What Is Bimagrumab?

1. How does bimagrumab work?

Muscle size is held in check by a family of signaling proteins. The best known is myostatin, but activin A and GDF11 send a similar “stop” message through the same receptors, called activin type II receptors (ActRIIA and ActRIIB). Bimagrumab binds those receptors and blocks all of these signals at once, rather than neutralizing a single one (Lach-Trifilieff and colleagues’ 2014 study).

In that early laboratory and animal work, the antibody enlarged mouse muscle more than an inhibitor aimed at myostatin alone. A mouse version of it still built muscle in animals bred without myostatin, which shows that the other ligands matter too. In cultured muscle cells, it blocked the wasting effects of both myostatin and activin A. These are animal and cell findings.

The fat effect works through a separate route. Activin E acts on fat cells with help from a second receptor, ALK7, and that signal is cut off when the type II receptors are blocked. The BELIEVE investigators describe the result as increased fat breakdown, with no apparent effect on food intake. GLP-1 drugs mainly work by lowering appetite, so the two mechanisms do not overlap.

How bimagrumab compares with other pathway blockers

Blocking the receptor shuts the door for every ligand that uses it. Ligand traps such as ACE-083 instead mop up the signals before they arrive.

2. What happened when bimagrumab was combined with semaglutide?

The BELIEVE trial is the main reason glp1.app readers hear about bimagrumab. Heymsfield and colleagues randomized 507 adults with obesity, averaging 107.5 kg, into nine groups for 48 weeks. Groups received placebo, IV bimagrumab at 10 or 30 mg/kg every 12 weeks, weekly semaglutide at 1.0 or 2.4 mg, or a combination. An open-label extension then ran to week 72.

Weight loss over time

Bimagrumab alone lost less weight on the scale than semaglutide alone. That is expected: it was adding muscle while removing fat, so the two partly cancelled out in body weight. Fat loss at week 72 was about the same in the two single-drug groups.

Where the weight came from

The more useful question for anyone on a GLP-1 is what kind of tissue was lost. At week 48, the trial reported these lean-mass changes by type of group:

In plain terms, roughly 29% of the weight lost on semaglutide 2.4 mg alone was not fat, compared with under 8% when high-dose bimagrumab was added. Visceral fat, the fat packed around abdominal organs, also fell more in the combination groups than with semaglutide alone.

Did strength improve?

Mostly not measurably. Grip strength improved significantly in only one group, bimagrumab 30 mg/kg plus semaglutide 1.0 mg, which gained 4.8 kg compared with 1.7 kg on placebo. Lean mass on a body scan includes water and organ tissue, not only muscle, so preserving it is not the same as proving better strength or function. Section 4 explains why this distinction matters for this drug in particular.

What the animal work added

Before BELIEVE, Nunn and colleagues tested the pairing in mice with diet-induced obesity. Bimagrumab added roughly 10% lean mass, and with semaglutide it increased fat loss while keeping lean mass. This is mouse data and was a rationale for the human trial, not a substitute for it.

3. Is bimagrumab being tested with tirzepatide?

Yes. Lilly agreed in July 2023 to buy Versanis Bio, which had moved bimagrumab into obesity research, for up to $1.925 billion, and named bimagrumab as Versanis’ lead asset (Lilly and Versanis announcement). Lilly’s current program pairs it with its own drug, tirzepatide.

The registry record for NCT06643728 describes a phase 2 trial in 252 people with obesity who do not have diabetes. All study drugs are given by subcutaneous injection, and the record lists doses only as “Dose 1” and “Dose 2.” The primary outcome is percent change in body weight at week 24. Primary data collection finished on January 16, 2026, the full study is expected to end in January 2027, and no results had been posted as of the record’s May 2026 update.

Lilly also registered a phase 2b study for type 2 diabetes, then pulled it with zero participants enrolled, “for strategic business reasons” (NCT06901349). Until tirzepatide data are published, any claim about how bimagrumab performs with tirzepatide goes beyond the evidence. The semaglutide findings cannot simply be transferred to a different drug and a different route.

4. Does more lean mass mean more strength or function?

So far, no. Novartis originally developed bimagrumab for muscle-wasting conditions, and every trial that measured function found the same gap: muscle grew, but performance did not improve beyond what control groups achieved.

A follow-on extension of RESILIENT lasting up to two years enrolled 211 people; it was ended early and again found no gain in mobility. Those results ended development for muscle diseases.

Healthy young men show the same pattern. After two weeks with a leg in a cast, 24 men received one 30 mg/kg dose or placebo. At 12 weeks, thigh volume was 5.1% above the pre-cast level with bimagrumab versus −0.1% with placebo, yet knee strength recovered equally in both groups within six weeks (Rooks and colleagues’ immobilization study). No trial has studied trained athletes.

For GLP-1 users, this history suggests caution. Keeping lean mass during weight loss may still prove worthwhile, but whether it protects strength, mobility or long-term health has not been shown.

5. What did trials find for blood sugar and type 2 diabetes?

In a phase 2 study lasting 48 weeks, 75 people with type 2 diabetes (mean BMI 32.9) were assigned at random to IV bimagrumab 10 mg/kg (capped at 1,200 mg) every four weeks or placebo, with diet and exercise counseling for both (Heymsfield and colleagues’ 2021 trial).

Two limits apply. Only the 58 people (77%) who completed treatment were analyzed, and the authors reported 80% confidence intervals rather than the conventional 95%.

A 2026 meta-analysis by Shao and colleagues combined four trials (268 people in total) that compared the drug with placebo in obesity, insulin resistance or type 2 diabetes. Against placebo, bimagrumab cut weight by 4.85 kg and fat mass by 4.72 kg, added 1.66 kg of lean mass and lowered HbA1c by 0.13 points. It also raised LDL cholesterol by 0.47 mmol/L, about 18 mg/dL.

One mouse study points the other way. Using an antibody that blocks the same receptors, Carlsson and colleagues saw worse glucose tolerance, larger hearts loaded with glycogen, fattier livers and mice that ran less on their wheels. The authors note the conflict with human results and suggest species differences may explain it.

6. Is bimagrumab approved or legal?

Bimagrumab has no approval from the FDA or any other regulator. Its registered studies span sarcopenia, hip fracture, COPD, cancer-related weight loss, inclusion body myositis and obesity (ClinicalTrials.gov listing). As an unapproved biologic, it is legitimately available only inside these trials.

The US Schedule III list of anabolic steroids does not include it, so it is not a federally controlled substance. That does not make an unapproved antibody legal to sell as a medicine.

In sport, the WADA 2026 Prohibited List bans agents that prevent activin receptor IIB activation under class S4.3, at all times, and names bimagrumab as an example. S4.3 substances are classed as non-specified.

7. What doses were studied, and how long does it last?

The doses below come from trials and are listed only to explain the research. They are not recommendations, and no dose is approved.

Why there is no simple half-life

In phase 1 work, a single 30 mg/kg IV dose produced peak blood levels of 982 μg/mL in older adults and 944 μg/mL in adults with obesity (Rooks and colleagues’ pharmacokinetic study). Clearance was nonlinear because part of the drug is removed when it binds its receptors. A standard half-life therefore could not be calculated, and higher doses stayed in the blood disproportionately longer. The lean-mass effect also persisted longer at 30 mg/kg than at 3 mg/kg.

Another phase 1 study, in 91 people aged at least 70, compared monthly IV infusions with weekly subcutaneous injections. About 40% of a subcutaneous dose reached the bloodstream, and weekly injections gave exposure and body-composition effects comparable with monthly infusions (Petricoul and colleagues). Every regimen except the lowest (52.5 mg) raised lean mass by 4–6%, or 1.5–2 kg, and fat mass fell about 2–3 kg. Because the drug lacks a fixed half-life, a simple half-life model cannot predict how long it stays active.

8. What are the side effects and risks?

Common side effects

In BELIEVE, the most frequent problems through week 48 were muscle spasms (mostly cramps), diarrhea and acne (Heymsfield and colleagues, 2026):

Other trials show the same picture. The pooled Shao meta-analysis estimated a roughly tenfold higher risk of muscle spasms, about five times the risk of diarrhea and nearly six times the risk of stopping treatment early compared with placebo.

Cholesterol and lab changes

LDL cholesterol rose in every bimagrumab group during the first 12 weeks of BELIEVE. At week 72 it remained 17.6% above baseline with bimagrumab alone but was back near baseline (+0.1%) with the semaglutide 2.4 mg combination. What this means for long-term heart risk is unknown.

Other blood-test shifts in BELIEVE’s bimagrumab arms included higher creatine kinase and alkaline phosphatase, short-lived increases in ALT, AST (liver enzymes) and lipase (a pancreatic enzyme), and a drop in magnesium that did not leave the normal range. Pancreatitis was rare and occurred at similar rates across groups. The 2021 diabetes trial recorded one pancreatitis case on bimagrumab, plus one person with high lipase and abdominal pain that turned out to be due to gallstones.

Bone, heart and hormones

• Bone. Hip bone density fell about 2% with semaglutide 2.4 mg and in most combination groups, versus 0.8% with placebo. The authors suggest reduced loading on bone after weight loss as a possible reason.
• Heart. A six-month trial in 68 healthy adults aged 60 to 86 used cardiac MRI and saw no meaningful shift in the heart’s main pumping chamber, either in its mass or its ejection fraction, with 10 mg/kg IV (Rooks and colleagues’ cardiac study). Longer and larger studies are lacking.
• Hormones. In women aged 55 to 75, FSH dropped by about 42 IU/L at week 8 versus placebo, reflecting activin’s role in pituitary control, and recovered once the drug cleared. Neither men nor women showed changes in testosterone or adrenal androgens (Garito and colleagues). Effects on fertility and pregnancy are unknown.

9. What can actually be measured or tracked?

The trials judged bimagrumab mainly by DEXA scans, which split the body into fat mass, lean mass and bone. An ordinary scale reports only total weight, and as BELIEVE shows, body weight alone can understate a drug that removes fat while adding muscle. Waist circumference, which fell 9 cm in the 2021 trial, is a simpler measure the trials also reported.

The safety signals in the trials were mostly found through blood tests: LDL cholesterol, creatine kinase, liver enzymes, lipase and magnesium, plus FSH in women. Those are clinical measurements that belong with a doctor, particularly for anyone already monitoring lipids or pancreatic symptoms on a GLP-1.

For the bigger question of whether preserved lean mass matters, the trials suggest tracking function, not just composition. Grip strength, walking tests and repeated strength performance are what the studies measured, and they are where bimagrumab has so far fallen short. Logging GLP-1 doses, protein intake and training alongside these measures, as described in the GLP-1 tracker guide, makes any change easier to interpret, though it cannot replace a controlled trial.

10. Bimagrumab FAQ

• Is bimagrumab FDA approved?

  No. Bimagrumab has not been approved by the FDA or any other regulator for any use. It is an investigational drug that is legitimately available only to people enrolled in clinical trials.

• Can bimagrumab be used with semaglutide?

  Only in a research setting so far. The BELIEVE phase 2 trial tested IV bimagrumab with weekly semaglutide and found more fat loss and better lean-mass preservation than semaglutide alone, along with more muscle spasms, diarrhea and acne. No approved combination exists.

• Is bimagrumab being tested with tirzepatide?

  Yes. Eli Lilly runs a phase 2 trial of subcutaneous bimagrumab with tirzepatide in 252 people with obesity but without diabetes. Its main outcome is percent change in body weight at week 24, and results had not been posted on ClinicalTrials.gov as of its last update.

• Does bimagrumab prevent muscle loss on GLP-1 drugs?

  In BELIEVE, lean mass fell 4.7% to 6.9% with semaglutide alone but only 0.8% to 2.3% when bimagrumab was added. Lean mass on a scan is not the same as strength or function, and only one combination group showed a significant grip-strength gain.

• What are the side effects of bimagrumab?

  Muscle spasms, diarrhea and acne were the most frequent. Trials also reported higher LDL cholesterol, temporary rises in liver and pancreatic enzymes, and higher rates of stopping treatment than placebo. FSH fell in women during treatment.

• How long does bimagrumab stay in your system?

  No single half-life has been established. The antibody is cleared partly by binding to its own receptors, so its removal is nonlinear and researchers could not calculate a standard half-life. Higher doses remained in the blood disproportionately longer.

• Is bimagrumab a steroid or a SARM?

  No. It is a fully human monoclonal antibody that does not act on the androgen receptor. In a study of adults aged 55 to 75 it did not change testosterone or adrenal androgens.

• Is bimagrumab banned in sport?

  Yes. The WADA 2026 Prohibited List bans agents that prevent activin receptor IIB activation at all times under class S4.3 and names bimagrumab as an example.

11. Sources