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 will inhibit certain effects produced by exogenous histamine. This activity appears rapidly. Cetirizine also inhibits the effects produced by endogenous histamine released in vivo by an agent.
It inhibits the cutaneous reaction induced by VIP (Vasoactive Intestinal Polypeptide) and substance P, neuropeptides which are believed to take part in the allergic reaction.
Cetirizine inhibits the histamine-mediated "early" phase of the cutaneous allergic reaction. It also significantly reduces the migration of inflammatory cell such as eosinophils and the release of mediators associated with the "late" cutaneous allergic response.
Cetirizine markedly reduces bronchial hyperreactivity to histamine in the asthmatic patient. It also reduces the allergic reaction induced by specific allergens. These effects are obtained without any central effects being demonstrated either by psychometric tests or by a quantified EEG.
Peak blood levels are reached within one hour after oral administration of cetirizine. The plasma half-life is approximately 10 hours in adults, 6 hours in children aged 6 to 12 years, and 5 hours in children aged 2 to 6 years. These data are consistent with the urinary excretion half-life to the drug. The cumulative urinary excretion represents about two thirds of the dose given in both adults and children. Consequently, the apparent plasma clearance in children is higher than that measured in adults.
Pharmacokinetics: Absorption is very consistent from on subject to the next.
Plasma levels are linearly related to the dose given.
Cetirizine is strongly bound to plasma proteins.