** Shares of drug developer Denali Therapeutics DNLI.O down 7.5% at $17.86 premarket

** Denali and partner Biogen BIIB.O say they will discontinue development of their Parkinson's drug after it failed to meet the main goal in a mid-stage study

** Say the drug, BIIB122, did not slow disease progression compared with placebo

** William Blair analysts say the data is disappointing, but has minimal impact on their company theses

** "We had always viewed the program as high risk and had modeled a 10% probability of success for the program" - William Blair

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

** As of last close, DNLI up 16.9% YTD