A muscle-sparing antibody blunted lean-mass loss during tirzepatide weight loss in a phase 2 trial

In the 102-patient EMBRAZE trial, adding the investigational antibody apitegromab to tirzepatide (Zepbound) spared 1.9 kg of lean mass versus placebo while total weight loss held steady — but the 24-week proof-of-concept study measured body composition by scan, not strength or function.

The most cited worry about GLP-1-class weight-loss drugs is that the pounds they shed are not all fat. A meaningful share of the loss is lean mass — muscle — which matters for strength, metabolism and, in older patients, the ability to stay independent. A phase 2 trial now reports that pairing the incretin drug tirzepatide (Zepbound) with an experimental antibody aimed squarely at muscle biology can blunt that lean-mass loss without sacrificing the weight loss itself.

The drug is apitegromab, a fully human monoclonal antibody from Scholar Rock that selectively inhibits the activation of myostatin, a protein that restrains muscle growth. Apitegromab is investigational and not approved for any use; it is available only through clinical trials. In the randomized, double-blind, placebo-controlled EMBRAZE study (NCT06445075), 102 adults with overweight or obesity and without diabetes were randomized 1:1 to tirzepatide plus apitegromab (10 mg/kg) or tirzepatide plus placebo, with every participant receiving tirzepatide throughout, according to the trial registry and the published report.

What the trial found

The primary endpoint was the change from baseline in total lean body mass, measured by DXA scan, at 24 weeks. At week 24, apitegromab produced a least-squares mean of 1.9 kg less lean-mass loss than placebo (80% confidence interval 1.2–2.7; P = 0.001) — “despite similar total body weight loss between groups,” the authors write, a result they frame as 54.9% retention of lean mass relative to placebo (DOI).

Selective targeting of myostatin preserved lean mass when combined with tirzepatide — and, critically, did so without blunting overall weight loss.

That last point is the crux. The goal is not simply more muscle; it is to make weight loss “higher quality” — proportionally more fat, less muscle — while keeping the scale moving. The pharmacodynamics tracked the mechanism: in apitegromab-treated participants, trough drug concentrations and total latent myostatin both rose over time and plateaued at roughly 16 weeks.

On safety, the two arms looked alike. Adverse events occurred in 39 of 51 apitegromab-treated participants (76%; 95% CI 63–86%) versus 36 of 51 placebo-treated participants (71%; 57–81%). Serious adverse events were balanced, affecting one of 51 participants (2%; 0–10%) in each arm. The authors describe apitegromab as “well tolerated.”

The caveats worth keeping

This is a proof-of-concept study, and its limits are real. The trial enrolled 102 people and ran 24 weeks — short relative to how long patients stay on weight-loss therapy, and too brief to show whether preserved DXA-measured lean mass translates into preserved strength, physical function or hard clinical outcomes. The primary endpoint was reported with an 80% confidence interval rather than the conventional 95%, a wider margin for declaring an effect that readers should weigh. Lean body mass by DXA is also a surrogate: it captures tissue quantity, not muscle quality or performance. And the study excluded people with diabetes and capped age at 65, so it says nothing yet about the older, frailer patients in whom muscle loss is most consequential.

EMBRAZE establishes that myostatin inhibition can shift the composition of incretin-driven weight loss in a controlled setting. Whether that shift improves how patients actually feel and function will take larger, longer trials with functional endpoints to answer.