What it is
Apitegromab is not a peptide. It is a fully human monoclonal antibody that blocks myostatin, the protein that limits skeletal muscle growth.
Its distinguishing feature is where in the pathway it acts. Myostatin is made as an inactive precursor and has to be cut before it can signal. Apitegromab binds the pro- and latent forms — the precursors — rather than the mature molecule. That is the mechanism the structural work published in the Journal of Biological Chemistry in 2020 set out to establish, and it is what separates this antibody from the others in its class.
It is in this library because it is searched beside the muscle and GLP-1 peptides, and because it has something most compounds here do not: a completed phase 3 trial with a published result.
The phase 3, and why the answer is two answers
SAPPHIRE (NCT05156320) enrolled 188 patients aged 2–21 with nonambulatory type 2 or type 3 spinal muscular atrophy, at 48 hospitals across nine countries, all of them already taking nusinersen or risdiplam. The primary endpoint was the change in the Hammersmith Functional Motor Scale-Expanded at 12 months, in the 2–12 age group.
| Comparison at 12 months | Difference in HFMSE points | 95% CI | p |
|---|---|---|---|
| Apitegromab pooled (20 + 10 mg/kg) vs placebo | 1.8 | 0.30 to 3.32 | 0.019 |
| Apitegromab 20 mg/kg vs placebo | 1.4 | −0.34 to 3.13 | 0.11 |
Lancet Neurology, 2025 (DOI 10.1016/S1474-4422(25)00225-X).
Both rows are from the same trial and the same paper. The comparison that reached significance is the one that pools two doses; the comparison of the higher dose against placebo on its own does not, and its interval crosses zero.
This is worth stating plainly because it is the part that disappears in summary. A sentence saying the phase 3 met its primary endpoint is accurate. So is a sentence saying the 20 mg/kg dose did not separate from placebo. The trial's own published interpretation contains both, in that order.
The underlying numbers also explain what the difference is made of: apitegromab groups gained 0.6 points on average over the year and the placebo group lost 1.2. Most of the gap is decline that did not happen.
Safety, in the reported record, was unremarkable: adverse-event rates were similar between arms and consistent with the disease and its background therapy — fever in 26% of apitegromab participants against 28% on placebo, nasopharyngitis 25% against 23% — and no patient discontinued because of an adverse event.
A registry that says less than the literature
ClinicalTrials.gov carries no posted results for SAPPHIRE, a completed trial whose full report is in a major journal. This library notes the pattern rather than explaining it: for this compound, the registry is the less complete source, which is the reverse of the usual failure and a reason to check both.
Seven records exist under the name, all sponsored by Scholar Rock:
| Study | Phase | Participants | Subject |
|---|---|---|---|
| NCT03921528 | 2 | 58 | TOPAZ, types 2 and 3 SMA — completed |
| NCT05156320 | 3 | 188 | SAPPHIRE — completed |
| NCT05626855 | 3 | 238 | long-term extension for trial completers |
| NCT07047144 | 2 | 52 | children under two years old |
| NCT06445075 | 2 | 102 | EMBRAZE, with tirzepatide in obesity |
| NCT07435129 | 2 | 60 | facioscapulohumeral muscular dystrophy, from 2026-07 |
| NCT06877689 | — | — | expanded access, listed as no longer available |
Queried 2026-09-20.
The obesity trial, and the number it produced
EMBRAZE (NCT06445075) is the study that brought this antibody into the same conversation as the GLP-1 drugs. 102 adults with overweight or obesity were randomised one-to-one to tirzepatide plus apitegromab 10 mg/kg or tirzepatide plus placebo.
At week 24:
- 1.9 kg less lean-mass loss than placebo (80% confidence interval 1.2 to 2.7, p=0.001).
- Total body weight loss was similar between the groups — which is what makes the lean-mass figure mean something.
- Reported as 54.9% retention of lean mass relative to placebo.
- Adverse events in 39 of 51 (76%) apitegromab participants against 36 of 51 (71%) on placebo; one serious adverse event in each arm.
Nature Medicine, 2026 (DOI 10.1038/s41591-026-04440-4). Note the 80% confidence interval — a wider-than-conventional interval, which the paper states, and which is ordinary for a proof-of-concept study but is not the 95% most readers assume.
One detail that travels badly and belongs here: the doses are not the same across the two programmes. Spinal muscular atrophy used 20 mg/kg and 10 mg/kg; the obesity trial used 10 mg/kg alone.
Where it stands
An investigational antibody with more human evidence behind it than almost anything else in this library, a phase 3 whose result is genuinely split between two pre-specified comparisons, a clean safety record across both programmes, and no approved product found in the FDA's databases on the date this entry was checked.
What it is not is a peptide, and what it has not done is establish that the muscle it preserves changes how anybody functions — which, for the obesity trial in particular, is the question the lean-mass kilogram is a stand-in for.
Related entries in this library: bimagrumab blocks the receptor the whole family signals through; trevogrumab binds the same protein after activation rather than before; tirzepatide is the drug EMBRAZE tested it beside. The wider class sits in the muscle-preservation entries.
Last checked
2026-09-20. The seven ClinicalTrials.gov records were read through the v2 API, the PubMed record through E-utilities, and the FDA status through the openFDA label and Drugs@FDA endpoints with a nusinersen control query, all on that date.