A multicenter, open-label study evaluating safety and clinical outcomes in children (1.4–7.5 years) with Hunter syndrome receiving idursulfase enzyme replacement therapy

PURPOSE: The primary objective of this study was to determine the safety of idursulfase in Hunter syndrome patients aged 5 years or younger. METHODS: Idursulfase (0.5 mg/kg) was administered intravenously on a weekly basis (52 infusions per patient) in an open-label study. Safety monitoring included adverse events, anti-idursulfase antibodies, vital signs, physical examination, 12-lead electrocardiogram, concomitant medications or procedures, and laboratory testing (clinical chemistry, hematology, and urinalysis). The following exploratory efficacy outcomes were assessed at baseline and at weeks 18 or 36 or 53: urinary glycosaminoglycan levels, liver or spleen size, developmental milestones, and growth indices. Pharmacokinetic parameters were assessed at week 27. RESULTS: Twenty-eight boys aged 1.4–7.5 years were enrolled (one discontinued for noncompliance) in the study. All the patients reported adverse events (16 patients (57%) reported possibly or probably treatment-related adverse events). The only severe adverse event was sleep apnea (two patients); others were mild or moderate. Sixteen patients had infusion-related adverse events, a similar proportion as previously reported. Thirteen patients (46%) experienced at least one serious adverse event: pyrexia and bronchopneumonia were the most common (three patients each). No clinically important drug-related changes in laboratory parameters or vital signs or electrocardiograms were reported. Nineteen patients (68%) developed anti-idursulfase immunoglobulin G antibodies. Growth rates remained within normal age-related ranges. Developmental quotients were lower than normal but remained stable. By week 18, organ size and urinary glycosaminoglycan levels decreased as compared with baseline and remained stable throughout the study. CONCLUSION: Idursulfase safety, tolerability, and efficacy were similar to that previously reported in males ≥5 years.