Adds details throughout

May 21 (Reuters) - Biogen BIIB.O and Denali Therapeutics DNLI.O said on Thursday they will discontinue development of their Parkinson's drug after it failed to meet the main goal in a mid-to-late stage study.

The drug, BIIB122, did not slow disease progression compared with placebo, missing the main goal of delaying worsening of symptoms measured by a combined score of movement and daily function. It also failed to show benefit on other key measures.

BIIB122 was tested in 648 patients with early-stage Parkinson's disease and, despite showing expected biological activity and an acceptable safety profile, did not demonstrate clinical benefit.

Denali will continue testing the experimental drug in patients with a specific genetic mutation linked to Parkinson's.

Shares of Biogen fell more than 1% in after-hours trading.