Spinal muscular atrophy (SMA) is a genetic condition that affects the nerve cells (motor neurons) in your spinal cord that control muscle movement. Over time, the loss of these nerve cells leads to progressive muscle weakness and muscle wasting (atrophy), which can affect the ability to sit, stand, walk, or even breathe and swallow, depending on how severe it is.
Medicines that target the underlying genetic cause of SMA, often called survival motor neuron 2 (SMN2)-targeted treatments, have changed the outlook for many people with this condition. But these medicines work on nerve cells, not muscle itself, so some muscle weakness and atrophy can remain even with treatment.
Isembyld (apitegromab-mstn) is a newer medicine designed to work differently. Rather than targeting the nerve cells affected by SMA, Isembyld targets muscle tissue directly by blocking a protein that normally limits muscle growth. It's meant to be used alongside an SMN2-targeted medicine, not in place of one.
The FDA approved Isembyld in September 2026. This article covers how Isembyld works, who it's approved for, how it's given, and what clinical studies found. You may also be interested in reading about Isembyld side effects.
How Does Isembyld Work for SMA?
To understand how Isembyld works, it helps to know what's happening in the body with SMA.
SMA happens when the body doesn't make enough of a protein called SMN (survival motor neuron protein). Without enough SMN protein, motor neurons in the spinal cord gradually break down, which weakens the connection between the brain and the muscles. Over time, muscle fibers that lose their nerve connection stop working properly and shrink.
Isembyld doesn't act on motor neurons or the SMN protein at all. Instead, it targets a different protein in the body called myostatin. Myostatin's normal job is to put the brakes on muscle growth, keeping muscles from getting too big. Isembyld binds to two inactive forms of myostatin before they're able to turn into their active form. By blocking this activation step, Isembyld releases the brakes on muscle growth, which can help muscles grow larger and stronger.
Because Isembyld works on muscle tissue itself rather than on nerve cells, it's designed to complement SMN2-targeted medicines such as nusinersen (Spinraza) or risdiplam (Evrysdi) rather than replace them. The idea is that while an SMN2-targeted medicine helps slow the loss of motor neurons, Isembyld helps build strength in the muscle fibers that are still connected and working.
Is Isembyld Right for Me?
Isembyld is approved for adults and children 2 years and older who have SMA and are already being treated with an SMN2-targeted medicine, such as nusinersen or risdiplam. It's meant to be added to that treatment, not used by itself.
Your health care provider will consider many things when deciding whether Isembyld is a good fit for you or your child, including how long you've been on an SMN2-targeted medicine, your current level of muscle function, and any history of low bone density or bone fractures, as Isembyld carries a risk of fractures (more on this in the side effects article). Isembyld hasn't been studied in children younger than 2 years.
How Is Isembyld Given?
Isembyld isn't a pill or a shot you give yourself. It's given as an IV infusion, meaning a health care provider delivers it through a small tube placed in a vein, usually at an infusion center, clinic, or hospital. Each infusion takes about one to two hours.
The recommended dose is 10 milligrams per kilogram (mg/kg) of body weight, given once every four weeks. Your health care provider will calculate the right dose based on current weight, and the dose may be adjusted over time as a child grows.
If a dose is missed, there are two options: Isembyld can be given as soon as possible and the every-four-week schedule restarted from that point, or the missed dose can simply be skipped and the next dose can be given on the original schedule. Your health care provider will help decide which approach makes sense. Don't stop Isembyld or change how often you receive it without talking to your health care provider first.
How Was Isembyld Studied for SMA?
Isembyld's approval was based on a clinical trial known as SAPPHIRE, which enrolled 188 participants aged 2-21 years with genetically confirmed SMA (type 2 or type 3) who were not able to walk independently. Everyone in the trial was already being treated with nusinersen or risdiplam and had been on that treatment for several months to years before joining the study.
The main group studied and the group the dosing recommendation is based on was 156 children aged 2-12 years. At the start of the study, participants averaged 8 years of age, and close to half (46%) were female. Most participants were White (71%), and a smaller number identified as Hispanic or Latino (8%), Asian (6%), or Black or African American (1%).
Participants had a wide range of starting muscle function, measured using a scale called the Hammersmith Functional Motor Scale Expanded (HFMSE), which scores 33 different movements (such as sitting up, rolling over, or standing) from 0 to 66, with a higher score meaning better motor function.
Participants were randomly assigned to one of three groups: Isembyld at the approved 10 mg/kg dose, Isembyld at a higher 20 mg/kg dose (twice the approved dose), or a placebo (which is a look-alike infusion with no active medicine in it). The study was double-blind, meaning none of the participants, their families, or the study team members knew who was receiving which treatment. Everyone continued taking their usual nusinersen or risdiplam throughout the study. Motor function was measured regularly over one year.
What Are the Main Benefits of Isembyld Based on Studies?
The study's main goal was to measure the change in HFMSE score after one year of treatment, comparing Isembyld 10 mg/kg to a placebo.
After one year:
- Children who received Isembyld 10 mg/kg had an average HFMSE score that went up by about 1 point from where they started.
- Children who received a placebo had an average HFMSE score that went down by about 1.2 points from where they started.
That's a difference of about 2.2 points favoring Isembyld. In other words, motor function tended to improve slightly in children who received Isembyld, while it tended to decline slightly in children who received a placebo, even though everyone in the study continued their SMN2-targeted treatment the whole time.
Your results may differ from what was seen in clinical studies.
Are There Additional Potential Benefits of Isembyld Based on Studies?
Researchers also looked at how many children had a more noticeable jump in motor function, specifically a gain of 3 points or more on the HFMSE scale, because surveys of families and caregivers suggest that gains in this range are often meaningful in everyday life (things such as being able to roll over in bed or feed yourself more easily).
About 34% of children who received Isembyld 10 mg/kg reached this 3-point improvement, compared with about 14% of children who received a placebo. This suggests children treated with Isembyld were notably more likely to have a meaningful jump in motor function than those who didn't receive it.
Researchers also studied Isembyld at a higher 20 mg/kg dose and in older participants (aged 13-21 years). Results trended in the same direction as the approved 10 mg/kg dose, though those results weren't considered conclusive on their own, partly because fewer people were studied in those groups. Testing in the study also showed that the 10 mg/kg and 20 mg/kg doses had a similar effect on myostatin in the blood. This is one reason the lower, 10 mg/kg dose is the one that's approved: it appears to provide the same benefit as the higher dose.
Your results may differ from what was seen in clinical studies.
How Long Were the Studies? Is There Longer-Term Data?
The main part of the SAPPHIRE study, comparing Isembyld to a placebo, lasted one year. After that, nearly all participants (about 98%) chose to continue into an open-label extension study, meaning everyone in the extension knows they're receiving Isembyld and no one is given a placebo. This extension is ongoing and is expected to provide more information about how Isembyld works over a longer period of time.
An earlier, smaller study of Isembyld also followed participants for up to four years and saw continued improvements in motor function over that time, which is part of the longer-term picture supporting Isembyld's use.
How Can I Get Isembyld? Is Cost Assistance Available?
Isembyld requires a prescription and is given by a health care provider through an IV infusion, so you won't be able to pick it up at a regular pharmacy. Your health care provider's office or an infusion center will typically coordinate scheduling and insurance details for you.
Scholar Rock, the drugmaker of Isembyld, may offer cost support options depending on your health insurance. Visit ISEMBYLD.com or call 877-473-6295 to learn more about what support may be available to you.
