A new phase 3 clinical trial testing the long-term safety and efficacy of apitegromab in patients with type 2 or type 3 spinal muscular atrophy (SMA) is now open. 

The open-label, multicenter extension study sponsored by Scholar Rock will test the experimental treatment in an estimated 260 participants, aged 2 years and above, who have completed the TOPAZ or SAPPHIRE trial. Participants will receive 20 mg/kg of apitegromab by intravenous infusion every 4 weeks for 104 weeks.

The trial’s primary outcome measures are the long-term safety and tolerability of apitegromab as measured by the incidence of treatment-emergent and serious adverse events.

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The secondary outcome measure is the long-term efficacy of the potential therapy as measured by any changes in motor function on the Hammersmith Functional Motor Scale Expanded (HFMSE), Revised Upper Limb Module (RULM), and Revised Hammersmith Scale (RHS), as well as the number of World Health Organization (WHO) motor development milestones attained and the results of the 6-minute walk test and 30-second sit-to-stand test.

The trial will also assess the presence of any antibodies against apitegromab in the blood. vFinally, the trial will further characterize the pharmacokinetics and pharmacodynamics of apitegromab as well as its effects on patient/caregiver-reported disability, fatigability, and suicidal ideation and behavior.

The trial is not yet recruiting participants. The estimated study start date is April 24, 2023, and the estimated completion date is January 1, 2027.

Apitegromab is an experimental, fully human immunoglobulin G4 monoclonal antibody. It specifically binds to pro- and latent myostatin, the inactive precursor forms of myostatin, thereby inhibiting its activation. Since myostatin negatively regulates muscle growth and strength, it is hoped that apitegromab could improve patients’ motor function.


Long-term safety & efficacy of apitegromab in patients with SMA who completed previous trials of apitegromab-ONYX (ONYX). ClinicalTrials.gov. November 25, 2022. Accessed December 12, 2022.