A series of disappointments that drove Sage Therapeutics Inc.’s stock down by 85% since August of 2023 has evolved into an unsolicited takeout offer by partner Biogen Inc., followed by a lawsuit filed by Sage a week later. The two parties, both of Cambridge, Mass., first partnered in a $1.52 billion deal in 2020, primarily to develop Zurzuvae (zuranolone) for depression. The deal included Biogen taking a 10.2% equity stake in Sage, paying $104.14 per share, or $650 million total.