Apitegromab
An antibody that keeps myostatin from activating, added to SMN treatment to build motor function in SMA
Apitegromab is a monoclonal antibody that binds the inactive precursor forms of myostatin, the protein that restrains muscle growth, and stops them being activated. It is a protein, not a peptide drug. FDA approved it as Isembyld on September 11, 2026 for spinal muscular atrophy in people 2 and older already on an SMN2-targeted treatment.
What is it?
SMN2-targeted drugs slow the loss of motor neurons in SMA; apitegromab aims at the muscle instead, by blocking the activation of myostatin. Myostatin restrains muscle growth. Rather than neutralising the active hormone, the antibody binds its precursor forms and holds them in the latent state, so less active myostatin is released where muscle needs it.
In the phase 3 SAPPHIRE trial, the two doses combined improved motor function over placebo in nonambulatory type 2 and 3 SMA, although the 20 mg/kg arm alone did not reach significance, and 10 mg/kg is the labelled dose. The label reports the main group, children aged 2 to 12 already on nusinersen or risdiplam: at a year, 10 mg/kg improved the Hammersmith motor score by 2.2 points more than placebo, with a nominal p-value of 0.01. The 20 mg/kg dose added nothing and is not recommended.
The label's one warning is fractures: in the pivotal study 9% on 10 mg/kg had a fracture, against 2% on placebo, including femur fractures, and in an open-label extension at 20 mg/kg 9% had fractures, five of them serious femur fractures. Most affected children already had low bone density, earlier fractures or contractures. Its commonest side effects were respiratory and viral infections, vomiting, cough and headache.
How the label reached the age range matters. The controlled evidence comes from children aged 2 to 12; approval for adults and teenagers rests on latent myostatin levels and pharmacokinetic modelling, plus a 20 mg/kg trend in patients aged 13 to 21. Target engagement looked the same at 10 and 20 mg/kg, consistent with the higher dose adding nothing. Only 1 of 128 treated patients formed anti-drug antibodies, at a single time point, and none were neutralising.
The longest follow-up comes from the phase 2 TOPAZ study. At 36 months, 35 nonambulatory patients with a mean age of 7.3 had gained an average of 4.0 points on the Hammersmith motor scale and 2.4 on the upper-limb scale from baseline, and 28 of 32 had kept or improved their motor milestones. TOPAZ was the study that led to a randomised, placebo-controlled phase 3 trial, so these figures show durability rather than an effect against placebo.
It is also being tested in obesity: in the phase 2 EMBRAZE trial, adding it to tirzepatide preserved 1.9 kg more lean mass than placebo over 24 weeks with similar weight loss, a use that is not approved.
Mechanism of Action
Apitegromab is a monoclonal antibody against the proforms of myostatin: it keeps myostatin latent rather than neutralising the active hormone (label).
Myostatin kept latent
Binds pro- and latent myostatin and blocks the protease cleavage that would release active myostatin.
Muscle, not neurons
Works alongside SMN2-targeted drugs, which protect motor neurons; it is approved only for patients already on one.
Antibody kinetics
Terminal half-life about 31.2 days; steady state after about 16 to 20 weeks of four-weekly infusions, with about twofold accumulation (label).
Published Research
Phase 1, 2 and 3 trials read from their PubMed abstracts; dosing and warnings from the Isembyld label read October 1, 2026.
SAPPHIRE
In nonambulatory type 2 or 3 SMA, the combined 20 and 10 mg/kg groups improved motor function significantly against placebo, but 20 mg/kg alone did not; no patient stopped for adverse events, and headache was similar (21% against 20%).
TOPAZ
In the phase 2 trial in later-onset types 2 and 3 SMA, apitegromab improved motor function; the commonest events were headache (24.1%), fever, upper respiratory infection and cough, with no deaths or serious adverse reactions.
Phase 1 in healthy adults
Single and repeated doses raised serum latent myostatin in a dose-dependent, sustained way, showing target engagement, with no clinically meaningful safety changes and no anti-drug antibodies.
TOPAZ at 36 months
In 35 nonambulatory patients, motor scores rose by a mean 4.0 (HFMSE) and 2.4 (RULM) points from baseline at 36 months, and 28 of 32 kept or gained motor milestones; fever and nasopharyngitis were the commonest adverse events.
EMBRAZE: muscle during tirzepatide weight loss
In 102 adults with overweight or obesity on tirzepatide, adding apitegromab 10 mg/kg for 24 weeks cut lean-mass loss by 1.9 kg against placebo with similar total weight loss (54.9% of lean mass retained relative to placebo); adverse events were 76% against 71%. This use is investigational.
How it locks myostatin
Structural work showed the antibody binds a sequence-divergent region of the myostatin prodomain, stabilising the latent form and limiting access to the protease sites that would release active myostatin; one antibody grips one myostatin dimer.
Preclinical safety
Weekly intravenous doses up to 100 mg/kg in monkeys for 4 weeks and 300 mg/kg in rats for up to 26 weeks caused no treatment-related adverse findings, and an 8-week juvenile rat study showed no effects on development, movement or reproduction.
Dosing Data
Isembyld is 10 mg/kg given as an intravenous infusion over 60 to 120 minutes every four weeks, diluted in saline, alongside an SMN2-targeted treatment.
Educational information, not medical advice. Each dose above is tagged with its source: an approved label, a published human study, animal data, or amounts reported by users. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Full medical disclaimer.
Side Effects & Contraindications
Reported Side Effects
From the Isembyld label (version 3).
Contraindications & Cautions
From the Isembyld label (DailyMed, effective September 14, 2026).
Legal Status
Approved in the US in September 2026, as an add-on for people with SMA aged 2 and older who are already taking an SMN2-targeted drug.
Frequently Asked Questions
?Is apitegromab a peptide?
No. It is a monoclonal antibody, a large protein; it sits in this encyclopedia beside the peptide and protein drugs used in muscle and growth disorders.
?Does it replace SMN treatment?
No. The label covers patients already receiving an SMN2-targeted treatment; it is added on.
?How long does it last?
The label estimates a terminal half-life of 31.2 days, which fits dosing every four weeks.
?Who was in the trials?
TOPAZ enrolled people with later-onset type 2 and 3 SMA; SAPPHIRE enrolled nonambulatory patients with type 2 or 3 SMA.
?Why is 20 mg/kg not used?
It was tested in the pivotal trial but showed no additional benefit over 10 mg/kg, so the label does not recommend it. Fractures also occurred at 20 mg/kg in the open-label extension.
?Is it approved for adults?
Yes, for adults and children 2 and older who are already on an SMN2-targeted treatment. The randomised evidence comes from children 2 to 12; use in older patients is supported by drug levels, myostatin measurements and a consistent trend at 20 mg/kg in patients 13 to 21.
?Can it be used in pregnancy?
There are no adequate human data. Antibodies like it cross the placenta, especially late in pregnancy, and in rats exposure through pregnancy and nursing affected the offspring's reproductive and neurobehavioural function at clinically relevant levels.
?What if an infusion is missed?
The label says to give it as soon as possible and then restart the four-weekly schedule from that dose.
References
- [fda-pi] Isembyld (apitegromab-mstn) injection US prescribing information (DailyMed version 3, effective September 14, 2026); read October 1, 2026.
- [pubmed] Crawford TO, et al. "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." Lancet Neurol, 2025;24(9):727-739. PMID: 40818473.
- [pubmed] Crawford TO, et al. "Safety and Efficacy of Apitegromab in Patients With Spinal Muscular Atrophy Types 2 and 3: The Phase 2 TOPAZ Study." Neurology, 2024;102(5):e209151. PMID: 38330285.
- [pubmed] Barrett D, et al. "A Randomized Phase 1 Safety, Pharmacokinetic and Pharmacodynamic Study of the Novel Myostatin Inhibitor Apitegromab (SRK-015): A Potential Treatment for Spinal Muscular Atrophy." Adv Ther, 2021;38(6):3203-3222. PMID: 33963971.
- [pubmed] Crawford TO, et al. "Long-term efficacy, safety, and patient-reported outcomes of apitegromab in patients with spinal muscular atrophy: results from the 36-month TOPAZ study." Front Neurol, 2024;15:1419791. PMID: 39105058.
- [clinical-trial] Pratley RE, et al. "Apitegromab for lean mass preservation during tirzepatide-induced weight loss: a randomized, double-blind, placebo-controlled phase 2 trial." Nat Med, 2026;32(7):2673-2678. PMID: 42260100.
- [pubmed] Dagbay KB, et al. "Structural basis of specific inhibition of extracellular activation of pro- or latent myostatin by the monoclonal antibody SRK-015." J Biol Chem, 2020;295(16):5404-5418. PMID: 32075906.
- [pubmed] Welsh BT, et al. "Preclinical Safety Assessment and Toxicokinetics of Apitegromab, an Antibody Targeting Proforms of Myostatin for the Treatment of Muscle-Atrophying Disease." Int J Toxicol, 2021;40(4):322-336. PMID: 34255983.
Sources & Citations
Studies were read from their PubMed abstracts; the Isembyld label (DailyMed version 3, effective September 14, 2026) was read October 1 and October 6, 2026.
Medical Disclaimer: This content is for educational and informational purposes only. It is not medical advice, and nothing here is an instruction to obtain or use any compound. Talk to a licensed clinician about your own situation.