Sage Therapeutics Inc. sued Biogen Inc. on Friday, a week after the biotech giant made an unsolicited offer to acquire the smaller neuroscience company for about $469 million.

Biogen’s proposed acquisition comes after two difficult years of regulatory and clinical challenges, during which shares of Sage Therapeutics have fallen by more than 90%.

The $7.22-per-share deal for 89.8% of Sage is a 30% premium on the company's share price on Friday and is estimated by Reuters to be worth around $442 million. Biogen has a longstanding alliance in place with Sage and co-markets Zurzuvae (zuranolone), which is FDA-approved for post-partum depression (PPD).