For the past three years, nearly every breakthrough in obesity medicine has traced back to a single class of drugs: GLP-1 receptor agonists like semaglutide and tirzepatide. Now, a phase 2 clinical trial published in Nature Medicine in early 2026 shows that a fundamentally different drug, one that never touches the GLP-1 receptor, can drive significant fat loss while preserving the lean muscle that GLP-1 drugs tend to strip away. The drug is bimagrumab, and the trial behind it is forcing obesity researchers to rethink how narrowly they have been designing treatments.
The BELIEVE trial: fat loss without the muscle penalty
The BELIEVE trial (registered as NCT05616013) was a randomized, blinded phase 2 study that enrolled adults with obesity across multiple treatment arms: bimagrumab alone, semaglutide alone, the two drugs combined, and placebo. The primary endpoint was absolute change in body weight, but the composition of that weight loss turned out to be the more striking finding.
Bimagrumab is a monoclonal antibody that blocks activin type II receptors, disrupting myostatin and activin signaling. That pathway governs muscle breakdown and fat storage, and it has nothing to do with the incretin hormones that GLP-1 drugs mimic. In the trial, participants who received bimagrumab alongside semaglutide lost body fat at rates comparable to or exceeding semaglutide alone, but they retained substantially more lean muscle mass. The monotherapy arm, bimagrumab without any GLP-1 drug, also produced meaningful fat reduction versus placebo.
The trial was sponsored by Versanis Bio, which Eli Lilly acquired in 2023 for up to $1.93 billion. Lilly’s investment signals that one of the largest players in the GLP-1 market sees commercial potential in a mechanism that complements, rather than competes with, its own incretin pipeline. The full study protocol, statistical analysis plan, and prespecified endpoints are publicly available through the ClinicalTrials.gov registry, giving independent scientists the documentation needed to evaluate the results on their own terms.
Mice without appetite circuits still lose weight on incretin drugs
A separate line of evidence, published in the International Journal of Obesity, tested what happens when you give semaglutide, tirzepatide, and retatrutide to mice that have been genetically engineered to lack the MC4R receptor, a central node in the brain’s appetite-regulation network. The POMC-MC4R pathway is widely considered the main downstream route through which these drugs suppress hunger.
The mice still lost weight. All three drugs produced quantified body weight reductions over the dosing period, even with that critical signaling pathway knocked out. The finding does not overturn the importance of MC4R signaling in humans, but it demonstrates that incretin-based drugs engage multiple biological routes simultaneously. Vagal nerve signaling, direct effects on gastric emptying, and other central nervous system pathways likely contribute to their efficacy in ways that are not yet fully mapped.
For drug developers, the implication is practical: if GLP-1 drugs do not depend on a single downstream cascade, then combining them with agents that target entirely separate biology could unlock additive or even synergistic effects rather than simply doubling down on the same mechanism.
Preclinical work confirms bimagrumab operates on its own track
Supporting the clinical findings, a peer-reviewed preclinical study indexed on PubMed showed that bimagrumab altered body composition and metabolic parameters in diet-induced obese mice, including during concurrent semaglutide treatment. The mouse data demonstrated that ActRII blockade and GLP-1 agonism produce additive rather than redundant effects: blocking myostatin preserved skeletal muscle while semaglutide reduced appetite and body fat. The combination shifted the ratio of what the animals lost, tilting weight reduction toward fat and away from lean tissue.
This matters because muscle loss during rapid weight reduction is one of the most persistent clinical concerns with current obesity drugs. Older adults, people with sarcopenia, and patients already at risk of frailty stand to lose functional capacity alongside body fat. A drug that selectively protects muscle while a GLP-1 agonist handles the fat could change the risk-benefit calculation for millions of patients.
Amgen’s MariTide: antagonism and agonism in one molecule
Amgen’s AMG 133, known as MariTide, represents yet another design philosophy. According to peer-reviewed preclinical and phase 1 data, MariTide is a bispecific molecule that blocks the GIP receptor while simultaneously activating the GLP-1 receptor. Rather than stimulating two incretin receptors the way tirzepatide does, MariTide suppresses one and stimulates the other, producing dose-dependent weight loss in early human testing.
Amgen has disclosed interim phase 2 data at medical conferences showing continued weight loss with monthly or less frequent dosing, a potential advantage over weekly injections. However, no phase 2 results have yet appeared in peer-reviewed journals, so the full dataset, including adverse events and durability of effect, has not been subjected to independent scrutiny. The concept of mixing receptor antagonism with agonism within a single molecule is distinct from both pure GLP-1 drugs and non-incretin approaches like bimagrumab, and it adds a third strategic lane to the obesity pipeline.
A regulatory precedent already exists for non-GLP-1 obesity drugs
The FDA has already approved one obesity treatment that bypasses the GLP-1 receptor entirely. Setmelanotide, developed by Rhythm Pharmaceuticals, is an MC4R agonist approved for weight management in people with obesity caused by specific rare genetic defects in the POMC, PCSK1, or LEPR genes. The approval, while limited to those monogenic conditions, established that targeting the melanocortin pathway can produce clinically meaningful weight loss when the right patients are identified.
That precedent sets a floor, not a ceiling. Regulators required statistically significant weight loss and clear functional benefit in a narrowly defined population. For drugs aimed at the broader obesity population, the bar will be higher: safety margins must be wider, long-term tolerability more certain, and the benefit over existing GLP-1 options clearly demonstrated. Bimagrumab and MariTide will both have to clear those hurdles in phase 3 programs.
What the evidence does not yet show
The BELIEVE results are phase 2 data. The sample size and treatment duration are smaller than what regulators require for approval, and the trial was designed to establish proof of concept and identify the right dose, not to confirm long-term safety or durability. Whether bimagrumab’s muscle-sparing benefits hold up over years of treatment, and whether serious adverse effects emerge at scale, will depend on phase 3 studies that have not yet reported. The Nature Medicine publication did not detail significant safety signals, but phase 2 trials are generally underpowered to detect rare adverse events.
The MC4R knockout mouse findings are mechanistically interesting but carry the standard limitations of animal research. The degree to which alternative signaling pathways driving weight loss in engineered mice operate the same way in humans with intact but variable MC4R function is unknown. Translation from mouse metabolism to human disease has a mixed track record, and these results should be treated as hypothesis-generating rather than clinically definitive.
Long-term cardiovascular outcomes data do not exist for any of these newer approaches. Semaglutide has accumulated cardiovascular safety evidence through the SELECT trial and other large outcomes studies, but bimagrumab, MariTide, and combination strategies pairing incretin drugs with muscle-pathway agents are too early in development to have equivalent safety profiles.
Where obesity pharmacology goes from here
The practical question facing patients and clinicians is not whether GLP-1 drugs will be replaced, but whether they will gain meaningful partners. Incretin-based therapies remain the backbone of medical weight management, supported by extensive clinical data and years of real-world use. Bimagrumab and MariTide are best understood as potential complements that exploit different biology: muscle preservation through ActRII blockade, altered energy partitioning, and appetite circuits that do not depend entirely on the melanocortin pathway.
Cost and access will shape adoption as much as efficacy data. Combination regimens involving two powerful biologics raise questions about cumulative side effects, injection burden, and price. If future trials show that bimagrumab provides only modest improvements over GLP-1 monotherapy, insurers may resist broad coverage. But if it delivers clearly superior outcomes in specific groups, such as older adults losing dangerous amounts of muscle or patients with partial resistance to incretin drugs, it could carve out a role that no GLP-1 drug alone can fill.
Phase 3 readouts for bimagrumab and further peer-reviewed data on MariTide are expected over the next one to two years. Until those results arrive, the non-GLP-1 and combination strategies emerging from labs and early-stage trials should be viewed as scientifically grounded but still unproven at the scale that matters most: large, long-duration human studies with hard clinical endpoints. The biology is promising. The proof is still being built.
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*This article was researched with the help of AI, with human editors creating the final content.
