Muscle-Boosting Drugs Near Approval for Children With SMA

Scholar Rock’s muscle-boosting antibody apitegromab faces an FDA decision by September 30 for children and adults with spinal muscular atrophy. If the agency says yes, it would be the first myostatin blocker sold in the United States, given by vein every four weeks to people who already take SMN-targeted SMA drugs.

The obesity combinations that brought this 30-year idea back to life are still in mid-stage trials. The most aggressive Regeneron stack already pushed more than a quarter of adults off treatment.

A Rare-Disease Drug, 28 Days From a Decision

Today is September 2, so the clock has 28 days left. Scholar Rock says it is ready to launch as soon as the agency acts, with commercial vials already filled at a backup U.S. plant. David L. Hallal, the company’s board chair and chief executive, said the firm still plans a U.S. start in the third quarter if approval comes.

The delay was a factory problem, not a failed efficacy file. The FDA sent a complete response letter on September 23, 2025, after inspectors flagged Catalent Indiana, a fill-finish site later folded into Novo Nordisk. Those findings were not specific to apitegromab. Scholar Rock resubmitted, then, after an April 2026 reinspection was classed Official Action Indicated, pulled that plant from the file on agency guidance.

Europe is now behind. After the same plant notice on August 7, Scholar Rock withdrew its EU application on August 20 and said it will file again with the backup plant. Japan’s medicines agency has said no extra local trials are required, and a Japanese filing is planned by year end. About 35,000 people with SMA have already received an SMN-targeted drug, which is the add-on market Scholar Rock is aiming at first.

THE SMA FILE, IN ORDER

1. September 23, 2025: FDA issues a complete response letter tied to the Catalent Indiana fill-finish plant, not to apitegromab’s trial data.
2. May 7, 2026: The agency accepts the resubmission and sets a September 30 action date.
3. August 7, 2026: Scholar Rock is told the April plant inspection is Official Action Indicated and drops that site from the U.S. file.
4. August 21, 2026: The company says the backup plant can supply a launch, while the EU file is reset and a Japan filing is lined up for year end.
5. September 30, 2026: The FDA’s decision date for the first muscle-targeted SMA drug.

The clinical case behind that file is modest on paper and large for the people in it. SAPPHIRE enrolled 188 nonambulatory patients aged 2 to 21 who were already on nusinersen or risdiplam; 128 got apitegromab and 60 got placebo. In the main group of 156 children aged 2 to 12, the drug beat placebo by 1.8 points on the Hammersmith Functional Motor Scale-Expanded at 12 months (p=0.019). Among those children, 30.4% on apitegromab gained at least 3 points, versus 12.5% on placebo. No one quit because of side effects. Fever, colds, cough, vomiting and headache ran at similar rates on drug and placebo.

A Third of GLP-1 Weight Loss Is Lean Tissue

The money and the internet arrived for a different reason. GLP-1 drugs cut a lot of weight, and a large slice of that weight is lean tissue, which is everything that is not fat or bone, so muscle plus water and other soft tissue. When people stop the shots, fat often returns faster than the lean mass does, which is the fear Brendan Gabriel, a researcher at the University of Aberdeen, has flagged for patients who start and stop treatment.

Regeneron puts the global obesity count above a billion and has built its pitch around the quality of the loss, not only the number on the scale. George D. Yancopoulos, the company’s president and chief scientific officer, put the trade-off in plain language when the COURAGE interim data landed.

Recent advancements have resulted in patients being able to lose significant amounts of body weight. Unfortunately, this weight loss comes at the cost of muscle loss, and we know muscle is important to overall health.

George D. Yancopoulos, M.D., Ph.D., president and chief scientific officer, Regeneron COURAGE release

That lean share is not a GLP-1-only quirk. Large diet-driven cuts also throw off a lot of lean tissue, which is why the add-on muscle drugs have to do more than reprint a known fraction. They have to change the mix, keep people on therapy, and, if the claim is strength, show it on a test that is not a one-day gym max.

David Glass, who works on these antibodies at Regeneron, has said the point of the work is stairs, bathrooms and shops in later life, because strength starts falling in the 30s and the body’s own brakes make exercise pay off less. Muscle also stores most of the sugar after a meal, which is why Gabriel talks about metabolic health in the same breath as frailty.

Regeneron’s Triple Combo Lost Nearly a Third of Patients

COURAGE is Regeneron’s test of that idea in people with obesity, pairing semaglutide with trevogrumab, an antibody against myostatin (GDF8), and, in one arm, garetosmab against activin A. The registry lists 1,005 adults in the COURAGE study, now active and no longer recruiting, with primary completion on June 16, 2026 and a full finish estimated for October 21.

The June 2, 2025 interim cut, taken when half the patients had reached week 26, is the dataset the field is still arguing over. Semaglutide alone cut 7.9 pounds of lean mass, which was 34.5% of the weight lost on the scale. Adding trevogrumab spared about 50 to 80 percent of lean mass and took more fat. The three-drug arm looked best on the DXA scan and worst in the clinic.

COURAGE WEEK 26 INTERIM, ON TREATMENT

The triplet also posted a 10.1% rate of severe events and a 6.7% rate of serious ones. Two people in that arm died, one of an undetermined cause with several heart-risk factors and one of cardiac arrest with known heart disease. Regeneron said it has not found a causal link. Glass has already said there are side effects that would make the company not want to go forward with that three-drug mix, and the firm is still trying to learn why the harm was so sharp.

Trevogrumab plus semaglutide, without garetosmab, is the arm that still looks usable. It halved lean loss, added some fat loss, and at the lower dose quit at about the same rate as semaglutide alone. That is a preservation drug, not a body-recomp miracle, and it is still an interim slice of a trial that has not yet posted its maintenance phase.

Bimagrumab Added Fat Loss Without the Dropout Spike

Eli Lilly’s bimagrumab takes a wider swing. It blocks activin type II receptors, so it hits myostatin and several related ligands at once. In the BELIEVE phase 2 trial of 507 adults, the high-dose pairing with weekly semaglutide 2.4 mg cut body weight 22.1% at week 72, against 15.7% on semaglutide and 10.8% on bimagrumab. Lean mass fell 2.9% on the combo, 7.4% on semaglutide, and rose 2.5% on bimagrumab alone. 92.8% of the combo’s weight loss was fat. Total fat mass dropped 45.7% on the combo versus 27.8% on semaglutide, and visceral fat fell 58.2% versus 35.8%.

Common bimagrumab effects in that trial were muscle spasms, diarrhea and acne. Semaglutide’s were the usual nausea, diarrhea, constipation and fatigue. The pairing did not produce COURAGE-style dropout. Lilly still killed a separate phase 2b of bimagrumab plus tirzepatide in people with obesity and type 2 diabetes in September 2025, citing strategy, so the receptor-blocker path is not a straight line to a Zepbound combo pill.

Scholar Rock’s own obesity test is smaller and cleaner. EMBRAZE randomized 102 adults to tirzepatide plus apitegromab 10 mg/kg or tirzepatide plus placebo for 24 weeks. Apitegromab saved 54.9 percent of the lean mass that tirzepatide would have taken, a 4.2-pound gap (p=0.001). Fat loss was similar, 18.8 pounds versus 17.7. Total weight loss was 12.3% with the antibody and 13.4% without it. Lean tissue made up 14.6% of the loss on the combo and 30.2% on tirzepatide alone. The antibody was generally well tolerated, in line with its SMA work.

THREE ADULT PROGRAMS, SIDE BY SIDE

The split matters for what the FDA would even be asked to approve in obesity. EMBRAZE kept muscle and did not add weight loss. COURAGE and BELIEVE, which reach further into the same receptor family, took more fat and more total pounds. A muscle-only label, without extra loss on the scale, is a harder regulatory story than a combo that both spares lean tissue and deepens fat loss.

Why Human Muscle Grew Only 10 Percent

The science that launched this field still hangs over every table. In the 1990s, U.S. researchers found myostatin, blocked it, and watched mouse muscles swell to two or three times normal size. The same pathway explains double-muscled Belgian Blue cattle and bully whippets, and in 2004 a 4-year-old boy with unusually large muscles was found to carry myostatin mutations. First-generation blockers then disappointed in people.

Glass’s account is that humans have a second brake, activin A, that binds the same receptors, so myostatin-only drugs never had the mouse effect. Even dual blockade seems to top out around 10% more muscle in people, against about 50% in mice. He has said he cannot explain the gap. Early drugs also grabbed other members of the same growth-factor family and ran into problems such as clotting.

HOW THE NEW SHOTS TRY TO STAY ON TARGET

• Precursor blockade: Apitegromab binds pro- and latent myostatin in skeletal muscle before the growth factor turns on, which is the selectivity Gabriel likes.
• Two-ligand blockade: Trevogrumab hits myostatin and garetosmab hits activin A, which matched broader receptor blockade in animals and then ran into adult tolerability in COURAGE.
• Receptor blockade: Bimagrumab shuts the shared ActRII door, which is less selective and so far has the largest fat-loss add-on in an obesity trial.

Heart muscle was the standing fear for long-term use. A Regeneron Genetics Center analysis of function-disrupting myostatin variants in 1.1 million people, published in March, did not find a cardiovascular harm signal. Carriers had more lean mass, higher grip strength, less body fat, and, on MRI of 77,572 UK Biobank volunteers, more than 10% extra muscle in some groups such as the glutes. If anything, left-ventricular wall thickness ran a touch lower, the opposite of hypertrophic cardiomyopathy. That is lifelong, partial loss of function in mostly healthy adults, not a weekly antibody in older patients on GLP-1s, but it is the best human safety map the field has.

Lean mass on a DXA scan is still not a deadlift. None of the obesity studies has shown that the people who kept more lean tissue were stronger. Glass argues that the usual strength tests are short, motivation-heavy efforts that swing from day to day. SAPPHIRE is the one large trial that measured real movement, and it did so in children with SMA, not in adults trying to keep muscle on semaglutide.

The Steroid Swap the Internet Already Bought

Gym talk has already priced these antibodies as a cleaner steroid. That reading skips the dose, the route, the 10% human cap, and the fact that the only phase 3 win is a 1.8-point motor score in nonambulatory SMA. Price will keep any approved product inside specialist clinics for a long time, and an IV every four weeks is a poor black-market object next to a tablet. If a few people did move off illicit anabolics onto a licensed myostatin drug, Glass has noted, the harm from “roid rage” and heart damage would fall. That is a side effect of success, not the design brief.

The louder online claim, that these shots will mint a nation of easy muscle, also runs past the biology. Blocking the brake does not replace progressive training, protein, or the years it takes to grow tissue. It shifts the balance a little, and in people the shift has been a few percent, not a doubling. COURAGE’s 34.5% lean share on semaglutide sits in the same band as other large weight cuts, so the unique-theft story that sells add-on muscle drugs is thinner than the marketing around it.

Glass still treats the jacked-fantasy traffic as a distraction from a growing older population that cannot climb stairs. He wants the drugs, if they clear safety, in that group, and he has said that simply holding onto muscle would help. Gabriel has called apitegromab the most promising of the selective shots because it is tuned to the muscle-specific precursor. Neither of them is selling a bodybuilding season.

What is actually on the calendar is narrower. Scholar Rock has vials in a backup plant and a U.S. launch team waiting on September 30. Regeneron is still sorting a triplet it does not want to take forward. Lilly has the fattest composition data and a cancelled tirzepatide combo. Older patients, and the people cycling on and off GLP-1s, remain the medical case. The first product that may exist in a month is an SMA antibody for children who already take nerve-targeted drugs, and it will be judged on whether those children move a little better, not on whether anyone looks like a 1970s bodybuilder.

Disclaimer: This article is news reporting and analysis of clinical trials and regulatory filings, and it is for information only. It is not medical advice, a treatment recommendation, or a suggestion to start, stop, or combine prescription weight-loss or muscle drugs. Anyone considering a change in therapy should speak with a licensed physician who knows their history, current medicines, and lab work. Trial figures, dropout rates, and approval timelines reflect the company statements, registry entries, and papers cited here as of September 2, 2026, and they can change with new data or an FDA action.