** Shares of drugmaker Biogen BIIB.O fall 1.3% to $189 premarket

** Japan's Eisai and Biogen say the U.S. Food and Drug Administration has extended its review of their marketing application for a weekly under-the-skin version of their Alzheimer's drug Leqembi

** The new target action date is August 24 - BIIB

** The companies are seeking approval for the once-weekly subcutaneous injection, branded as leqembi iqlik, to be used as a starting dose for patients with early Alzheimer's disease

** The agency has not flagged any concerns so far about the drug's potential approval as a starting dose, companies say

** The drug, Leqembi, is already approved in the United States as an intravenous infusion

** Up to last close, stock up 8.7% YTD