Monoclonal antibodies

Summary

A survey of the three monoclonal antibodies that come up alongside GLP-1s in the conversation about muscle and fat: bimagrumab, trevogrumab, and garetosmab. None of the three is a peptide, and the difference is one of order of magnitude: the garetosmab molecule weighs about 146 kDa versus 4,113.58 g/mol for semaglutide, it is produced in Chinese hamster ovary cell culture instead of synthesized, and the label says to store between 2 °C and 8 °C, not freeze, and not shake. The only published phase 2 trial is BELIEVE, with 507 adults: bimagrumab given intravenously every 12 weeks, alone, lost 9.3 kg over 48 weeks versus 14.2 kg for semaglutide alone and 17.8 kg for the combination. The only one of the three with an FDA-approved label is garetosmab, and not for weight: it's for fibrodysplasia ossificans progressiva, a rare disease, given as a 60-minute infusion every four weeks. The garetosmab phase 1 trial in healthy obese people was withdrawn before it started, and so was the bimagrumab-with-tirzepatide one.

A monoclonal antibody is not a peptide, and the difference is one of order of magnitude

A monoclonal antibody is a large protein, built by a living cell, designed to latch onto a single target. A peptide is a short chain of amino acids, and most of what this site covers comes from chemical synthesis. The difference between the two isn't one of degree — it's one of size, manufacturing, and logistics, and that's what decides what can be done with a vial.

So as not to discuss this in the abstract, I read both labels in the FDA's label database on September 12, 2026 and put side by side the only antibody on this page that has an approved label and the best-known peptide on this site. The semaglutide label I read is the one for the tablet presentation — RYBELSUS and OZEMPIC tablets, in the same document —, not the injectable pen; the molecule is the same, it's the way it's taken that changes.

The math this table hides

• 146 kDa divided by 4,113.58 g/mol gives about 35. The antibody is 35 times heavier than the semaglutide molecule. It isn't a variation within the same family of substances; it's a different category of thing.
• 10 mg/kg in an 80 kg person is 800 mg per infusion. The math is mine, done using the per-kilogram dose on the label. The PASATRU vial contains 300 mg in 5 mL — so one dose takes more than two and a half vials. Compare that with the 14 mg of the highest-dose semaglutide tablet, and with the micrograms in which the rest of this site measures peptide.
• One is swallowed on an empty stomach; the other goes into a vein for an hour, at a healthcare facility. This row of the table is the reason there is no reconstitution table here: there's nothing to reconstitute, and the next step isn't an insulin syringe.

The name stopped identifying the class

The market learned to recognize antibodies by the ending -mab, for monoclonal antibody. That has stopped being the rule. In the executive summary of the WHO's 73rd International Nonproprietary Names Consultation (Working Doc. 21.533, November 2021), the panel of experts approved a new scheme that divides monoclonal antibodies into four groups — and decided to drop the -mab stem for good, to avoid confusion now that the other groups have gained their own suffix.

The names on this page all predate that decision, and so they still end in -mab. I write them here in Portuguese form, with the ending -mabe, when the text runs in Portuguese; in the queries and tables I use the form the WHO published: bimagrumab, trevogrumab, and garetosmab.

How much evidence really exists

The numbers below were gathered by me on September 12, 2026. The query for each row is next to the number, so that anyone can repeat it and prove me wrong. The three antibodies are the ones that come up in the same conversation as GLP-1s when the subject is lean mass: bimagrumab, which blocks type II activin receptors; trevogrumab, targeting myostatin; and garetosmab, targeting activin A.

The class is enormous; these three are not

Monoclonal antibody is among the most studied things in medicine, and that's exactly the contrast that matters to anyone arriving here from a training forum: the size of the class's literature doesn't transfer to the specific molecule someone is considering.

What the only published trial measured

Of the three, only bimagrumab has a phase 2 trial published in an obesity journal: BELIEVE, published in Nature Medicine in March 2026 (NCT05616013). 507 adults with obesity were randomized into nine groups and treated for 48 weeks, with an open-label extension to week 72.

The design is the most important piece of information on the page, and it almost never appears when the study is cited on social media: bimagrumab was given by intravenous infusion every 12 weeks, at 10 or 30 mg/kg, while semaglutide was weekly subcutaneous, at 1.0 or 2.4 mg.

The average weight changes at week 48, as the article itself reports them: −9.3 kg with bimagrumab 30 mg/kg alone, −14.2 kg with semaglutide 2.4 mg alone, −17.8 kg with both combined at the high dose, versus −3.3 kg with placebo — all with P < 0.001 against placebo. The common adverse events with bimagrumab were muscle spasms, diarrhea, and acne.

Read what this result doesn't say. The antibody alone lost less weight than semaglutide alone. What the field's hypothesis actually supports is something else — the composition of the weight lost —, and that's the question the trevogrumab phase 2 trial, COURAGE (NCT06299098), set as its primary endpoint: percentage change in fat mass, lean mass, and weight. There are 1,005 participants, start date March 13, 2024, primary completion projected for June 16, 2026, and no published results that I have found — PubMed returns two articles for the drug name, and neither is a trial.

The only one with a label is for a disease almost no one has

Garetosmab has an FDA-approved label under the brand name PASATRU (BLA 761508, Regeneron), with an effective date of August 14, 2026 in the record I read. The indication has nothing to do with weight: it's to reduce the formation of new heterotopic ossification lesions and flare-ups in adults with fibrodysplasia ossificans progressiva, a rare disease in which soft tissue turns into bone.

It's the most useful example on this entire page, because it shows what it means for a molecule of this class to reach the market: a 60-minute infusion every four weeks, at a healthcare facility; a single-dose vial of 300 mg in 5 mL; store between 2 °C and 8 °C, in the original packaging, protected from light, without freezing or shaking. The most common adverse reactions, with an incidence of 10% or more, are abscess, acne, increased hair growth, madarosis, oral ulcers, and epistaxis.

The label also carries the number no vial peptide has: 1.3% (1 of 76) of patients treated for up to 76 weeks developed antibodies against the drug itself. A large protein is seen by the immune system; the label measures that and publishes it.

What was withdrawn before it began

• NCT06901349 — bimagrumab with tirzepatide, from Eli Lilly: withdrawn. The reason recorded on ClinicalTrials.gov is Study terminated for strategic business reasons.
• NCT06970405 — garetosmab in healthy obese men and postmenopausal women, phase 1: withdrawn. Reason recorded: Sponsor Decision. It was the only garetosmab study I found outside of rare disease.
• What's still standing: NCT06643728, on bimagrumab with tirzepatide for weight management, with 252 participants, is active and no longer recruiting; and NCT05933499, conducted by Massachusetts General Hospital with 63 participants, is recruiting.
• A withdrawn trial is not evidence of harm. It's evidence that the answer won't come soon — and, in a field where enthusiasm runs far ahead of the data, knowing which questions stopped being asked is worth as much as knowing the answers that exist.

Anti-doping: the class already has a detection method

Anyone training to compete has a concrete data point here, and it's an old one. According to the article by Sakellariou and colleagues in Scientific Reports in 2025, inhibitors of activin receptor signaling pathways are included in sections S2 (Peptide hormones, growth factors, related substances and mimetics) and S4 (Hormone and metabolic modulators) of the World Anti-Doping Agency Prohibited List.

The same paper describes a method capable of detecting nine of these substances in serum and plasma for doping control, garetosmab among them, with a detection limit between 10 and 50 ng/mL. And bimagrumab already had its own method published in 2018, in Proteomics Clinical Applications, by the same Cologne group.

In other words: detection isn't behind the molecule. It's ahead of it.

What this survey did not do

• I did not check Brazilian registration for any of the three. This site's Brazilian sweep is from September 4 and did not cover this class. Any statement I made today about Brazilian registration of these antibodies would be a guess. What is verified is the FDA label: one of the three has one, two don't.
• I did not check whether any of them is sold as research material. This site grew out of a source that catalogs peptide vials; I did not go to any vendor to check whether antibodies circulate through the same channel, and I will not claim either that they do or that they don't.
• I read abstracts and labels, not the full articles. The BELIEVE numbers are the ones the abstract itself publishes. I did not go to the supplementary material, did not separate per-protocol from intention-to-treat analysis, and did not assess risk of bias.
• I didn't cover the class. The same query that returned 149 distinct generic names in the FDA database shows the scale of what was left out: this page covers three antibodies, chosen because they come up in the body-composition conversation, and says nothing about the others.
• There is no reconstitution table, community dose, or cycle structure on this page, and that's deliberate. Publishing a homemade regimen for a molecule that only exists as a hospital infusion would give it an appearance of protocol that the evidence and the pharmaceutical form itself do not support.

References

1. Heymsfield SB et al. Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial. Nat Med. 2026;32(3):869-882 — BELIEVE, NCT05616013, 507 participants, 48 weeks. PMID 41772149.
2. BELIEVE record on ClinicalTrials.gov, with the nine-group design and the intravenous bimagrumab doses every 12 weeks.
3. COURAGE record on ClinicalTrials.gov — trevogrumab, with or without garetosmab, added to semaglutide; 1,005 participants; fat mass and lean mass among the primary endpoints.
4. Record of the bimagrumab-with-tirzepatide study withdrawn by Eli Lilly, with the stated reason.
5. Record of the garetosmab phase 1 study in healthy obese people, withdrawn by sponsor decision.
6. Approved PASATRU label (garetosmab-grts), BLA 761508, in the FDA's public label database — class, molecular mass, route, dose, storage, adverse reactions, and immunogenicity.
7. Sakellariou P, Walpurgis K, Thomas A et al. Combined detection of inhibitors of the activin receptor signaling pathways (IASPs) by means of LC-HRMS/MS for human doping control. Sci Rep. 2025;15:19887 — sections S2 and S4 of the Prohibited List and the method for nine substances. PMID 40481031.
8. Walpurgis K, Thomas A, Dellanna F, Schänzer W, Thevis M. Detection of the Human Anti-ActRII Antibody Bimagrumab in Serum by Means of Affinity Purification, Tryptic Digestion, and LC-HRMS. Proteomics Clin Appl. 2018;12(3):1700120. PMID 29226558.
9. Executive summary of the WHO's 73rd INN Consultation, Working Doc. 21.533, November 2021 — the four monoclonal antibody groups and the decision to drop the -mab stem.
10. World Anti-Doping Agency Prohibited List, the same source used on the other pages of this site.
11. The FDA's public label database, used for the class counts and to confirm the absence of registration for bimagrumab and trevogrumab.

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