A randomized, double-blind phase 2 trial published in June 2026 reveals that the monoclonal antibody apitegromab may help adults on tirzepatide preserve lean body mass. The study, involving 102 participants, found that those taking the myostatin-blocking drug lost significantly less lean mass than those on a placebo while achieving similar total weight reduction.
The Challenge of Lean Mass Loss During Weight Management
Weight-loss medications, particularly GLP-1 receptor agonists like tirzepatide, have transformed obesity treatment, yet they introduce a distinct physiological trade-off. As reported by The Guardian, research suggests that 25% to 40% of the total weight lost by patients on these therapies is not fat, but lean body mass—a category that includes essential muscle tissue.
This loss is more than an aesthetic concern. Lean mass is metabolically active, burning more calories than fat, and plays a crucial role in maintaining physical strength. Prof Alexander Miras, an obesity expert at Ulster University, noted the potential functional decline associated with this trend.
“This means that [people] may be less able to lift heavy weights, for example, or walk up a hill. This new medication may help reduce the effects of GLP-1-based drugs on muscle strength and therefore improve functionality even further compared to someone not on the new medication who is just taking tirzepatide.”
Prof Alexander Miras, Ulster University, via The Guardian
The clinical concern stems from the observation that rapid, drug-induced weight loss often lacks the stimuli—such as mechanical loading—required to signal the body to spare protein stores. Data published in the Journal of Clinical Endocrinology & Metabolism indicates that without intervention, the reduction in skeletal muscle mass during caloric deficits can lead to a decline in basal metabolic rate, potentially complicating weight maintenance efforts once pharmacological support is withdrawn. Regulatory bodies, including the FDA, have increasingly signaled interest in body composition endpoints for metabolic drugs, though no current standard of care exists for the concurrent preservation of lean mass during GLP-1 therapy.
How Apitegromab Modifies Muscle Retention
The trial, known as EMBRAZE, investigated whether blocking myostatin—a protein that naturally inhibits muscle growth—could mitigate the catabolic effects of rapid weight loss. According to Nature, the study was conducted across seven sites in the United States, including locations such as the ProSciento Clinical Research Unit in Chula Vista and the Translational Research Institute in Orlando.
The EMBRAZE design employed a randomized, double-blind, placebo-controlled approach to isolate the impact of apitegromab (also known as SRK-015) in patients already stabilized on a tirzepatide regimen. The study investigators, led by researchers affiliated with Scholar Rock—the developer of the therapy—administered the drug intravenously. The mechanism of action involves binding to the latent form of myostatin, preventing it from activating the ActRIIB receptor, which is a key pathway for muscle atrophy. By neutralizing this protein, the drug aims to shift the body’s metabolic partitioning toward fat oxidation while sparing muscle protein, even in the presence of the significant caloric deficit induced by tirzepatide.
For more on this story, see How to Prevent Muscle Loss on GLP-1 Weight Loss Drugs.
The findings, which were published in Nature Medicine, tracked 102 participants over 24 weeks. The results demonstrated a clear divergence in body composition outcomes:
- Apitegromab group: Participants lost an average of 1.6kg of lean mass, representing 14.6% of their total weight loss.
- Placebo group: Participants lost an average of 3.5kg of lean mass while on the same weight-loss regimen.
In essence, the addition of apitegromab was associated with a 55% greater retention of lean mass compared to the placebo, even though the total weight lost between the two groups remained similar, as detailed by Science News. Notably, the study monitored safety markers including liver enzymes and blood pressure, reporting no significant differences in adverse events between the treatment and placebo groups, though the small sample size limits the power to detect rare side effects.
Clinical Context and Future Research Needs
While the results are statistically significant, the medical community remains cautious. Prof Naveed Sattar, a cardiometabolic medicine expert at the University of Glasgow, emphasized that the trial is an early-stage assessment. Because the study was limited in duration and primarily featured female participants, larger and longer-term trials are essential to confirm long-term safety and actual health outcomes. The current data does not confirm whether the preservation of lean mass correlates with improved physical performance metrics, such as handgrip strength or gait speed, which are often used as proxies for frailty in older adult populations.
This follows our earlier report, Exercise Outperforms Protein Powder for Muscle Strength in Older Adults – New Study.
“This is an early-stage trial that suggests novel drugs can help mitigate muscle mass loss with prescribed tirzepatide. However, it’s too early to say whether this actually benefits peoples health or ability to move or function better. In the meantime, people prescribed these drugs should be supported to increase their physical activity, as this can help maintain muscle mass in a physiological way. Importantly, activity should also be framed as something enjoyable and sustainable, rather than purely as a medical add‑on.”
Prof Naveed Sattar, University of Glasgow, via The Guardian
Furthermore, the regulatory pathway for apitegromab remains in the investigational phase. The drug has not yet received approval from the FDA or the European Medicines Agency for the indication of lean mass preservation in the context of anti-obesity medication. Investigators have indicated that phase 3 trials would need to include a more diverse demographic—incorporating a wider range of ages and comorbidities—to determine if the efficacy observed in the EMBRAZE study holds true for the general population of patients struggling with obesity. Researchers are also looking to determine if the drug’s effect persists after the discontinuation of the therapy, or if lean mass is rapidly lost once the myostatin-blocking effect is removed.
For patients currently utilizing weight-loss injections, the guidance remains centered on holistic management. Until further clinical data validates the use of myostatin inhibitors for this specific purpose, experts continue to stress that resistance training and sustainable physical activity remain the primary tools for protecting muscle health. Patients should consult their healthcare provider regarding their specific weight management plan and any concerns about changes in muscle strength. A physician can help determine if existing metabolic markers and physical performance baselines warrant a discussion about evolving therapeutic options or if lifestyle modifications—such as supervised strength training—are the appropriate course of action.
