Biogen shares slide as diranersen data raise dosing question

Biogen (Nasdaq: BIIB) saw its shares fall 8% on Tuesday after a fuller look at mid-stage results for its experimental Alzheimer's therapy diranersen renewed questions over the US biotech major's decision to push the drug into late-stage testing despite a failed primary endpoint.

The detailed findings from the Phase II CELIA study, presented at the Alzheimer's Association International Conference (AAIC) in London, confirmed a result that has puzzled the field: the lowest of three doses produced the biggest clinical benefit, and higher doses did not. The trial had been designed to show that more drug meant more benefit, and on that measure it missed.