Enantiomer-selective pharmacokinetics and metabolism of ketorolac in children

OBJECTIVE: To compare the pharmacokinetics and metabolism of R(+)- and S(-)- ketorolac in children. METHODS: Children from 3 to 18 years old received 0.6 mg/kg racemic ketorolac intravenously. Serial blood samples were obtained for 12 hours, and urine was collected for 12 to 24 hours. Racemic ketorolac was measured in plasma, and racemic ketorolac, para-hydroxyketorolac, and ketorolac glucuronide were measured in urine by HPLC. S(-)- and R(+)-ketorolac were measured in plasma; S(-)- and R(+)-ketorolac and ketorolac glucuronide were measured in urine by chiral HPLC separation. Plasma pharmacokinetic parameters for racemic drug and both enantiomers were determined for each patient. RESULTS: Clearance of racemic ketorolac in children was approximately 2 times the clearance reported in adults. Clearance of the S(-) enantiomer was 4 times that of the R(+) enantiomer. Terminal half-life of S(-)-ketorolac was 40% that of the R(+) enantiomer, and the apparent volume of distribution of the S(-) enantiomer was greater than that of the R(+) form. Recovery of S(-)-ketorolac glucuronide was 2.3 times that of the R(+) enantiomer. CONCLUSION: The higher clearance in children suggests that the weight-adjusted dose of ketorolac may have to be greater for children to achieve plasma concentrations comparable to those of adults. Because of the greater clearance and shorter half-life of S(-)-ketorolac, pharmacokinetic predictions based on racemic assays may overestimate the duration of pharmacologic effect. Enantiomeric pharmacokinetic differences are best explained by stereoselective plasma protein binding. Selective glucuronidation of the S(-) enantiomer suggests that stereoselective metabolism may also be a contributing factor.