A pedestrian walks past Biogen Inc. headquarters in Cambridge, Massachusetts, U.S., in 2021. Biogen Inc. shares soared after its controversial Alzheimer’s disease therapy was approved by U.S. regulators, a landmark decision that stands to dramatically change treatment for the debilitating brain condition. Adam Glanzman/Bloomberg file

Biogen’s studies of its drug for a rare form of amyotrophic lateral sclerosis suggest the drug has a “treatment effect,” the Food and Drug Administration’s staff said in documents released ahead of a meeting out of outside advisers this week.

Although Biogen’s drug failed both the main and secondary goals of a final-stage trial, “there appear to be generally consistent trends favoring tofersen across different statistical methods,” the FDA staff said Monday in documents released on the agency’s website. Patients with ALS, sometimes called Lou Gehrig’s disease, are willing to accept risk given the severity of the condition and lack of effective treatments, the staff wrote.

More drugmakers are targeting incurable diseases like ALS, often forcing the FDA to grapple with difficult questions of whether to clear medicines backed by questionable data for desperate patients. The Wednesday meeting of an independent panel of FDA advisers may give hints as to how aggressively the agency is willing to move toward giving expedited approval for experimental drugs that offer promise but haven’t yet shown a clear benefit for such patients.

While a trial of tofersen didn’t prove it slows ALS, the drug did decrease levels of a protein associated with brain injury. The usual approach to confirming whether a drug works after such unclear results would be to conduct a second, larger trial. But the form of ALS that tofersen seeks to treat is so rare that a second big trial in symptomatic patients “does not appear to be feasible at this time,” the FDA said.

Biogen has proposed following patients in the original trial to assess whether the people who received the drug from the beginning survived longer than others who started taking it later, the FDA said. The agency is weighing whether to authorize the drug using a controversial mechanism called accelerated approval or to consider granting full approval, according to the briefing document.

The briefing documents read favorably and suggest approval of tofersen is likely, RBC Capital Markets analyst Luca Issi wrote in a note to clients. Shares of Biogen rose 0.7% Monday at noon in New York. Shares of Biogen’s partner Ionis Pharmaceuticals rose 1.5%.

ALS destroys nerve cells that control voluntary muscle movement, including breathing. There is no cure for the lethal disorder, which the Centers for Disease Control and Prevention estimates afflicts roughly 30,000 people in the U.S. Mutations in the SOD1 gene are found in about 10% to 20% of inherited cases of ALS and only 1% to 2% of sporadic cases, according to the ALS Association. While uncommon, these mutations can cause people to decline particularly rapidly.