The FDA wants more data, extends its review of aducanumab

We have posted often about the drama surrounding approval of the monoclonal antibody treatment for Alzheimer's disease: aducanumab.  You will recall from Beating the Dementia Monster that I auditioned for the phase 3 trial, but I didn't make the cut.  There were several trials, but they were all terminated when it appeared that the treatment was ineffective.  But then (as we discussed in December 2019), Biogen reevaluated the data and found that, given the right dose, it really was (somewhat) effective.  So they applied to the FDA for approval for clinical use.

So how did the FDA respond?  Well, their advisory committed didn't think that Biogen had made their case.  In November 2020 we discussed the committee's recommendation that Biogen's application be rejected.  But this was advisory only, and the FDA continued to review Biogen's data and their application.  As they continued their review, pressure on the FDA to approve aducanumab mounted.

So where does this stand today?  On January 29, Biogen announced that the FDA had requested additional data and analysis from Biogen in order to complete the review.  Such a request automatically triggers a three-month extension of the deadline for the FDA to complete their review.  The new deadline is June 7, 2021.

What is the nature of the new information, and what does the request mean?  I haven't been able to find out.