The U.S. Food and Drug Administration approved a new drug Tuesday to treat a rare form of amyotrophic lateral sclerosis, or ALS.
The drug Qalsody (tofersen) is expected to help people with a very specific mutation, SOD1, which applies to only 2% of the ALS population.
In a statement, the FDA said it estimates "there are fewer than 500 patients with SOD1-ALS in the United States."
Among those who qualify, the drug has the potential to slow muscle degeneration by targeting SOD1 messenger RNA, or mRNA, genetic material that tells the body how to make proteins to reduce the protein being made.
Studies showed that the drug reduced plasma neurofilament light (NfL), a blood-based biomarker of axonal (nerve) injury and neurodegeneration, according to the FDA.
"Patients receiving Qalsody had nominally significant reductions in plasma NfL concentration at Week 28 compared to the placebo arm," the FDA said in a statement.
"The genetic form of ALS caused by the mutation SOD1 can be one of the most aggressive forms of the disease, and it not only affects the person who is experiencing the disease but half the members of their family," Dr. James Berry, chief of the division of ALS and motor neuron diseases at Massachusetts General Hospital, told ABC News.
The gene was first discovered at the hospital in the 1990s and ALS patients at MGH are currently receiving this drug as part of a trial.
"This really brings hope for finally changing the paradigm for how we treat people with ALS, and what that disease course looks like; it's incredibly exciting," Berry said.
The drug is given via a lumbar puncture -- or a spinal injection -- with three doses in 14-day intervals, followed by monthly dosing.
Researchers determined Qalsody to be safe with the most common side effects being pain, fatigue, joint and muscle pain and increased white blood cells in cerebrospinal fluid.
The drug was given conditional approval by the FDA under the fast track designation. Biogen, which makes the drug, will still need to complete additional and larger studies on its effectiveness.
"I don't really have doubts that there's efficacy," Dr. Colin Quinn, a neurologist at Penn Medicine, who wasn't involved in the trial, told ABC News. "They've continued to collect data through an open label extension, which has been quite encouraging."
Although this drug is for a subset of ALS patents, Quinn and others hope it can pave the way for treatments for patients with sporadic ALS, meaning it comes on suddenly rather than due to a genetic mutation.
Treatments "to silence RNA that you don't want to be there has potential for broad application in ALS," he said. "And, in fact, there are multiple trials now looking at that very thing. So just proving that this approach works at all, I think, is really important."
It comes after the FDA approved another drug from Biogen, Aduhelm, in 2021 to treat Alzheimer's disease despite a lack of evidence that it is effective.
ALS, also sometimes known as Lou Gehrig's disease, is a neurodegenerative disease that affects nerve cells in the brain, leading to weakness and paralysis.
The disease often begins with patients experiencing muscle twitching and weakness in one arm or leg followed by having difficulty swallowing or slurring speech.
Patients' conditions eventually decline to a point at which they are unable to move, speak, eat or even breathe on their own.
According to a 2017 estimate from the Centers for Disease Control and Prevention, about 32,000 people in the U.S. live with ALS.
There is no cure, but some already approved treatments may help. It is always fatal, according to the ALS Association.
"ALS once, not many years ago, was considered a disease that we really would have a hard time making any headway against," Berry said. "We now have an effective therapy that's been approved for people with SOD1 ALS and we really are changing the way that we can care for people through clinical trials. It's an amazing time."