Mechanism of Action
To understand how the drug works, we need to review 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, it activates the enzyme phospholipase C. This leads to a sharp increase in intracellular calcium ion ($Ca^{2+}$) concentration. The calcium signal triggers a cascade of reactions: smooth muscle contraction and increased vascular wall permeability.
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 shuts down predominantly the $H_1$ pool responsible for allergy and virtually does not affect other types of histamine receptors (e.g., $H_2$).
Cellular signaling pathway: $$\text{Histamine} + H_1 \text{ receptor} \xrightarrow{\text{G-protein}} \text{Phospholipase C} \uparrow \text{Ca}^{2+} \rightarrow \text{Smooth muscle contraction \& Edema}$$
Pharmacological Effects
As an agent affecting immune processes and suppressing inflammation, cetirizine exerts a potent anti-allergic effect. By blocking the receptors, it prevents the development of histamine-mediated effects:
- Prevents smooth muscle spasm.
- Halts the increase in vascular permeability, thereby preventing tissue edema.
Contrasting with first-generation drugs (such as chloropyramine, diphenhydramine, or clemastine), its main pharmacodynamic difference is the absence of a pronounced sedative effect. Second-generation drugs poorly cross the blood-brain barrier (BBB), thus causing no drowsiness or central nervous system depression. Additionally, they possess a longer duration of action.
Indications for Use
The drug is prescribed for relieving allergy symptoms whose pathogenesis is driven by histamine release and $H_1$ receptor activation. Unlike mast cell stabilizers (e.g., cromoglicic acid), which prevent the release of mediators itself, cetirizine works "on the front line" by blocking target organ receptors after histamine has already been released.
Administration and Prescription
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) once daily at bedtime.
- Syrup: 1 dessert spoonful (approx. 10 mL equivalent) 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. ```