Garetosmab (REGN2477) Australia: What Does the Research Show?
Garetosmab is an experimental monoclonal antibody that blocks activin A, another important regulator of skeletal muscle mass. Research suggests its effects become particularly powerful when activin A blockade is combined with myostatin inhibition.
Last updated: August 2026Garetosmab, also known as REGN2477, has become an important part of current muscle growth research because it targets a protein called activin A.
Activin A is one of several signalling proteins capable of restricting skeletal muscle growth through activin type II receptors.
This places it in the same broader biological system as myostatin, although the two proteins are not the same.
Research from Regeneron suggests that myostatin and activin A are two of the most important negative regulators of adult muscle mass.
When researchers block both proteins at the same time using trevogrumab and garetosmab, the effect on muscle can be substantially greater than targeting either pathway alone.
If myostatin is one brake on muscle growth, activin A appears to act as another. Garetosmab is designed to reduce the activin A brake.
What is garetosmab?
Garetosmab is a fully human monoclonal antibody developed by Regeneron Pharmaceuticals.
Its main target is activin A.
Activin A belongs to the transforming growth factor beta family of signalling proteins and can interact with receptors involved in regulating skeletal muscle.
Garetosmab is designed to bind activin A and reduce its biological activity.
The drug has been investigated in several different research areas, including muscle biology and fibrodysplasia ossificans progressiva.
Garetosmab does not directly block myostatin. It blocks activin A, which acts through overlapping receptors involved in the control of muscle mass.
What does REGN2477 mean?
REGN2477 is Regeneron’s development code for garetosmab.
Older scientific papers and clinical trial records may therefore refer to the compound using either name.
- Garetosmab
- REGN2477
- REGN-2477
These names refer to the same experimental monoclonal antibody.
What is activin A?
Activin A is a naturally occurring signalling protein involved in numerous biological processes.
One of its functions appears to be the regulation of skeletal muscle mass.
Like myostatin, activin A can activate ActRIIA and ActRIIB receptors.
Activation of these receptors contributes to signals that restrict muscle growth.
Researchers initially knew that broadly blocking activin type II receptors could produce substantial muscle growth.
The problem was that these receptors interact with many different proteins.
Broad receptor blockade could therefore interfere with biological functions that researchers did not necessarily want to alter.
This led scientists to investigate which specific proteins were responsible for most of the muscle effect.
Research increasingly pointed toward two major candidates:
- Myostatin / GDF-8
- Activin A
How does garetosmab work?
Garetosmab binds to activin A and prevents it from producing its normal signalling effect.
By reducing activin A activity, researchers can reduce one of the signals passing through activin type II receptors that contributes to the restriction of muscle growth.
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activin A
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Published research describes garetosmab as a fully human IgG4 monoclonal antibody targeting activin A.
More recent research has also noted that it inhibits activin A related heterodimers containing INHBA, including activins AB and AC.
Is garetosmab a myostatin inhibitor?
No.
This is an important distinction.
Garetosmab targets activin A.
Trevogrumab targets myostatin / GDF-8.
Targets activin A and reduces activin signalling through the muscle regulation pathway.
Targets GDF-8 or myostatin, another major negative regulator of skeletal muscle.
They are commonly discussed together because both proteins signal through overlapping activin receptors.
Research suggests that blocking both can produce a stronger muscle effect than blocking either protein individually.
Is garetosmab a peptide?
No.
Garetosmab is a monoclonal antibody, not a conventional peptide.
It is a large biological protein manufactured using pharmaceutical biotechnology.
This is very different from the smaller synthetic research peptides commonly discussed online.
A product advertised as an activin blocker or muscle growth research compound should not automatically be assumed to contain pharmaceutical grade garetosmab or material equivalent to REGN2477 used in clinical trials.
What did garetosmab do in animal studies?
Some of the most important early research into REGN2477 examined how activin A contributes to adult muscle mass.
A 2017 study published in Nature Communications investigated garetosmab alone and in combination with the anti-myostatin antibody trevogrumab.
Activin A and myostatin blockade
In cynomolgus monkeys, blocking activin A alone caused relatively small increases in lean mass.
Blocking myostatin alone produced a similar effect.
When the two antibodies were combined, the increase was much larger.
At the maximal tested doses in one experiment, combined REGN2477 and REGN1033 treatment increased lean body mass by approximately 9.1%.
In a longer experiment, the combination produced an approximately 13.7% increase in lean body mass.
Researchers also reported a large increase in biceps muscle weight and confirmed that muscle fibres had become larger.
The study suggested that activin A may play a particularly important role in regulating muscle mass in primates, and that targeting both activin A and myostatin could reproduce much of the muscle growth seen with broader activin receptor blockade.
Has garetosmab been tested in humans?
Yes.
One particularly important Phase 1 trial studied garetosmab and trevogrumab in healthy postmenopausal women.
The trial was randomized, double blind and placebo controlled.
Researchers tested:
- Trevogrumab alone
- Garetosmab alone
- The two antibodies together
- Placebo
The findings were published in Nature Communications in 2025.
Combination treatment produced dose dependent increases in thigh muscle volume measured by MRI.
The muscle increase was greater with dual blockade than with either antibody alone.
Researchers also observed increases in broader measures of lean body mass and decreases in fat mass.
The study provided direct clinical evidence that myostatin and activin A work together to regulate human muscle mass and that blocking both pathways can produce greater than additive increases in muscle.
Did the muscle stay after treatment stopped?
This is one of the most interesting findings from the Phase 1 study.
After combination treatment was withdrawn, the increase in thigh muscle volume gradually declined.
Researchers reported that muscle size had returned toward baseline by approximately week 28.
This suggests that continued pathway inhibition may be required to maintain the muscle effect.
The study suggests the additional muscle is not necessarily permanent. When myostatin and activin A signalling returned, the body appeared to move back toward its previous level of muscle mass.
Why combine garetosmab with trevogrumab?
This is arguably the most important reason garetosmab has become relevant to muscle research.
Myostatin inhibition alone does not appear to reproduce the full effect of blocking activin type II receptors.
Researchers therefore suspected that another protein must be contributing to the remaining muscle restriction.
Activin A emerged as one of the strongest candidates.
GDF-8
of muscle growth
reduced
In animal research, combined inhibition produced a synergistic increase in muscle mass.
Human Phase 1 research later showed the same general pattern, with greater than additive increases in muscle volume when both antibodies were used together. :contentReference[oaicite:1]{index=1}
Garetosmab, trevogrumab and semaglutide
More recently, the combination has been studied alongside semaglutide.
The goal is not simply to create larger muscles.
Researchers are trying to change the composition of weight loss.
GLP-1 receptor agonists can cause substantial fat loss but can also reduce lean mass.
Regeneron’s approach is to combine a weight loss drug with medicines that reduce muscle loss.
Semaglutide reduces body weight. Trevogrumab reduces myostatin signalling. Garetosmab reduces activin A signalling. Researchers are testing whether this produces greater fat loss while protecting more muscle.
What did the COURAGE trial show?
Regeneron’s Phase 2 COURAGE trial tested semaglutide alone and combinations involving trevogrumab and garetosmab in adults with obesity.
The interim 26 week data provided a useful comparison of the different approaches.
In the interim analysis, around one third of semaglutide induced weight loss came from lean mass.
Trevogrumab combinations prevented roughly half of the lean mass loss compared with semaglutide alone.
The interim trevogrumab, garetosmab and semaglutide group preserved substantially more lean mass.
In Regeneron’s June 2025 interim analysis, the triple combination of semaglutide, trevogrumab and garetosmab preserved approximately 80.9% of the lean mass that was lost with semaglutide alone.
It also produced greater fat loss.
The triple combination group lost approximately 30.0 lb of total body weight compared with approximately 23.0 lb with semaglutide alone in that interim dataset.
Fat mass loss was approximately 25.4 lb in the triple combination compared with 15.3 lb with semaglutide alone. :contentReference[oaicite:2]{index=2}
What do we know about garetosmab safety?
This is where the results become more complicated.
The stronger body composition effect of adding garetosmab came with a less favourable tolerability profile.
In the COURAGE interim results, treatment emergent adverse events occurred more frequently in the triple combination group than with semaglutide alone or semaglutide plus trevogrumab.
Severe treatment emergent adverse events were also higher in the triple combination group.
Regeneron’s completed 26 week update subsequently described the trevogrumab and semaglutide combination as generally well tolerated, while noting that the triple combination had a substantially higher rate of discontinuation related to tolerability issues and other adverse events. :contentReference[oaicite:3]{index=3}
Adding garetosmab appears capable of strengthening the muscle preserving effect, but the available human data also suggest that broader pathway inhibition may come with additional safety and tolerability costs.
Why has activin A raised reproductive concerns?
Activin A is involved in more than skeletal muscle.
In the Phase 1 research program, investigators initially selected healthy postmenopausal women partly because preclinical toxicology findings raised questions about possible reproductive effects of activin A blockade.
This does not establish that garetosmab causes infertility in humans.
It does show why targeting activin A requires greater caution than simply thinking of it as another muscle growth pathway. :contentReference[oaicite:4]{index=4}
Can garetosmab build muscle by itself?
The evidence suggests that activin A inhibition can influence muscle mass, but garetosmab appears particularly interesting when combined with myostatin inhibition.
In primate studies, garetosmab alone produced relatively modest increases in lean mass.
Trevogrumab alone produced a similar modest effect.
Combining the two produced a much larger response. :contentReference[oaicite:5]{index=5}
Garetosmab is probably better understood as part of a dual pathway strategy than as a standalone “muscle building drug”. Much of the most dramatic research involves simultaneous activin A and myostatin inhibition.
Garetosmab vs trevogrumab
| Feature | Garetosmab | Trevogrumab |
|---|---|---|
| Development code | REGN2477 | REGN1033 |
| Main target | Activin A | Myostatin / GDF-8 |
| Drug type | Monoclonal antibody | Monoclonal antibody |
| Direct myostatin inhibitor? | No | Yes |
| Muscle research | Yes | Yes |
| COURAGE obesity trial | Studied in triple combination | Main muscle preserving antibody |
| Current role | Additional activin A blockade | Selective myostatin blockade |
What is the current research status?
Garetosmab remains an investigational pharmaceutical compound.
It is still included in Regeneron’s COURAGE research program alongside trevogrumab and semaglutide.
ClinicalTrials.gov lists COURAGE as a Phase 2 study involving 1,005 participants.
As of the update posted on 8 May 2026, the trial was listed as active, not recruiting. :contentReference[oaicite:6]{index=6}
The trial includes treatment groups receiving semaglutide, trevogrumab and intravenous garetosmab during part of the study. :contentReference[oaicite:7]{index=7}
Is garetosmab available in Australia?
Garetosmab should currently be considered an investigational pharmaceutical medicine, not an established muscle building treatment available to Australian consumers.
Its use in obesity and muscle preservation remains part of clinical research.
It should not be confused with supplements or research chemicals marketed online as activin inhibitors.
Online availability does not establish that a product contains authentic REGN2477 or that it is equivalent to garetosmab manufactured for regulated pharmaceutical trials.
Garetosmab research summary
| Question | Current evidence |
|---|---|
| What is garetosmab? | Fully human monoclonal antibody |
| Other name | REGN2477 |
| Primary target | Activin A |
| Is it a myostatin inhibitor? | No |
| Developer | Regeneron Pharmaceuticals |
| Animal muscle evidence? | Yes |
| Human muscle evidence? | Yes, particularly when combined with trevogrumab |
| Obesity research? | Yes, in the COURAGE Phase 2 program |
| Established Australian muscle treatment? | No |
The bottom line
Garetosmab is one of the most interesting compounds in the current muscle regulation field, but it is important to describe it correctly.
It is not a myostatin inhibitor.
It is an activin A antibody.
Research suggests that activin A works alongside myostatin to restrict adult skeletal muscle mass.
Blocking either pathway individually can affect muscle, but the strongest experimental effects have generally appeared when both are blocked together.
In non-human primates, combined garetosmab and trevogrumab treatment produced substantial increases in lean mass.
Human Phase 1 research later confirmed that dual blockade can produce greater than additive increases in muscle volume.
The COURAGE obesity trial has added another layer of evidence, with the triple combination of semaglutide, trevogrumab and garetosmab showing particularly strong preservation of lean mass during weight loss.
However, those stronger body composition effects also came with greater tolerability concerns.
Garetosmab strengthens the case that muscle mass is controlled by more than myostatin alone. Activin A appears to be another major brake, but blocking it may also involve more complex safety trade-offs.
Frequently asked questions
What is garetosmab?
Garetosmab is an investigational fully human monoclonal antibody developed by Regeneron that targets activin A.
Is REGN2477 the same as garetosmab?
Yes. REGN2477 is the development code for garetosmab.
Is garetosmab a myostatin inhibitor?
No. Garetosmab targets activin A. Trevogrumab is the Regeneron antibody that directly targets myostatin or GDF-8.
Is garetosmab a peptide?
No. Garetosmab is a monoclonal antibody rather than a conventional research peptide.
Can garetosmab increase muscle?
Activin A blockade can influence muscle mass, but some of the strongest results have occurred when garetosmab is combined with the myostatin antibody trevogrumab.
Why are garetosmab and trevogrumab combined?
Myostatin and activin A both contribute to signals that restrict muscle growth. Research suggests that blocking both pathways together produces a stronger muscle effect than blocking either individually.
What did the COURAGE study show?
Interim Phase 2 results showed that the combination of semaglutide, trevogrumab and garetosmab preserved substantially more lean mass than semaglutide alone, although the triple combination also produced more tolerability concerns.
Is garetosmab available in Australia?
Garetosmab remains an investigational pharmaceutical compound and is not an established muscle building medicine for Australian consumers.
Explore related research
Explore how myostatin controls skeletal muscle and the main drugs being developed to target the pathway.
View the research hub → Related compound Trevogrumab (REGN1033)Learn about the anti-myostatin antibody commonly studied alongside garetosmab.
Read the trevogrumab guide →Research sources
- Latres E, et al. Activin A more prominently regulates muscle mass in primates than does GDF8. Nature Communications. 2017. View study →
- Gonzalez Trotter D, et al. GDF8 and activin A are the key negative regulators of muscle mass in postmenopausal females: a randomized Phase I trial. Nature Communications. 2025. View study →
- Mastaitis JW, et al. GDF8 and activin A blockade protects against GLP-1 induced muscle loss while enhancing fat loss in obese male mice and non-human primates. Nature Communications. 2025. View study →
- Regeneron Pharmaceuticals. Interim Results from the Phase 2 COURAGE Trial. June 2025. View results →
- Regeneron Pharmaceuticals. Complete 26 week results from the Phase 2 COURAGE trial. September 2025. View results →
- ClinicalTrials.gov. COURAGE Phase 2 trial. NCT06299098. View trial →