Apitegromab meets primary endpoint in phase III SAPPHIRE study in patients with spinal muscular atrophy (SMA)

The study achieved its primary endpoint demonstrating a statistically significant and clinically meaningful improvement for apitegromab versus placebo in motor function as measured by the gold standard HFMSE in SMA patients on chronic dosing of standard of care therapies (either nusinersen or risdiplam). Based upon the similar pharmacological profile of the 20 mg/kg and 10 mg/kg doses of apitegromab, the statistical analysis plan was prespecified to analyze both the combined dose (10 mg/kg and 20 mg/kg) compared to placebo, and 20 mg/kg dose each compared to placebo. Statistical significance is achieved per the prespecified statistical analysis plan (Hochberg multiplicity adjustment) where the p-value (≤0.025) is more rigorous if only one prespecified analysis crosses the statistical significance boundary of  less than 0.05.

i.  In the main efficacy population (ages 2-12), the mean difference in change from baseline in HFMSE was 1.8 points (p=0.0192) for all patients receiving apitegromab 10 mg/kg and 20 mg/kg (n=106) compared to placebo (n=50). Patients receiving 20 mg/kg of apitegromab (n=53) showed a 1.4 point mean difference compared to placebo (p=0.1149). 

ii. The prespecified analysis of the 10 mg/kg dose showed that patients receiving 10 mg/kg of apitegromab (n=53) showed an improvement of 2.2 points (nominal p=0.0121) compared to placebo.

• iii. Based upon PK/PD data from the SAPPHIRE trial, similar levels of target engagement were observed for the 10 mg/kg and 20 mg/kg dose groups.

Motor function outcomes were meaningful and consistent across the main efficacy population and in the ages 13-21 exploratory population, favored apitegromab (n=22) compared to placebo (n=10). Thirty percent of patients receiving apitegromab had  greater than 3 point improvement in HFMSE versus 12.5% of patients on placebo. Patients receiving apitegromab demonstrated early motor function improvement compared to placebo from the first measured time point at 8 weeks, benefit expanded at 52 weeks as measured by HFMSE. Following trial completion, 98 percent of SAPPHIRE patients (185/188) enrolled in the ongoing ONYX open-label expansion study.

Treatment with apitegromab was well-tolerated across all age groups. There were no clinically relevant differences in the adverse event profile by dose, 10 mg/kg versus 20 mg/kg. No new safety findings were observed in the SAPPHIRE clinical trial; the profile was consistent with that observed in the Phase II TOPAZ clinical trial, including an extension study which had over four years of treatment as of the cut-off date. Serious adverse events (SAEs) were consistent with the underlying disease and current standard of care received by patients; no SAEs were assessed as related to apitegromab. There were no study drug discontinuations due to adverse events.

The FDA has granted Fast Track, Orphan Drug, and Rare Pediatric Disease designations, and the European Medicines Agency (EMA) has granted Priority Medicines (PRIME) and Orphan Medicinal Product designations, to apitegromab for the treatment of SMA. The Company plans to submit a U.S. Biologics License Application (BLA) and a European Union marketing authorisation application (MAA) in Q1 2025.

Analyses of the full Phase III SAPPHIRE data are ongoing, and Scholar Rock plans to present detailed results at an upcoming medical conference in early 2025. Preliminary baseline characteristics from the trial will be presented during a poster presentation at the upcoming 29^th Annual Congress of the World Muscle Society on Friday, October 11, 2024, being held in Prague, Czech Republic.