** Brokerage Truist downgrades drugmaker Biogen BIIB.O to "hold" from "buy", trims PT to $142 from $199

** New PT implies a 13.6% upside to the stock's last close

** Truist says the downgrade is due to weak growth prospects in Biogen'ss existing products and no low-risk new drug in development

** Brokerage is no longer optimistic about Biogen's Alzheimer's drug opportunity, and expects only modest growth for Leqembi

** Expects limited growth for Zurzuvae and believes Skyclarys sales have peaked and says Biogen's late-stage pipeline is seen as high-risk

** As of last close, BIIB stock down 18.3% YTD

(Reporting by Padmanabhan Ananthan)

((Padmanabhan.Ananthan@thomsonreuters.com))