Drugmaker Biogen will buy all outstanding shares of Sage Therapeutics that it does not already own for $7.22 apiece, a filing showed on Friday, sending the latter's stock up 34.6% in premarket trading Monday.

As per the filing, Biogen has a 10.2% stake in Sage Therapeutics. The offer price, representing a 30% premium to the stock's close on Friday, values Sage's equity at $441.7 million, according to calculations.

Biogen has been navigating a slow pick-up in sales for its Alzheimer's drug Leqembi in the United States after concerns over cost, efficacy and side effects.

Sage, whose shares fell 74.9% last year, abandoned the development of its drug dalzanemdor after multiple trial failures. The company said in November that it will focus on its postpartum depression drug, Zurzuvae, which it developed in partnership with Biogen.

In July, Sage also scrapped the development of another neurological disorder drug it was developing with Biogen.