Evidence review: Open Peptide Project (verified against primary PubMed, ClinicalTrials.gov & FDA records) · Last checked 2026-10-27. ← Back to Evidence Library · Trial & Regulatory Tracker →
Apitegromab (Isembyld, SRK-015): what the evidence says
At a glance
What it is: a fully human monoclonal antibody that stops latent myostatin from being activated. It is a large antibody, not a peptide. FDA approved it as Isembyld on Sep 11, 2026. R1, R2
What the evidence says: Human in the SAPPHIRE phase 3 trial (188 patients aged 2–21 with nonambulatory type 2 or 3 SMA already on nusinersen or risdiplam), children 2–12 gained 1.8 more points on a motor-function scale than placebo over 12 months. The higher dose alone did not reach significance. R2
Biggest risks: in SAPPHIRE, adverse events were similar to placebo and no one stopped because of them. R2 UNVERIFIED the approved label's warnings (label not opened).
Rating
| Human Human evidence | Strong: SAPPHIRE phase 3 RCT (n = 188) R2 |
| Animal Animal / lab evidence | Not reviewed |
| Regulatory | FDA-approved R1 · not on FDA's 503A or 503B bulk-substance category lists and not on FDA's compounding safety-risk page R3, R4, R5 · named on WADA S4.3 R6 |
| Established human dose | Yes, label dose (see Study details) |
Evidence verdict: Spinal muscular atrophy (with SMN therapy): Strong (FDA-approved; one phase 3 RCT) · Healthy muscle gain: Unknown (no data)
This page reports what studies used. It is not dosing advice.
Legend: HUMAN data · ANIMAL / LAB data · UNKNOWN · UNVERIFIED: not yet confirmed against a primary source
About the evidence: approval rests mainly on one phase 3 trial with a modest average benefit, added on top of SMN therapy. There is no evidence in healthy people or bodybuilding.
What is it?
Related pages: taldefgrobep-alfa, bimagrumab, trevogrumab, follistatin.
Does it work?
- Human SAPPHIRE: +1.8 HFMSE points vs placebo (p = 0.019) in ages 2–12. R2, R7
- Human Builds on the phase 2 TOPAZ trial. R2, R8
- Unknown Healthy adults or athletes: no data.
Is it safe?
- Human Most common events (fever, colds, cough, vomiting) at similar rates to placebo. R2
Gray-market vials: extra risks
A gray-market vial has no quality control and no guarantee of identity, strength or sterility. We test retail vials blind under the Open Peptide Standard; see /tests/. A passing test covers one batch on one date and does not mean the compound is safe.
Is it legal?
- US: FDA-approved (BLA 761463), prescription. R1
- Compounding: not on FDA's 503A or 503B bulk-substance category lists and not on FDA's compounding safety-risk page. R3, R4, R5
- Sport: WADA 2026 S4.3 names apitegromab; banned at all times. R6
Study details
Study details
These are amounts researchers used, not recommendations.
| Study | People / model | Route | Amount used | Follow-up | Reported | Citation |
|---|---|---|---|---|---|---|
| SAPPHIRE 2025 | 188 nonambulatory SMA type 2/3, ages 2–21 | IV every 4 weeks | 10 or 20 mg/kg | 12 months | +1.8 HFMSE points vs placebo (ages 2–12) | PMID 40818473 |
Full reference list (PubMed, CT.gov, FDA and WADA checked Oct 27, 2026)
- R1 Drugs@FDA, Isembyld (apitegromab-mstn), Scholar Rock, original approval Sep 11, 2026, application BLA761463 (openFDA drugsfda record checked Oct 27, 2026). https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=BLA761463
- R2 Crawford TO, et al. Lancet Neurol. 2025. PMID 40818473
- R3 FDA, 503A bulk drug substances categories list (updated May 14, 2026). https://www.fda.gov/media/94155/download
- R4 FDA, 503B bulk drug substances categories list (updated Mar 21, 2025). https://www.fda.gov/media/94164/download
- R5 FDA, Certain bulk drug substances for use in compounding that may present significant safety risks (content current Apr 22, 2026). https://www.fda.gov/drugs/human-drug-compounding/certain-bulk-drug-substances-use-compounding-may-present-significant-safety-risks
- R6 WADA. The 2026 Prohibited List, S4.3 Agents preventing activin receptor IIB activation. https://www.wada-ama.org/sites/default/files/2025-09/2026list_en_final_clean_september_2025.pdf
- R7 ClinicalTrials.gov NCT05156320, SAPPHIRE phase 3, completed (CT.gov API v2, checked Oct 27, 2026). https://clinicaltrials.gov/study/NCT05156320
- R8 ClinicalTrials.gov NCT03921528, TOPAZ phase 2, completed (CT.gov API v2, checked Oct 27, 2026). https://clinicaltrials.gov/study/NCT03921528
Disclaimer
This page is for education and harm reduction, not medical advice or a recommendation to use Apitegromab (Isembyld, SRK-015). This is a prescription medicine. Use it only under the care of a licensed clinician; do not buy it from research-chemical or gray-market sellers. Report problems to FDA MedWatch (https://www.fda.gov/safety/medwatch).