Supernus Pharmaceuticals signed a definitive agreement to acquire fellow biopharmaceutical company Sage Therapeutics for $8.50 a share in a deal worth an initial $561 million. Sage stock closed last Friday at $6.60 a share, and was up 36% to $9.09 Monday after the deal was announced. The main draw of the deal for Supernus was Sage’s Zurzuvae, the first and only oral medication approved by the Food and Drug Administration for treatment of postpartum depression.