Bimagrumab: The Weight Loss Drug That Builds Muscle Instead of Burning It

There is a persistent myth about weight loss medications: that losing weight on GLP-1 drugs means losing muscle too, and that there is nothing you can do about it. The "Ozempic face" and "Ozempic body" concerns have made this one of the most common objections patients raise.

The muscle loss part is real. It is not a myth. When you lose significant weight through any method, including GLP-1 medications, diet, or even bariatric surgery, roughly 25-40% of what you lose is lean mass rather than fat. On semaglutide 2.4 mg, clinical data shows about 7.4% lean mass loss alongside the weight reduction. That is a meaningful trade-off.

But the idea that nothing can be done about it? That is changing.

Bimagrumab is a monoclonal antibody that works through an entirely different mechanism from GLP-1 drugs. It blocks the signals that tell your body to break down muscle. In a Phase 2b trial, patients who took bimagrumab alongside semaglutide lost 22.1% of their body weight, and 92.8% of that weight loss came from fat. Their lean mass loss was just 2.9%, compared to 7.4% with semaglutide alone.

This article explains how bimagrumab works, what the clinical data shows, and what it could mean for patients concerned about muscle loss during weight loss treatment.

A completely different mechanism

Bimagrumab does not work like GLP-1 drugs at all. It does not suppress appetite. It does not slow gastric emptying. It does not affect blood sugar.

Instead, bimagrumab is a fully human monoclonal antibody that blocks activin type II receptors (ActRIIA and ActRIIB) on muscle and fat cells. These receptors normally receive signals from myostatin, activin A, and other proteins that tell your muscles to break down and your fat cells to store more fat.

By blocking those receptors, bimagrumab does two things simultaneously:

It promotes muscle growth. Without the breakdown signals from myostatin and activin, muscle protein synthesis increases. In clinical trials, patients on bimagrumab alone actually gained 2.5-3.6% lean mass while losing fat.

It promotes fat loss. ActRII blockade also affects adipose tissue, promoting brown adipose tissue differentiation and increasing fat oxidation. This is an appetite-independent effect, meaning the fat loss happens through direct tissue-level changes, not because patients eat less.

This is why bimagrumab is described as complementary to GLP-1 medications rather than competing with them. GLP-1 drugs reduce how much you eat. Bimagrumab changes what your body does with the tissue it has. The combination targets weight loss from two independent directions.

The original proof of concept: JAMA Network Open (2021)

Bimagrumab's potential for body composition was first demonstrated in a Phase 2 trial published in JAMA Network Open in January 2021. The trial enrolled 75 adults with type 2 diabetes and obesity (mean BMI 32.9, mean age 60.4 years) and ran for 48 weeks.

Patients received bimagrumab 10 mg/kg by intravenous infusion every four weeks versus placebo.

The key finding: bimagrumab reduced fat mass by one-fifth while simultaneously increasing lean mass. No GLP-1 medication does both. The body weight change of 6.5% was modest compared to GLP-1 drugs, because muscle gain partially offset fat loss on the scale. But the body composition change was dramatic.

This trial proved the concept. The question was whether combining bimagrumab with a GLP-1 drug could get the best of both: the appetite suppression and large-scale weight loss from GLP-1, plus the muscle preservation and fat-targeted weight loss from bimagrumab.

The BELIEVE trial: combination with semaglutide

The Phase 2b BELIEVE trial, presented at the American Diabetes Association Scientific Sessions in June 2025, answered that question. It enrolled 507 adults with overweight or obesity (without type 2 diabetes) and tested bimagrumab combined with semaglutide over 72 weeks.

Bimagrumab was given as an intravenous infusion at weeks 4, 16, 28, and 40 (roughly every 12 weeks). Semaglutide was given as a once-weekly subcutaneous injection at either 1.0 mg or 2.4 mg.

Weight loss results at 72 weeks

What these numbers mean

The combination produced more weight loss than either drug alone (22.1% vs 15.7% for semaglutide, 10.8% for bimagrumab). But the quality of that weight loss was the real story.

With semaglutide alone, 28.2% of the weight lost was lean mass. With the combination, only 7.2% was lean mass. That is a 67% improvement in muscle preservation.

Patients on bimagrumab alone lost 100% fat, no lean mass at all, and actually gained 2.5% lean mass. But the total weight loss was only 10.8%, much lower than what GLP-1 drugs achieve.

The combination gives you the large-scale weight loss of semaglutide (22.1%) with nearly all of it coming from fat (92.8%) rather than a mix of fat and muscle.

Nearly 70% of participants in the combination group lost at least 20% of their body weight. 94% achieved at least 30% fat mass reduction.

Development history

Bimagrumab has had an unusual journey to get here:

Novartis originally developed it for rare muscle-wasting conditions, specifically sporadic inclusion body myositis (sIBM). It failed a Phase 2b/3 trial for sIBM in 2016 because while it improved muscle mass, it did not improve physical function measures like walking distance.

Versanis Bio, a biotech startup founded in 2021 with $70 million in Series A funding, licensed bimagrumab from Novartis and repositioned it for obesity. The JAMA Network Open trial that demonstrated its body composition effects was the basis for this pivot.

Eli Lilly acquired Versanis Bio in July 2023 for up to $1.925 billion. Lilly now develops bimagrumab (designated LY3985863) as a complement to their tirzepatide (Zepbound) franchise.

Current development status

Bimagrumab is still in Phase IIb. It has not entered Phase 3 trials. No NDA filing has been made or announced. If Phase 3 begins in 2026-2027, approval would be at the earliest around 2029-2030.

Eli Lilly terminated one of two planned combination trials with tirzepatide, but the obesity-only trial continues. The cancellation was described as strategic, not related to safety or efficacy concerns.

Side effects

Bimagrumab's side effects are distinct from GLP-1 medications because the mechanism is different:

The muscle spasms and acne are unique to bimagrumab's ActRII blocking mechanism. These side effects were generally mild to moderate, transient, and required minimal treatment.

In the BELIEVE trial, 9% of combination-treated participants discontinued due to adverse events over 72 weeks. No new safety signals emerged from combining the two drugs. No significant cardiac effects were found on ECG or echocardiography in any trial.

When used in combination with semaglutide, patients may experience both profiles: GI side effects from the semaglutide and muscle spasms/skin changes from the bimagrumab.

What this means for GLP-1 patients

Bimagrumab addresses the biggest clinical criticism of GLP-1 weight loss treatment: muscle loss. This matters most for:

Older patients. Age-related muscle loss (sarcopenia) is already a concern. Losing additional muscle through weight loss treatment can accelerate functional decline. Preserving muscle mass during weight loss is especially important for patients over 50-60.

Patients losing large amounts of weight. The more weight you lose, the more lean mass you lose in absolute terms. Patients targeting 20%+ weight reduction lose more muscle than those targeting 10%.

Patients concerned about metabolic health. Muscle is metabolically active tissue. Losing it can reduce resting metabolic rate, which may contribute to weight regain after stopping treatment. Preserving muscle may improve long-term weight maintenance.

It is worth noting that exercise during GLP-1 treatment also helps preserve muscle mass, though the effect is smaller than what bimagrumab demonstrated in trials.

Frequently asked questions

What is bimagrumab?

Bimagrumab is a monoclonal antibody that blocks activin type II receptors, preventing muscle breakdown signals from myostatin and activin. It promotes fat loss and muscle preservation through an appetite-independent mechanism, completely different from how GLP-1 drugs work.

Can bimagrumab be used with GLP-1 medications?

In the BELIEVE trial, bimagrumab combined with semaglutide produced 22.1% weight loss at 72 weeks, with 92.8% of weight loss from fat and only 2.9% lean mass reduction. The combination was well tolerated. Trials combining bimagrumab with tirzepatide are ongoing.

Is bimagrumab available?

No. Bimagrumab is in Phase IIb development. It has not been approved anywhere and is not available by prescription. Based on current timelines, approval could come around 2029-2030 at the earliest.

How is bimagrumab given?

In clinical trials, bimagrumab has been administered as an intravenous infusion roughly every 12 weeks. This is different from GLP-1 medications, which are self-administered via subcutaneous injection or oral pill.

Does exercise help with muscle loss on GLP-1s in the meantime?

Yes. Resistance training during GLP-1 treatment helps preserve muscle mass, though not to the same degree as bimagrumab showed in trials. Our guide to combining exercise with GLP-1 treatment covers practical strategies.

The bottom line

Bimagrumab is the most advanced drug specifically designed to preserve muscle during weight loss. The BELIEVE trial showed that combining it with semaglutide converts weight loss from roughly 72% fat / 28% lean mass to 93% fat / 7% lean mass, while also increasing total weight loss from 15.7% to 22.1%.

It is not available yet, and it may be several years before it reaches patients. But the concept it proves is important: muscle loss during obesity treatment is not an inevitable trade-off. It is a problem that can be addressed pharmacologically.

In the meantime, if you are concerned about muscle preservation during GLP-1 treatment, resistance training and adequate protein intake remain the best available strategies.

Bimagrumab is an investigational drug not approved by any regulatory authority. The clinical trial results discussed are from published peer-reviewed studies and conference presentations but may not reflect final outcomes. Weight loss and body composition results vary by individual. This article is for informational purposes only and does not constitute medical advice. Consult your doctor before starting any medication. Trimly is a MOH-licensed telehealth clinic (HCSA License R/25M0505/MDS/001/252).