Sage Therapeutics has brought a lawsuit against Biogen shortly after receiving an “unsolicited” offer from its neuroscience partner.

The lawsuit is sealed in Delaware’s Court of Chancery, but in a separate filing, Sage said it’s “seeking preliminary injunctive relief to enforce a standstill agreement and a trial on a paper record on an expedited basis,” according to Bloomberg Law.

“Sage has filed necessary paperwork with the Delaware court seeking a temporary restraining order to preserve our rights and enforce the standstill provision previously agreed to with Biogen,” a Sage spokesperson said in a statement to Fierce Pharma.

The two firms joined hands in late 2020 when Biogen signed on to jointly develop and commercialize Sage’s zuranolone, which in 2023 became the first oral treatment for postpartum depression, and another neuroscience candidate. Besides an upfront payment, Biogen took a stake in Sage at a price of $104.14 per share.

In the original pact, Biogen agreed not to acquire securities of Sage or seek a merger between the parties, subject to specified conditions, according to prior filings. The standstill restrictions terminate after meeting one of certain terms, such as the end of the original collab or the arrival of December 28, 2027.

More recently, Biogen on Jan. 10 offered to acquire the rest of Sage it doesn’t already own for an equity value of about $469 million. The per-share price came out to $7.22 this time, but the offer still represented a 30% premium to Sage’s closing price that day, according to a proposal letter penned by Biogen CEO Chris Viehbacher.

Sage’s spokesperson added that “any discussions between Biogen and Sage regarding an acquisition of Sage should be conducted in private.”

“The board’s evaluation of Biogen’s unsolicited proposal is ongoing, and they will continue to act in the best interest of Sage and its shareholders,” the company’s spokesperson said. “No decision has been made regarding Biogen’s proposal.”

However, after speaking with Sage’s management last week, Mizuho analyst Uy Ear said in an investor note that Sage seems “unlikely to accept Biogen’s take-out offer at this offer.”

Sage sees “blockbuster-plus” potential for Zurzuvae in postpartum depression and is happy with the current launch trajectory, according to Ear.

Biogen’s offer, at about $469 million, effectively assigns Sage a negative value because the Massachusetts biotech had about $569 million in cash and equivalents at the end of September. Biogen already owns about 6.2 million shares, or roughly 10%, of Sage.

The decline in Sage’s stock price reflects the setbacks the biotech has suffered in recent years. Zurzuvae’s value was significantly reduced after the FDA declined to approve it in the much larger indication of major depressive disorder. Then, Biogen last year walked away from a collaboration on SAGE-324 after the GABA-positive allosteric modulator had failed in an essential tremor test.

On the other hand, the Sage acquisition proposal is a “prudent financial move” for Biogen to effectively market Zurzuvae and to maximize the drug’s value, William Blair analysts said in their own note last week. BMO Capital Markets analyst Evan David Seigerman noted that such a deal would reduce collaboration expenses for Biogen.

Analysts at Citi called a potential transaction “favorable” for Biogen. Besides Zurzuvae, the Citi team argued that if Sage’s early pipeline, including SAGE-319 for behavioral symptoms associated with neurodevelopmental disorders and SAGE-324 for seizures in certain epilepsies, were to work, “it could contribute a significant portion of revenue.”

But neither William Blair nor BMO believes an acquisition of Sage would significantly change the narrative around Biogen, suggesting that the Big Biotech still needs additional deals to change its growth profile.