Cetirizine | Antihistamine | CAS 83881-51-0 | H₁ Blocker

Product Name: Cetirizine
CAS No.: 83881-51-0 (acid form)
Chemical Name: (±)-[2-[4-[(4-chlorophenyl)phenylmethyl]-1-piperazinyl]ethoxy]acetic acid
Molecular Formula: C₂₁H₂₅ClN₂O₃
Appearance: White or almost white crystalline powder
Solubility: Cetirizine dihydrochloride is freely soluble in water; practically insoluble in acetone and methylene chloride
Stability: Relatively stable when stored
Common salt: Dihydrochloride

1. Introduction

Cetirizine is a second-generation antihistamine, a selective H₁ histamine receptor blocker. It has anti-allergic effects, reducing itching, swelling, and rhinorrhea. It does not cause pronounced sedative effects at therapeutic doses.

Allergic diseases such as allergic rhinitis, conjunctivitis, and urticaria significantly reduce the quality of life of millions of people. Cetirizine, being an active metabolite of hydroxyzine, belongs to second-generation antihistamines. It effectively blocks histamine action at H₁ receptors, thereby reducing allergy symptoms, and has significantly lower sedative and anticholinergic potential compared to first-generation drugs.

2. Chemical Structure & Synthesis

Cetirizine is a carboxylated metabolite of hydroxyzine.

Synthesis: Cetirizine can be obtained by oxidation of the hydroxyzine side chain (e.g., using potassium permanganate or other oxidants) to a carboxyl group. The starting hydroxyzine is synthesized from 1-(4-chlorobenzhydryl)piperazine and 2-(2-chloroethoxy)ethanol.

3. Mechanism of Action

Cetirizine is a selective antagonist of peripheral H₁ histamine receptors. It competitively blocks histamine binding to these receptors, preventing the development of histamine-mediated effects:

• Capillary dilation and increased permeability

• Tissue edema

• Itching

• Smooth muscle contraction of bronchi and intestines (partially)

Cetirizine also has some anti-inflammatory effects, suppressing eosinophil migration. At therapeutic doses, it practically does not cross the blood-brain barrier, therefore it does not cause pronounced sedative effects and has no significant anticholinergic activity.

4. Pharmacokinetics

Rapidly absorbed from the gastrointestinal tract. Peak plasma concentration is reached in approximately 1 hour. Food intake does not significantly affect the extent of absorption but may slightly slow its rate. High plasma protein binding (approximately 93%). Virtually not metabolized in the body. Excreted primarily unchanged by the kidneys. Half-life is approximately 7–10 hours.

5. Applications

• Symptomatic treatment of seasonal and perennial allergic rhinitis and conjunctivitis – itching, sneezing, rhinorrhea, tearing, conjunctival hyperemia

• Chronic idiopathic urticaria – reduction of itching and rashes

• Allergic dermatoses accompanied by itching and rashes

• Hay fever (pollinosis)

• Angioneurotic edema (Quincke's edema) – as part of combination therapy

6. Conclusion

Cetirizine is an effective and well-tolerated second-generation antihistamine widely used to relieve symptoms of various allergic conditions. Its selectivity for H₁ receptors and low ability to cross the blood-brain barrier minimize the risk of sedative effects, making it convenient for use in active patients. Its long duration of action allows once-daily dosing.