Bimagrumab is a monoclonal antibody, not a peptide. This entry documents it in PeptideBlog.net’s related non-peptide compounds category (adjunct research area) because it keeps coming up alongside peptide-based GLP-1 drugs (semaglutide, tirzepatide) in discussions of preserving muscle during weight-loss therapy. Originally developed by Novartis for a muscle-wasting disease (sporadic inclusion body myositis) and shelved after a 2016 trial failure, it was revived by Versanis Bio and, since Eli Lilly’s 2023 acquisition of Versanis, is being tested intravenously alongside semaglutide and tirzepatide to blunt the lean-muscle loss seen with GLP-1 drugs. A published phase 2 trial (BELIEVE) found the combination with semaglutide produced more weight loss and proportionally more of it as fat loss than semaglutide alone, but a separate 2025 Lilly trial pairing bimagrumab with tirzepatide was halted early for an unstated reason, and bimagrumab itself remains unapproved for any use.
Not a peptide: why it is here
Bimagrumab (development code BYM338) is a full-length human monoclonal antibody, built from roughly 142,450 daltons’ worth of folded antibody protein, not a short synthesized chain of amino acids. That puts it in a different manufacturing and regulatory category from the peptides this site otherwise covers. PeptideBlog.net includes it, alongside a small number of other non-peptide compounds, because readers researching muscle preservation during GLP-1 weight loss frequently encounter it in the same conversations as peptides like retatrutide or tirzepatide, and an accurate, sourced page is more useful to them than silence.
What is bimagrumab?
Bimagrumab is a human monoclonal antibody that was originally developed by Novartis, working with Morphosys, and given FDA breakthrough therapy designation in 2013 for sporadic inclusion body myositis (sIBM), a rare, progressive disease that weakens skeletal muscle. In a large placebo-controlled phase 2b/3 trial (RESILIENT, NCT01925209, 251 patients), bimagrumab increased lean body mass and muscle strength on secondary measures but did not meet its primary endpoint, a change in six-minute walking distance at week 52; Novartis reported the miss in April 2016 and the sIBM program did not advance to approval.
Novartis subsequently explored bimagrumab in other muscle-wasting and metabolic settings, including sarcopenia in older adults, muscle loss after hip fracture, cachexia in lung and pancreatic cancer, COPD-related cachexia, and type 2 diabetes with obesity, all as intravenous infusions, before licensing the molecule to the obesity-focused biotech Versanis Bio in 2021. Eli Lilly acquired Versanis Bio in 2023 for up to $1.9 billion specifically for bimagrumab’s potential in cardiometabolic disease, and the drug’s newest and most publicized research role is as a muscle-preserving add-on to GLP-1 receptor agonist weight-loss drugs. As of this writing bimagrumab is not approved by any regulator for any indication.
How does it work?
Plain-English explanation
The body has a natural braking system on muscle growth, run mainly by a protein called myostatin and related activin-family proteins. These proteins act through a receptor on muscle cells called ACVR2B (activin receptor type 2B). Bimagrumab is an antibody that physically occupies that receptor, blocking myostatin and activin from telling muscle cells to stay small. With the brake released, muscle tissue tends to grow, which is why bimagrumab increases lean mass in several of its trials, sometimes even without any exercise or diet intervention.
Technical explanation
Bimagrumab is a human IgG1 monoclonal antibody that binds ACVR2B (and cross-reacts with the closely related ACVR2A) with high affinity, acting as a competitive antagonist of myostatin (GDF-8), GDF-11, and activin A signaling through that receptor. Blocking ligand binding at ACVR2B blocks downstream SMAD2/3 phosphorylation, the intracellular signaling cascade that normally restrains muscle protein synthesis and promotes muscle protein breakdown, and this de-repression drives skeletal muscle hypertrophy. The antibody is given by intravenous infusion and has a long circulating half-life; in the obesity trials described below, doses of 10 mg/kg and 30 mg/kg were used with loading doses at weeks 1 and 4 followed by dosing every 12 weeks.
Potential benefits and research applications
Preserving lean mass during GLP-1 weight loss
What is being investigated: whether adding bimagrumab to a GLP-1 receptor agonist (semaglutide or tirzepatide) can produce weight loss that is proportionally more fat and less muscle than the GLP-1 drug alone. How the effect might occur: ACVR2B blockade directly promotes muscle growth or preservation, working through a mechanism unrelated to and additive with appetite suppression. Evidence: the phase 2 BELIEVE trial (NCT05616013, 507 adults with obesity, published in Nature Medicine) found that at week 48, the combination of bimagrumab 30 mg/kg plus semaglutide 2.4 mg produced greater absolute weight loss than semaglutide alone, with a much smaller proportional loss of lean mass in the combination arm than in the semaglutide-alone arm. Strength: this is a real, published, randomized, placebo-controlled phase 2 human trial. Limitation: it is a single phase 2 trial; no phase 3 data exist, and the drug is combined with, not compared as a standalone weight-loss agent against, an approved GLP-1 drug.
Fat loss and visceral fat reduction
What is being investigated: whether bimagrumab, alone or combined with a GLP-1 drug, shifts weight loss composition toward fat rather than lean tissue, including visceral (abdominal) fat specifically. How the effect might occur: by sparing muscle rather than by directly burning more fat, so a larger share of any given amount of weight lost is fat by default. Evidence: BELIEVE trial DXA (dual-energy X-ray absorptiometry) data reported markedly greater proportional fat-mass loss and visceral adipose tissue reduction in the combination arm than with semaglutide alone. Strength: measured with DXA body-composition scanning in a randomized trial, a relatively objective method. Limitation: DXA-based body composition findings from a single mid-size phase 2 trial have not yet been replicated in an independent or larger study.
Muscle strength and mass in sarcopenia and muscle-wasting disease
What is being investigated: bimagrumab’s original purpose, restoring or preserving muscle mass and strength in conditions such as sporadic inclusion body myositis, age-related sarcopenia, post-hip-fracture muscle loss, and cancer- or COPD-related cachexia. How the effect might occur: direct ACVR2B blockade driving muscle hypertrophy, independent of any weight-loss drug. Evidence: multiple Novartis-sponsored phase 2 trials showed measurable increases in lean body mass by DXA, sometimes accompanied by improved muscle strength, but the pivotal sIBM trial (RESILIENT) did not translate that lean-mass gain into a meaningful improvement in physical function (six-minute walking distance), its primary endpoint. Strength: lean-mass gains have been shown repeatedly across several human trials in different populations. Limitation: no muscle-wasting indication for bimagrumab has reached approval; the drug’s inability to move the needle on functional outcomes in sIBM, despite objective muscle growth, was a well-documented setback for the “more muscle equals better function” hypothesis in that specific disease.
What dosage information circulates?
Figures in this section summarize amounts and schedules reported in published research or circulating online. Their inclusion documents what is reported and does not establish that a regimen is verified, safe, effective, or appropriate.
Bimagrumab has never been available to consumers or prescribed outside of clinical trials, so unlike many peptides discussed elsewhere on this site, there is no gray-market or self-administered dosing pattern to report. Every dose figure below comes from company-sponsored, registered clinical trials; no established or reliably sourced dosing information exists for any use outside a trial setting, and none should be inferred from these figures.
Amounts studied in human research
In the BELIEVE obesity trial, bimagrumab was given by intravenous infusion at 10 mg/kg or 30 mg/kg, with loading doses at week 1 and week 4, then every 12 weeks thereafter, over a 48-week primary treatment period (with an optional extension to week 72), alone or combined with open-label subcutaneous semaglutide at 1.0 mg or 2.4 mg weekly. In the earlier RESILIENT sIBM trial, bimagrumab was dosed intravenously at multiple dose levels (up to 10 mg/kg) approximately every four weeks for 52 weeks. A 2025 Lilly-sponsored trial combining bimagrumab with tirzepatide in obesity (NCT06901349) was halted less than a month after starting, with no reason disclosed; a related, still-active phase 2 bimagrumab-plus-tirzepatide trial (NCT06643728) has enrolled but not yet reported results.
Practitioner and community-reported protocols
Not consistently reported. Bimagrumab requires intravenous infusion at a clinical site and has never been sold or self-administered outside a trial, so this site did not identify a genuine, verifiable community or practitioner dosing thread for it, distinct from its own trial doses.
Combinations and related research
Formally studied combination: bimagrumab plus semaglutide, tested head-to-head against each drug alone and against placebo in the BELIEVE phase 2 trial described above; this is the best-sourced combination for bimagrumab. Formally studied but incompletely reported combination: bimagrumab plus tirzepatide has been the subject of at least two Lilly-sponsored phase 2 trials (NCT06901349, halted early with no reason given, and NCT06643728, active but unreported as of this entry’s research). No results from any tirzepatide-combination trial were available to review at the time of this entry. Community-reported combinations: none identified; because bimagrumab is not available outside formal trials, this site did not find self-directed combination protocols to document.
Side effects, risks, and limitations
In the BELIEVE obesity trial, bimagrumab-associated adverse events reported in secondary coverage included muscle spasms, diarrhea, and acne, while semaglutide-associated adverse events included nausea, diarrhea, constipation, and fatigue; treatment discontinuation rates were higher with bimagrumab monotherapy (roughly 14.0 to 21.4 percent across dose arms, per secondary reporting) than with the combination (roughly 5.3 to 12.5 percent), and three cases of pancreatitis were reported, described as balanced across treatment groups, with no deaths reported in that trial. In the earlier RESILIENT sIBM trial, bimagrumab increased objective lean muscle mass without producing a corresponding functional benefit on its primary endpoint; this entry’s sources do not provide a detailed breakdown of that trial’s adverse-event profile. As a monoclonal antibody rather than a peptide, bimagrumab also carries antibody-class risks such as infusion reactions and the possibility of immunogenicity (anti-drug antibody formation). Because bimagrumab requires intravenous infusion at a medical facility and has never been sold commercially, the purity, contamination, and mislabeling concerns this site flags for self-sourced injectable peptides do not apply to it in the same way.
Regulatory and developmental status
Bimagrumab is not approved by any regulator for any indication, as of this entry’s research (September 2026). It received FDA breakthrough therapy designation in 2013 for sporadic inclusion body myositis, but that program did not advance after its pivotal RESILIENT trial missed its primary endpoint in 2016. Novartis subsequently explored it in sarcopenia, post-hip-fracture muscle loss, cancer and COPD cachexia, and type 2 diabetes with obesity, none of which reached approval, before licensing the molecule to Versanis Bio in 2021. Eli Lilly acquired Versanis Bio in 2023 (a deal reported at up to $1.9 billion) and currently sponsors bimagrumab’s phase 2 development as a muscle-preserving adjunct to its own GLP-1 drugs. No phase 3 trial or regulatory filing for bimagrumab in any obesity-adjunct or other indication had been reported as of this entry’s research.
Frequently asked questions
Is bimagrumab a peptide?
No. It is a monoclonal antibody, a much larger, more complex protein than a peptide. It is included on this site as a related non-peptide compound because of how often it comes up in discussions of preserving muscle during GLP-1 weight loss.
What is bimagrumab?
A human monoclonal antibody, also known by its development code BYM338, that blocks a muscle-growth-limiting receptor called ACVR2B. It was originally developed by Novartis for a muscle-wasting disease, then acquired by Eli Lilly in 2023 through its purchase of Versanis Bio and is now being studied as an add-on to GLP-1 weight-loss drugs.
How does it work?
It blocks the ACVR2B receptor that myostatin and activin normally use to restrain muscle growth. With that brake released, muscle tissue tends to grow or is better preserved, even during periods of significant weight loss.
What dosage has been studied?
In the most relevant recent trial (BELIEVE), 10 mg/kg or 30 mg/kg by intravenous infusion, with loading doses at weeks 1 and 4, then every 12 weeks, combined with weekly subcutaneous semaglutide.
What dosage commonly circulates online?
None. Bimagrumab has never been sold or available outside clinical trials, so there is no community or practitioner dosing pattern to report.
Has it been studied in humans?
Yes, extensively, across more than a dozen registered trials since around 2011, most recently and most prominently in combination with semaglutide (published, phase 2) and separately with tirzepatide (unpublished or halted, phase 1 and 2).
Does it preserve muscle during GLP-1 weight loss?
In the one published phase 2 trial that tested this directly (BELIEVE), adding bimagrumab to semaglutide produced more total weight loss, and a much smaller proportional loss of lean mass, than semaglutide alone. This is a single phase 2 finding, not a settled, approved use.
Is it approved?
No, it is not approved anywhere for any use. It remains investigational, phase 2 stage, after its original sIBM program failed in 2016.
What remains unknown?
Why Eli Lilly halted its bimagrumab-plus-tirzepatide obesity trial after less than a month in 2025 (no reason was disclosed); whether bimagrumab will advance to phase 3 or ever reach approval for any indication; and why its clear lean-mass gains did not translate into functional improvement in the earlier sIBM trial.
Bottom line
Bimagrumab is a monoclonal antibody, not a peptide, that blocks the ACVR2B receptor to release a natural brake on muscle growth. It failed its original pivotal trial in a muscle-wasting disease in 2016 despite growing muscle on DXA scans, and it now has one published, positive phase 2 result (BELIEVE) showing that adding it to semaglutide increases weight loss and preserves substantially more lean mass than semaglutide alone. A separate 2025 trial pairing it with tirzepatide was halted early for an undisclosed reason, and a related tirzepatide-combination trial remains active but unreported, so its future in the GLP-1 muscle-preservation space is genuinely unresolved. It is not approved anywhere, has not been tested in a phase 3 trial, and no verifiable community dosing or discussion pattern exists for it the way it does for many peptides on this site.