Isembyld

Isembyld is used to treat spinal muscular atrophy (SMA) in adults and children 2 years of age and older who are currently receiving a survival motor neuron 2 (SMN2) targeted treatment. Isembyld targets muscle loss and is administered as an intravenous infusion once every 4 weeks. It is not known if Isembyld is safe and effective in children under 2 years of age.

SMA is a rare neuromuscular disease affecting approximately 1 in 10,000 births and a leading genetic cause of infant mortality. A faulty SMN1 gene deprives motor neurons of a protein they need to survive, causing progressive muscle weakness and wasting. SMN2-targeted therapies improve outcomes but leave muscle loss itself unaddressed.

Isembyld (apitegromab-mstn) was approved by the FDA on September 11, 2026. Approval was based on Study 1 (NCT05156320), which enrolled 188 people aged 2 to 21 years with 5q SMA who were already receiving nusinersen or risdiplam. They were randomly assigned to Isembyld 10 mg/kg (the recommended dose), Isembyld 20 mg/kg, or placebo, given as an infusion every 4 weeks.

• The main results come from the children aged 2 to 12 years. Motor function was measured with the Hammersmith Functional Motor Scale Expanded (HFMSE), which scores 33 activities such as sitting, standing, and walking out of a total of 66 points. A higher score means better motor function.
• After 1 year, children given Isembyld 10 mg/kg scored 2.2 points higher on average than those given a placebo (nominal p=0.0121). A gain of 3 points or more was reached by 34.2% of children on Isembyld, compared with 13.5% on placebo.

Before taking Isembyld, tell your healthcare provider about all of your medical conditions, including if you:

• have a history of low bone density or bone fractures.
• are pregnant or plan to become pregnant. If you are pregnant or are planning to become pregnant, ask your healthcare provider for advice before taking this medicine. It is not known if Isembyld will harm your unborn baby. Tell your healthcare provider right away if you become pregnant during treatment with Isembyld.
• are breastfeeding or plan to breastfeed. It is not known if Isembyld passes into breast milk. Talk to your healthcare provider about the best way to feed your baby while on treatment with Isembyld.

Tell your healthcare provider about all the medicines you take, including prescription and over-the-counter medicines, vitamins, and herbal supplements. Keep a list of them to show your healthcare provider, including your pharmacist, when you get a new medicine.

Isembyld may cause serious side effects, including:

• Fractures. Treatment with Isembyld may increase the risk of bone fractures, including serious fractures. Bone fractures may happen with or without a fall or other injury. Your healthcare provider may consider stopping treatment with Isembyld if you experience a bone fracture during treatment.

Common side effects

The most common side effects of Isembyld include:

• upper respiratory tract infections
• vomiting
• cough
• viral infections
• headache
• stomach flu (gastroenteritis)
• sore throat (pharyngitis)
• hypersensitivity

These are not all of the possible side effects of Isembyld. Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.

Isembyld (apitegromab-mstn) is a fully human IgG4 monoclonal antibody that targets proforms ofmyostatin and works by binding to promyostatin and latent myostatin, the inactive precursor forms of myostatin. By binding these proforms, it inhibits the activation of myostatin and blocks myostatin signaling. Myostatin is a negative regulator of skeletal muscle mass, so Isembyld acts on muscle rather than on the underlying motor neuron defect. It is used alongside an SMN2-targeted treatment, not in place of one.

• You will receive Isembyld from a healthcare provider through a needle placed in a vein (intravenous or IV infusion). The infusion will take about 1 to 2 hours to give you the full dose of medicine.
• You will receive Isembyld every 4 weeks.
• Do not stop treatment with Isembyld unless your healthcare provider tells you to.
• Your healthcare provider will determine the amount of Isembyld needed based on your current weight.
• If you miss a dose of Isembyld, either: Isembyld should be given (administered) as soon as possible, and then restart the dosing schedule 4 weeks after the missed dose is given, or the dose can be skipped, and the next dose can be given as originally scheduled.

Active ingredients: apitegromab-mstn

Inactive ingredients: histidine, L-histidine hydrochloride monohydrate, sodium chloride, polysorbate 80, and Water for Injection. Hydrochloric acid was added to adjust the pH.

Manufactured by Scholar Rock, Inc., 301 Binney St, Cambridge, MA 02142 USA. U.S. License No. 2372. For more information, go to www.ISEMBYLD.com or call 1-877-ISEMBYLD (1-877-473-6295).