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Pharmacology: Cetirizine, a human metabolite of Hydroxyzine, is a potent and selective antagonist of peripheral H1-receptors. In vitro receptor binding studies have shown no measurable affinity for other than H1-receptors.
Pharmacokinetics: Absorption: The steady-state peak plasma concentrations is approximately 300 ng/mL and is achieved within 1.0 ± 0.5 hour. The distribution of pharmacokinetic parameters such as peak plasma concentration (Cmax) and area under curve (AUC) is unimodal. The extent of absorption of Cetirizine is not reduced with food, although the rate of absorption is decreased. The extent of bioavailability is similar when Cetirizine is given as solutions, capsules or tablets.
Distribution: The apparent volume of distribution is 0.50 L/Kg. Plasma protein binding of Cetirizine is 93 ± 0.3%. Cetirizine does not modify the protein binding of Warfarin.
Biotransformation: Cetirizine does not undergo extensive first pass metabolism.
Elimination: The terminal half-life is approximately 10 hours and no accumulation is observed for Cetirizine following daily doses of 10 mg for 10 days. About two third of the dose are excreted unchanged in urine.
Linearity/Non-linearity: Cetirizine exhibits linear kinetics over the range of 5 to 60 mg.
Renal impairment: The pharmacokinetics of the drug was similar in patients with mild impairment (creatinine clearance higher than 40 mL/min) and healthy volunteers. Patients with moderate renal impairment had a 3-fold increase in half-life and 70% decrease in clearance compared to healthy volunteers.
Patients on hemodialysis (creatinine clearance less than 7 mL/min) given a single oral 10 mg dose of Cetirizine had a 3-fold increase in half-life and a 70% decrease in clearance compared to healthy volunteers. Cetirizine was poorly cleared by hemodialysis. Dosing adjustment is necessary in patients with moderate or severe renal impairment.
Hepatic impairment: Patients with chronic liver diseases (hepatocellular, cholestatic, and biliary cirrhosis) given 10 or 20 mg of Cetirizine as a single dose had a 50% increase in half-life along with a 40 % decrease in clearance compared to healthy subjects.
Dosing adjustment is only necessary in patients with hepatic impairment if concomitant renal impairment is present.
Elderly: Following a single 10 mg oral dose, half life increased by 50% and clearance decreased by 40% in 16 elderly subjects compared to the normal subjects. The decrease in Cetirizine clearance in these elderly volunteers appeared to be related to their decreased renal function.
Pediatric population: The half-life of Cetirizine was about 6 hours in children of 6-12 years and 5 hours in children 2-6 years. In infants and toddlers aged 6 to 24 months, it is reduced to 3.1 hours.
