Urinary excretion of chlorpheniramine and its metabolites in children

The pharmacokinetics and urinary excretion of chlorpheniramine were studied in 11 patients, aged 6-16 years, with allergic rhinitis. In these children, chlorpheniramine had a mean elimination half-life of 13.1 +/- 6.6 h, a mean clearance rate of 7.23 +/- 3.16 mL/min/kg, and a mean apparent volume of distribution of 7.0 +/- 2.8 L/kg. Over 48 h, the recovery in urine was as follows: chlorpheniramine, 11.3 +/- 6.7%; demethylchlorpheniramine , 23.3 +/- 11.1%; and didemethylchlorpheniramine , 9.6 +/- 9.4%. Urine flow rate and urine pH were uncontrolled and ranged from 2.2 to 113.3 mL/h and 5.1-7.9, respectively, over the 48-h period. In some children urine flow rate and pH were constant, while in others there was great variability. When drug and metabolite excretion rates versus both urine flow rates and pH values were analyzed by multiple linear regression, the results were significantly better (p less than or equal to 0.05) than when each factor was analyzed independently. The excretion rate of chlorpheniramine and its two demethylated metabolites decreased as urine pH increased and urine flow rate decreased. This information must be considered in future pharmacokinetic studies of this drug.