** Shares of drug developer Sage Therapeutics SAGE.O rise about 33% to $7.38 in extended trading
** Biogen BIIB.O sends letter to SAGE CEO offering to buy all outstanding shares of the company it does not already own
** BIIB offers to buy all SAGE shares for $7.22 per share in cash, implying a total deal value of $441.7 mln, according to Reuters calculations
** BIIB, which is SAGE's largest shareholder, currently owns 10.2% or over 6.24 mln shares of SAGE common stock
** The share price offered implies a 30% premium to SAGE's closing price on Friday
** SAGE stock fell ~75% in 2024
(Reporting by Vallari Srivastava in Bengaluru)
((Srivastava.Vallari@thomsonreuters.com;))