Pharmacokinetic Variability of Rufinamide and Stiripentol in Children with Refractory Epilepsy: A Retrospective Study of Therapeutic Drug Monitoring from the National Epilepsy Centers in Denmark and Norway

Background: Rufinamide and stiripentol, orphan drugs used in Lennox–Gastaut and Dravet syndromes, respectively, are antiseizure medications (ASMs), often administered to children; however, pharmacokinetic studies are lacking. The authors compared the pharmacokinetic variability of these drugs with respect to the dose, serum concentrations, comedication, age, and duration of treatment. Methods: Children and adolescents (<18 years) whose serum concentrations were measured were retrospectively identified from the therapeutic drug monitoring (TDM) databases at 2 national epilepsy centers in Norway and Denmark (2012–2021). Results: Data from 165 patients (56% boys/44% girls) treated with rufinamide and 52 patients (50% boys/50% girls) treated with stiripentol were included. For rufinamide, the median age was 10 (range 2–17) years, dose 23 (3–73) mg/d, and serum concentration 34 (3–227) µmol/L [8.1 mg/L (0.71–54.0 mg/L)]. For stiripentol, the median age was 8.5 (range 1–17) years, dose 37 (18–76) mg/d, and serum concentration 33 (4–113) µmol/L [7.7 mg/L (0.93–26.3 mg/L)]. The concomitant use of 1–9 other ASMs during the data collection was noted. Pharmacokinetic variability, calculated as the concentration/(dose/kg) ratio, ranged from 0.26 to 11.31 (µmol/L)/(mg/kg) for rufinamide and 0.17–1.52 (µmol/L)/(mg/kg) for stiripentol. The intraindividual coefficients of variation ranged widely, from 5% to 110% for rufinamide and 11%–117% for stiripentol. The treatment period was at least 5 years in 50% of patients. No statistically significant effects of age, sex, or ASM comedication were observed, possibly due to the small sample size and heterogeneous groups with variable seizure situations, comorbidities, and changes in comedication and physiology. Conclusions: This study demonstrates considerable pharmacokinetic variability in and between patients for both drugs and similar use in terms of age, burden of comedication and retention rates. TDM may be useful in the clinical setting to monitor and optimize treatment in this vulnerable patient group.