Small open-label study of ATR-258 saw fat loss, lean mass gains and better leg power, but no placebo group and just eight weeks long.
Losing weight on the new wave of obesity drugs often comes with a side order of lost muscle. Studies suggest that 25–45% of the weight lost on incretin-based medicines may be lean mass, according to Atrogi, a Swedish biotech that wants to change that.
On 1 October, the Stockholm company announced human data for ATR-258, an oral drug it calls first-in-class. The results are small, early and interesting, in roughly that order of confidence [1].
ATR-258 targets a receptor in skeletal muscle called the β2-adrenergic receptor. Older drugs that switch it on, known as β2-agonists, can boost muscle power but come with trade-offs, according to the study’s lead investigator. ATR-258 is “GRK2-biased,” which in simple terms means it is designed to push the receptor down one signaling route inside the cell and leave the others quieter. The hope is muscle benefits without the downsides.
Professor Morten Hostrup, who led the trial, has spent years studying this receptor. “What struck me most was the magnitude of the signal in lower-extremity power,” he said. “Having worked extensively with classical β2-agonists, I know they can enhance power, but not to the extent we observed here.”
13 is lucky for some
The trial ran eight weeks and enrolled 13 overweight or obese people. It was investigator-initiated, open-label and single-arm, so everybody got the drug, everybody knew it, and there was no placebo group for comparison. Participants stepped through three ascending dose levels, and the drug was well tolerated.
Lower-extremity muscle power improved by about 13% from baseline. Mean fat mass fell by 1.1 kg (about 2.4 pounds) and lean mass rose by 0.8 kg (about 1.8 pounds), both statistically significant. Those changes are modest in absolute terms. The company’s emphasis is on the combination of less fat, more muscle and a stronger body, all at once.
Without a control group, though, it’s hard to say how much of the change came from the drug. Eight weeks and 13 people is a short, small window. Atrogi calls the result first human proof-of-mechanism, a fair description for a study of this kind, and Hostrup himself calls it “a genuinely intriguing signal.”
What comes next
The data build on a paper published in Cell in June 2025, which described a Phase 1 trial of 69 people that looked at safety and tolerability in healthy volunteers and patients with type 2 diabetes. Atrogi now plans a Phase 2 trial in 2027 designed to demonstrate clinical proof-of-concept in muscle-sparing weight loss and sarcopenia, the loss of muscle mass and strength with age. Hostrup has been invited to present the initial findings at the International Conference on Cachexia, Sarcopenia and Muscle Wasting in Washington, DC, on 10–12 December.
“We are a Phase 2-ready biotech with a first-in-class, next-generation oral GRK2-biased β2-adrenergic modulator,” CEO Paul Little said.
“To see fat loss alongside an increase in lean muscle mass and improvement in muscle function represents a truly differentiated profile, and is highly encouraging,” Chief Scientific Officer Tore Bengtsson added. He also said the mechanism may deserve a look in sarcopenia and in rare diseases marked by substantial muscle loss and weakness.
Why longevity cares about legs
Muscle is easy to overlook in aging conversations until it goes missing. The release describes muscle mass and function as critical to mobility, metabolic health and physical resilience, and anyone who has watched an older relative struggle to stand from a chair knows what that means in daily life. Power in the legs, the thing Atrogi saw move most, plausibly matters a great deal for that kind of independence.
If a pill can protect or rebuild muscle while people lose fat, it speaks to two of the biggest threads in the field: the obesity-drug boom and the loss of strength with age. Right now, it’s a promising signal from 13 people. The 2027 trial is the one that will tell us whether it holds.
[1] https://www.atrogi.com/news/atrogi-announces-positive-human-data-for-atr-258