Pharmacokinetics of Belimumab in Children With Systemic Lupus Erythematosus

The phase 2 placebo‐controlled, double‐blind PLUTO trial characterized the pharmacokinetics of belimumab plus standard systemic lupus erythematosus (SLE) therapy in patients with childhood‐onset SLE (cSLE) and demonstrated similar efficacy and safety to that in adult SLE. Patients with active cSLE aged 5‐17 years were randomized to intravenous belimumab 10 mg/kg every 4 weeks (n = 53). A linear 2‐compartment population pharmacokinetics (popPK) model with first‐order elimination was developed, and an exploratory exposure‐response analysis assessed the impact of between‐patient exposure variability on clinical response (SLE Responder Index 4 [SRI4]) in week 52, and occurrence of serious adverse events during the study. The popPK model estimated clearance of 158 mL/day, steady‐state volume of distribution of 3.5 L, terminal half‐life of 16.3 days, and distribution half‐life of 0.8 days in the overall population. Fat‐free mass (FFM) better characterized the pharmacokinetics than total body weight and was more consistent with allometric scaling theory; belimumab pharmacokinetics were largely determined by FFM. Age, sex, disease activity, and concomitant medication had no impact on pediatric belimumab exposure after accounting for body size. Individual and median steady‐state pediatric pharmacokinetic profiles were similar to known adult profiles and pediatric exposure estimates for belimumab 10 mg/kg intravenously were consistent with adult exposures. Exposures were similar between SRI4 responders and nonresponders, and patients who did or did not experience a serious adverse event. There was no clinically relevant correlation between exposure and efficacy or safety, confirming belimumab 10 mg/kg intravenous dose every 4 weeks as appropriate for pediatric patients with cSLE.