Longer-term use and early initiation of Biogen Inc's experimental treatment for an inherited form of amyotrophic lateral sclerosis (ALS) was effective in slowing disease progression, according to a new analysis by the company published on Wednesday.

The Biogen drug, tofersen, is currently under priority review by the U.S. Food and Drug Administration with an approval decision expected by Jan 25. ALS, also known as Lou Gehrig's disease, breaks down nerve cells in the brain and spinal cord that make muscles work, leading to progressive paralysis and death. Its cause is largely unknown.

A combined 12-month analysis of data from a late-stage trial and an open-extension study of tofersen, showing more positive results than the Phase III trial alone in patients with the rare type of ALS, was published in the New England Journal of Medicine.

Biogen last October said the drug missed the main goal of the late-stage study, failing to show statistically significant improvement in the functional status of patients with fast-progressing ALS at six months.

The new analysis found that longer use of tofersen may help stabilize muscle strength and control and that a trial duration of more than 28 weeks may be required to determine the drug's true effect on patients.

"Those that started on tofersen early and were on the drug for about six months longer had greater preservation of function, higher scores on breathing, and greater strength," Dr Timothy Miller, the study's lead researcher and ALS Center director at Washington University School of Medicine in St. Louis, said in an interview.

"I'm enthusiastic about that response," he said, adding, "of course, it took longer than we initially expected." Biogen is seeking approval of tofersen for ALS patients with mutations in a specific gene that leads to the accumulation of toxic levels of a protein called SOD1. 

Around 2 per cent of the more than 31,000 ALS cases in the United States are linked to mutations of that gene. Most side effects in both the late-stage study and the follow-on portion were mild to moderate, including headache and back pain, the company said. Nearly 7 per cent of patients who received tofersen experienced serious neurological events, including spinal cord inflammation and swelling of the optic nerve.