GYM329
GYM329 (RO7204239), anti-latent myostatin sweeping antibody
Written by Aaron CuhaReviewed Sep 2026
Also known as: RO7204239, Anti-latent myostatin antibody
The one myostatin antibody now being tested where it matters most: alongside tirzepatide in people with obesity, and for insulin sensitivity in type 2 diabetes. No results yet.
Overview
Three anti-myostatin antibodies added lean mass and failed to improve function, all in people with muscular dystrophy or long-standing weakness. GYM329 is the first to be aimed squarely at the population this site's readers belong to, and that makes it the most consequential open question in the field.
Two things make it different. Mechanically it targets latent myostatin, the inactive precursor, and is engineered for antigen sweeping, which accelerates clearance of the bound target rather than simply neutralising it. And strategically it is being studied for insulin sensitivity and muscle composition in type 2 diabetes with overweight or obesity, and in a phase 2 combination with tirzepatide in 285 participants with obesity.
No human efficacy results are published. This page exists so that when they arrive, the reader already knows what three previous antibodies did and did not achieve.
Mechanism of action
An antibody engineered by Chugai and Roche against latent myostatin, the inactive precursor form, rather than the mature active protein. It uses antigen sweeping, an engineering approach that promotes uptake and degradation of the antibody-antigen complex so the target is actively cleared instead of merely blocked. The stated rationale is more complete and durable suppression than earlier neutralising antibodies achieved, which is the same reasoning that motivated domagrozumab's higher potency and did not work there.
Human evidence
No published human efficacy results. Registered trials exist in the populations that matter for this site's central question, and none has reported.
- A phase 2 combination trial with tirzepatide in 285 participants with obesity or overweight and at least one weight-related comorbidity is active and not recruiting.
- A trial in type 2 diabetes with overweight or obesity is studying insulin sensitivity and muscle composition, which is the lean-mass-preservation use case stated explicitly.
- A spinal muscular atrophy programme runs alongside risdiplam.
- The published record is preclinical: mouse disease models reporting enhanced muscle strength, plus a nonclinical safety evaluation.
What this does not tell you: Everything published about this antibody is animal work. The mouse result reporting enhanced strength is the finding that would matter most if it transfers, and transferring is precisely what the three earlier antibodies failed to do after similarly encouraging preclinical work. Treat the combination trial as the test, not the preclinical data as the answer, and note that no results are posted for any of these studies.
Reading the research record
Read the myostatin field as a set and one result repeats. Blocking myostatin reliably adds lean mass, by roughly a quarter to half a kilogram in the trials that measured it properly, and reliably fails to improve what a person can actually do. Stamulumab found no improvement in strength or function. Domagrozumab missed its primary endpoint and every secondary clinical endpoint. Landogrozumab added 0.43 kg of appendicular lean mass at p < 0.0001 and moved two of roughly seven physical performance measures. Bimagrumab, which this site covers separately, produced the largest body composition change in the class and did not deliver a matching functional gain either. The tissue appears. The capability does not follow.
GYM329 is the reason that pattern is not the end of the story. Every failed trial in this class was run in people with a damaged or long-weakened muscle, where releasing a growth brake may simply be the wrong lever. Nobody has tested it in the situation readers of this site actually face, which is rapid weight loss on an incretin drug taking lean tissue along with fat. That trial exists now and has not reported. When it does, the question to ask is the same one the earlier trials answered badly: did anyone become able to do something they could not do before.
The evidence, charted
Fig. 1 · evidence composition
3of 5 citations (60%) are in people
Counted from the citation list on this page. The Human count is the same number shown in the badge at the top. Both understate any literature larger than the sources we cite.
Fig. 2 · evidence over time
Evidence spans 4 distinct years, 2021 to 2026, counted from the citation list on this page.
Fig. 3 · legal status at a glance
US
Not approved
UK
Approved
AU
Approved
CA
Not approved
Approved in 2 of 4, prescription route in 0, not approved in 2. A jurisdiction's classification is a regulatory fact, not a verdict on the science; see Legal status below for the full text and any notes.
Fig. 4 · dose response
No human dose response curve exists
We draw this figure where the data supports it. For this compound in humans it does not, so the panel stays empty rather than borrowing an animal curve and implying it transfers.
Key studies & citations
- Animal2021
Novel myostatin-specific antibody enhances muscle strength in muscle disease models
The preclinical characterisation. Mouse disease models. Reports enhanced muscle strength, which is notable precisely because that is the endpoint the three earlier antibodies failed to move in humans. Animal work, and the site keeps that separate from human evidence for exactly this reason.
Scientific Reports - In vitro2023
Preclinical in vitro evaluation of immune suppression induced by GYM329, Fc-engineered sweeping antibody
In vitro. Worth reading for what it is about rather than as a routine safety box: the antigen-sweeping Fc engineering that makes this antibody clear its target also engages immune cells, and this study evaluates the resulting immune suppression. The mechanism that differentiates GYM329 from the antibodies that failed is also the mechanism that creates a safety question the others did not have.
Journal of Toxicological Sciences - Human2026
A study to assess efficacy, safety, tolerability, pharmacokinetics and pharmacodynamics of RO7204239 in combination with tirzepatide in participants with obesity or overweight with at least one weight-related comorbidity
Phase 2, 285 participants, active and not recruiting. The trial that tests whether blocking myostatin preserves what a GLP-1 drug takes. This is the single most relevant registered study to the muscle-loss question on this site.
ClinicalTrials.gov - Human2026
A study of GYM329 for insulin sensitivity and muscle composition in type 2 diabetes with overweight or obesity
The metabolic indication. Muscle is the main site of insulin-stimulated glucose disposal, so more muscle should improve insulin sensitivity. That chain has not been demonstrated with a myostatin antibody in humans.
ClinicalTrials.gov - Human2024
A study of RO7204239 in participants with spinal muscular atrophy
The neuromuscular indication, run alongside risdiplam. The population closest to where the earlier antibodies were tested and failed.
ClinicalTrials.gov
Frequently asked questions
Is this the drug that stops muscle loss on GLP-1s?
It is the drug being tested for that, in a phase 2 combination with tirzepatide in 285 people. No results are published. Three earlier antibodies in this class added lean mass without improving function, so the result worth waiting for is a functional one rather than a body composition one.
How is it different from the antibodies that failed?
It targets latent myostatin, the inactive precursor, and uses antigen sweeping to clear the target rather than only block it, which is intended to give deeper and more durable suppression. Note that domagrozumab was also more potent than its predecessor and that did not help, so potency alone has already failed once as an explanation.
Can I get it?
No. It is investigational and not approved anywhere. The only access is a clinical trial.
What should I do about muscle loss on a GLP-1 in the meantime?
The interventions with actual randomised support are resistance training and adequate protein, neither of which requires waiting for this antibody. The training and diet sections of this site cover what has been measured for both.