Safety and efficacy of apitegromab in nonambulatory type 2 or type 3 spinal muscular atrophy (SAPPHIRE): a phase 3, double-blind, randomised, placebo-controlled trial.

BACKGROUND: Approved spinal muscular atrophy therapies greatly improve clinical outcomes; however, substantial motor function deficits persist. Apitegromab, a fully human monoclonal antibody, selectively inhibits myostatin activation, improving muscle function. We aimed to assess the safety and efficacy of apitegromab in patients with nonambulatory type 2 or type 3 spinal muscular atrophy receiving nusinersen or risdiplam. METHODS: SAPPHIRE, a double-blind, placebo-controlled, phase 3 trial, was done in 48 hospitals in Belgium, France, Germany, Italy, Poland, Spain, the Netherlands, the UK, and the USA. Eligible participants were aged 2-21 years, had genetically documented SMN-deficient nonambulatory type 2 or type 3 spinal muscular atrophy, an estimated life expectancy greater than 2 years, Hammersmith Functional Motor Scale-Expanded (HFMSE) scores 10-45, and had received at least 10 months' nusinersen or at least 6 months' risdiplam therapy at screening. Participants aged 2-12 years were randomly assigned 1:1:1 to receive apitegromab 20 mg/kg, apitegromab 10 mg/kg, or placebo every 4 weeks; participants aged 13-21 years were randomly assigned 2:1 to receive apitegromab 20 mg/kg or placebo every 4 weeks. All participants, parents or caregivers, investigators, and site personnel were unaware of the treatment assignment. The primary endpoint, change from baseline in HFMSE at 12 months, was assessed in participants aged 2-12 years who received at least one dose of apitegromab or placebo and had at least one post-baseline evaluable HFMSE assessment (modified intention-to-treat set). Comparisons of the combined apitegromab dose (20 mg/kg and 10 mg/kg) versus placebo and the 20 mg/kg dose versus placebo were done with a mixed-effects model with repeated measurement. Safety was analysed in all participants who received at least one dose of apitegromab or placebo through evaluation of adverse events, physical examinations, vital signs and cardiac assessments, laboratory evaluations, and concomitant medications. SAPPHIRE is registered with ClinicalTrials.gov, NCT05156320, and is completed. FINDINGS: From March 28, 2022, to Sept 4, 2024, we enrolled 188 patients (156 in the population aged 2-12 years and 32 in the population aged 13-21 years); of whom 128 participants received apitegromab and 60 participants received placebo. At 12 months, least squares mean difference in HFMSE change from baseline was 1·8 (95% CI 0·30 to 3·32, p=0·019) points for participants aged 2-12 years receiving apitegromab versus placebo (least squares mean 0·6 vs -1·2). Least squares mean difference in HFMSE change from baseline was 1·4 (95% CI -0·34 to 3·13; p=0·11) for apitegromab 20 mg/kg versus placebo (least squares mean 0·2 vs -1·2). The incidence and severity of adverse events were similar between apitegromab and placebo, and consistent with spinal muscular atrophy and background spinal muscular atrophy therapy. The most frequently reported adverse events were pyrexia (apitegromab, 33 [26%] of 128 vs placebo, 17 [28%] of 60), nasopharyngitis (32 [25%] vs 14 [23%]), cough (30 [23%] vs 12 [20%]), vomiting (29 [23%] vs ten [17%]), upper respiratory tract infection (28 [22%] vs 18 [30%]), and headache (27 [21%] vs 12 [20%]). No patients discontinued due to adverse events. INTERPRETATION: Participants in the apitegromab treatment groups (combined 20 mg/kg and 10 mg/kg dose) achieved statistically significant improvements in motor function compared with placebo; however, the least squares mean difference was not significant between apitegromab 20 mg/kg and placebo. Overall, SAPPHIRE results build on findings from the phase 2 TOPAZ trial, showing improved motor function with a generally well tolerated safety profile, supporting the use of muscle-targeting therapy for spinal muscular atrophy. FUNDING: Scholar Rock.