemistry : Antihistamines

Antihistamines : H1-antagonists with classical structure : Tricyclic ring system or phenothiazines : 1. Promethazine HCl (Phenargen) 2. Trimeprazine (Temaril) 3. Methdilazine (Tacaryl HCl) - Synthesis and Drug Profile


H1-antagonists with classical structure 

Tricyclic ring system or phenothiazines  

1. Promethazine HCl (Phenargen)


Properties and uses: Promethazine hydrochloride is a white or faintly yellowish crystalline powder, highly soluble in water, soluble in alcohol and in methylene chloride. It may be used effectively in perennial and seasonal allergic rhinitis, vasomotor rhinitis, allergic conjunctivitis due to inhalant allergens and foods, and certain milder type of skin manifestations of urtricaria. It also possesses some anticholinergic, antiserotonergic, and marked local anaesthetic properties.

Assay: Dissolve the sample in a mixture of 0.01 M hydrochloric acid and alcohol and titrate against 0.1 M sodium hydroxide. Determine end point potentiometrically.

Dose: Usual dose is 20–50 mg per day.

Dosage forms: Promethazine hydrochloride injection I.P., Promethazine hydrochloride tablets I.P., B.P., Promethazine hydrochloride syrup I.P., Promethazine injection B.P., promethazine oral solution B.P.


2. Trimeprazine (Temaril)


Properties and uses: It is a white crystalline powder, soluble in water. It is used as histamine H1-receptor antagonist.

Dose: Usual dose for adults is 10–40 mg per day orally.


3. Methdilazine (Tacaryl HCl)


Properties and uses: It may be used for the symptomatic relief of urtricaria. It has also been used successfully for the treatment of migraine headache.

Dose: Usual dose for adults is 8 mg taken orally two to four times a day.

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