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Pharmacology: Cetirizine hydrochloride is a second generation, long-acting antihistamine. The drug has been characterized as a selective, peripheral H1-receptor antagonist. It is the carboxylic acid metabolite of hydroxyzine. The increased polarity may decrease distribution of the drug into the central nervous system (CNS), resulting in reduced potential for adverse CNS effects compared with some first generation antihistamine.
Pharmacokinetics: Cetirizine hydrochloride is rapidly absorbed from the gastrointestinal tract. Following oral administration of 10- or 20- mg doses of drug in healthy adult, peak plasma concentrations of 257-384 or 580 ng/ml, respectively, are achieved in about 1 hour. The antihistaminic effect of the drug (as measured by suppression of the wheal and flare response induced by intradermal injection of histamine) was apparent within 20 and 60 minutes in 50 and 95% of individuals, respectively, and persisted for about 24 hours.
Distribution of drug and its metabolites into human body tissues and fluids has not been fully elucidated. The substantial polarity of drug apparently limits distribution of the drug into the CNS. Drug is distributed into milk in humans and animals, approximately 93% bound to plasma proteins; protein binding appears to be independent of the concentration of the drug ranging from 25-1000 ng/ml, which includes usual therapeutic plasma concentrations.
Elimination may undergo biphasic elimination with an initial distribution half-life of about 3 hours and mean terminal elimination half-life about 8.3 hours (ranges: 6.5-10 hours). About 80% of the dose is excreted within 5 days, mainly (more than 50%) as unchanged drugs; most excretion occurs within 24 hours.
