Mechanism of action
To understand how the drug works, we need to recall the physiology of histamine receptors. They belong to the class of G protein-coupled receptors. Normally, when the allergy mediator (histamine) binds to the $H_1$ receptor, the enzyme phospholipase C is activated. This leads to a sharp increase in intracellular calcium ion ($Ca^{2+}$) concentration. The calcium signal triggers a cascade of reactions: smooth muscle contracts and vascular wall permeability increases.
Cetirizine (also known by the brand name Zyrtec) acts as a highly selective blocker. It selectively binds specifically to $H_1$ receptors, preventing histamine from triggering the intracellular cascade described above. It is important to note the high specificity of the drug: it predominantly turns off the $H_1$ pool responsible for allergy and has virtually no effect on other types of histamine receptors (e.g., $H_2$).
Pharmacological effects
As an agent affecting immune processes and suppressing inflammation, cetirizine exerts a potent antiallergic effect. By blocking the receptors, it prevents the development of histamine-mediated effects:
- Prevents smooth muscle spasm.
- Stops the increase in vascular permeability, thereby preventing tissue edema.
The main pharmacodynamic difference from first-generation drugs (such as chloropyramine, diphenhydramine, or clemastine) is the absence of a pronounced sedative effect. Second-generation drugs poorly cross the blood-brain barrier (BBB), thus causing no drowsiness or CNS depression. Furthermore, they have a longer duration of action.
Indications for use
The drug is prescribed to relieve allergy symptoms whose pathogenesis is driven by histamine release and $H_1$ receptor activation. Unlike mast cell membrane stabilizers (e.g., cromoglicic acid), which prevent the release of mediators themselves, cetirizine works "on the frontline" by blocking target organ receptors once histamine has already been released.
Prescribing specifics and formulation
The drug is administered orally. Food intake does not affect its efficacy. Tablets should be swallowed whole without chewing. Traditionally, the drug is recommended to be taken at bedtime.
Dosing regimens:
- Tablets: 1 tablet (10 mg or 0.01 g) once daily at bedtime.
- Syrup: 1 dessert spoonful once daily at bedtime.
Prescription examples:
For tablets: ```latin Rp.: Tab. Cetirizini 0,01 D.t.d. N. 10 S. Take orally, 1 tablet at bedtime, without chewing. ```
For syrup: ```latin Rp.: Sir. Cetirizini 1 mg/ml - 100 ml D.S. Take orally, 1 dessert spoonful at bedtime. ```
Briefly: answers to topical questions
For which specific allergic diseases is cetirizine indicated?
Cetirizine is used to treat a number of allergic conditions accompanied by itching, erythema, and edema.
- Allergic rhinitis — indicated for patients, including those undergoing tuberculosis chemotherapy.
- Allergic conjunctivitis — used when allergic inflammation of the conjunctiva occurs.
- Acute urticaria — prescribed for long-term outpatient use after acute exacerbation has been relieved.
What are the contraindications for the use of cetirizine?
According to the cited sources, restrictions on the use of cetirizine in children are as follows:
- Children under 6 months — contraindicated.
- Children under 6 years — the drug should be used strictly in the form of oral drops.
- Children aged 6 to 12 months — use is possible only under medical prescription and strict medical supervision.
Cetirizine is TARGETED (highly selective for $H_1$), works ALL day long (unlike the short-acting 1st generation), and doesn't put the brain to sleep (does not cross the BBB).
Class and exam questions
How does cetirizine fundamentally differ from diphenhydramine or chloropyramine?
Cetirizine is a second-generation drug. It is highly selective for $H_1$ receptors, practically does not cross the BBB (hence no sedative effect), and has a long duration of action, whereas first-generation drugs are short-acting and cause drowsiness.
How does the receptor blocked by this drug normally function?
The $H_1$ receptor is G protein-coupled. When binding to histamine, it activates phospholipase C, leading to an increase in intracellular $Ca^{2+}$, causing smooth muscle spasm and increased vascular permeability.
Does cetirizine block all histamine receptors in the body?
No. Antihistamines of this group have high specificity and predominantly block $H_1$ receptors involved in the allergic response.
What else to study on this topic
- Comparative pharmacokinetics of first- and second-generation antihistamines.
- The role of G proteins and phospholipase C in intracellular signal transduction.
- Other second-generation agents: loratadine and desloratadine.
- Alternative targets in allergy: mast cell membrane stabilizers and leukotriene inhibitors.
- Use of first-generation antihistamines in combination drugs (e.g., with interferon alfa-2a).
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