What’s happening? Reviewers at the US Food and Drug Administration (FDA) have said they have enough evidence to support the approval of Biogen’s investigational Alzheimer’s disease (AD) drug. Results from a late-stage trial of aducanumab, developed with Eisai, were said to be “exceptionally persuasive” and show the antibody treatment is effective in slowing decline in patients with early AD. A second study did not lead to the same conclusion but the reviewers said this did not undervalue the findings of the positive study. A final decision on approval is expected by March 2021.

Why does this matter? AD is the sixth leading cause of death in the US. An estimated 5.8 million Americans suffer from the disease and this number could rise to 14 million by 2050. Globally, the WHO puts this number at 30 million to 35 million cases, rising to over 100 million in 2050. There is a desperate need, therefore, for new and effective therapies.

This news marks a large turnaround for Biogen, which discontinued both Phase III trials of aducanumab in March 2019 owing to a futility analysis suggesting the drug would not perform better than a placebo. The company, however, bounced back in October of that year with a new analysis of a larger dataset from these studies showing a high dose of the drug was effective. In July, Biogen completed its Biologics License Application to the FDA, seeking approval for the treatment. The European Medicines Agency has also recently accepted a Marketing Authorisation Application for the drug.

Even though it appears the FDA staff largely concur the drug should be approved, one detractor, Tristan Massie, stated that evidence from the trials was not enough to show efficacy. On page 255 of the hefty Advisory Committee Briefing Document, he suggests a further trial is needed to confirm either a positive or negative outcome.

Another critic is Mayo Clinic neurologist David Knopman, who was recused from the meeting owing to his involvement in conducting clinical trials of the drug. He says the treatment cannot be proved effective based on trials with conflicting outcomes.

This opinion was also seconded by an independent advisory panel on 6 November, which echoed the sentiment there is not enough evidence to prove the drug works. The FDA doesn’t have to follow the advice of this panel, but normally does. Public commenters, including those suffering with Alzheimer’s and their carers, have urged its approval.

If the FDA decides to approve the drug, however, aducanumab will become the first treatment in the world to slow the progress of AD. Other drugs are available, but they only help improve symptoms. No new AD drugs have been approved in the US since 2003.

Aducanumab would most likely benefit the 2.2 million people with mild cognitive impairment or early-stage AD who live in the US. The drug is estimated to cost $50,000 per year, so if just a third of these were treated with it, Biogen could see annual sales of $36bn. This means the drug could become one of the biggest sellers ever – for comparison, the top-selling drugs in 2019 were AbbVie’s Humira (adalimumab) and Merck’s Keytruda (pembrolizumab) bringing in $19.6bn and $11.1bn, respectively.