The agency authorized the treatment via a policy that allows a drug to be fast-tracked onto the market under certain circumstances before there is conclusive proof that it works. Biogen will be required to provide confirmatory evidence, from ongoing clinical research, to keep the drug on the market.
The decision is the first conditional approval granted for a medication for A.L.S., or amyotrophic lateral sclerosis, which generally causes paralysis and death within a few years. Fewer than half the patients eligible for Qalsody survive more than three years after their diagnosis.
The approval is based on evidence that the drug can significantly reduce levels of a protein that has been linked to damage to nerve cells. Biogen has argued that these results are reasonably likely to help patients, even though the drug, in a clinical trial, did not significantly slow the progression of the disease, as measured by patients’ ability to speak, swallow and perform other activities of daily living.
Despite the uncertainty about its benefit, Qalsody’s approval is widely seen as more justifiable than that of Aduhelm, another drug from Biogen. Aduhelm prompted an outcry when the F.D.A. approved it in 2021 to treat Alzheimer’s despite a lack of evidence that it worked.
At a meeting last month, a panel of independent advisers to the F.D.A. unanimously recommended that the agency grant conditional approval of Qalsody, even though a majority of advisers concluded that there was not convincing evidence that it was effective.