Fluoxetine Pharmacokinetics in Pediatric Patients

The objective of this study was to evaluate the pharmacokinetic profile of fluoxetine (FLX) and its major metabolite, norfluoxetine (NORFLX), in children and adolescent patients undergoing psychiatric treatment. Twenty-one pediatric subjects--10 children (6-12 years) and 11 adolescents (13-18 years)--were administered 20 mg FLX for 60 days, with sparse blood samples taken throughout the open-label study. Subjects contributed 168 plasma concentrations. Pharmacokinetic parameters were estimated using a mixed effects nonlinear model. Mean steady-state FLX and NORFLX of 127 ng/mL and 151 ng/mL, respectively, were achieved in children and adolescents after 4 weeks of treatment, with high between-patient variability. FLX was 2-fold higher and NORFLX was 1.7-fold higher in children relative to adolescents; however, when normalized to body weight, FLX and NORFLX were similar for both age groups. Age, body weight, body mass index, and body surface area, modeled independently as continuous variables, significantly improved the population pharmacokinetic model when evaluated as patient factors. Body weight was the covariate retained in the final model. In conclusion, children have 2-fold higher FLX and NORFLX relative to adolescents that appear to be related to indices of body size. The accumulation profile and steady-state concentrations in adolescents appear similar to those in adults.