Pharmacology: Pharmacodynamics: In experimental animals, Cetirizine has been shown to be an anti-H1 agent devoid of any significant anticholinergic or antiserotonin effects. At pharmacologically active doses, it induces neither sedation nor behavioral changes. This may be explained by the fact that Cetirizine does not cross the blood-brain barrier.
It was shown in human pharmacology studies that Cetirizine would inhibit certain effects produced by endogenous histamine released in vivo by an agent, e.g. 48/80.
Finally, it inhibits the cutaneous reaction induced by VIP (Vasoactive Intestinal Polypeptide) and substance P, neuropeptides that are believed to take part in the allergic reaction. Cetirizine markedly reduces bronchial hyper-reactivity to histamine in the asthmatic patients. It also reduces the allergic reaction induced by specific allergens.
These effects are obtained without any central effects being demonstrated either by psychometric test or by a quantified EEG.
Pharmacokinetics: Peak blood levels of the order of 0.3 ug/ml are reached between 30 and 60 minutes after administration of a 10 mg dose of Cetirizine. Its plasma half - life is approximately 11 hours.
Absorption is very consistent from one subject to the next. Its renal clearance is 30 ml/minute and the excretion half-life is approximately 9 hours. Cetirizine is strongly bound to plasma proteins.