Mersana Therapeutics Stock Explodes Over 200% To Notch Best Day Ever As $285M Buyout Trumps Analyst Downgrades: Retail Cheers ‘Very Good Bargain’

• Mersana surged even as multiple firms downgraded the stock following Day One’s acquisition agreement.
• Analysts said competing bids are unlikely and noted Day One’s offer reflects confidence in Mersana’s assets.
• Retail traders called the deal a bargain for Day One and highlighted Mersana’s broader pipeline value.

Mersana Therapeutics logged its strongest trading session on record on Thursday, soaring even as a long list of Wall Street firms downgraded the stock after Day One Biopharmaceuticals agreed to acquire the company in a deal that could be worth up to $285 million.

Day One Biopharmaceuticals closed down 14% on Thursday, while Mersana Therapeutics finished the session up 209%.

Analysts Weigh In On Day One’s Bid

William Blair downgraded Mersana to ‘Market Perform’ from ‘Outperform’, saying it views it as unlikely that another bidder will emerge.

Guggenheim cut its rating to ‘Neutral’ from ‘Buy’, while LifeSci Capital shifted its view to ‘Market Perform’ from ‘Outperform.’ Leerink moved the stock to ‘Market Perform’ from ‘Outperform’ and set a $29 price target. Truist downgraded Mersana to ‘Hold’ from ‘Buy’ and lowered its target to $30.75 from $36, saying it believes the deal will go through and estimating the CVR’s value at $5.75. BTIG lowered its rating to ‘Neutral’ from ‘Buy’, and Wedbush also cut the stock to ‘Neutral’ from ‘Outperform.’

Oppenheimer said Day One paying up to $285 million for Mersana “is interesting.” The firm noted the acquisition “wasn't on the short list,” and said management had been “talking down PTK7,” and questioned whether Emi-Le may be positioned as its replacement. Oppenheimer added that Day One could have obtained a comparable asset from China at a lower cost and said the offer suggests management is “seeing something in Emi-Le.” The firm reiterated a ‘Perform’ rating on Day One.

Deal Terms

Under the merger agreement, Day One will launch a tender offer within 10 business days of Nov. 12. The deal requires that a majority of Mersana’s shares be tendered for the transaction to proceed, after which any remaining shares would be acquired in a follow-on merger for the same consideration. Once completed, Mersana would become a wholly owned subsidiary of Day One, and its stock would be delisted. 

The agreement received unanimous board approval, and holders representing about 8.5% of outstanding shares, including executives, directors, and Bain Capital Life Sciences affiliates, have signed support agreements to tender their shares.

Legal Review

Separately, law firm Ademi & Fruchter said it is reviewing the deal for potential breaches of fiduciary duty, citing insider benefits, deal protection,s and limits on competing offers.

Stocktwits Traders Flag ‘Cheap’ Valuations

On Stocktwits, retail sentiment for both Mersana and Day One was ‘extremely bullish’ amid ‘extremely high’ message volume.

One user said, “If they were able to buy Mersana Therapeutics ($MRSN), they could easily buy $CHRS; I'm sure many companies are interested, it's very cheap.”

Another user argued that Day One is getting what they called “a very good bargain” in the acquisition, saying the company is “effectively getting the entire pipeline of Mersana for about $40 million” and adding, “I am buying the stock.”

A third user noted that while many investors had expected Day One to be a buyout target, in this deal the company instead acted as “Apex predator,” adding that it would be “interesting to watch further manifestations in this new constitution.”

Mersana’s stock has more than tripled so far in 2025, while Day One’s stock has climbed 35% over the same period.

For updates and corrections, email newsroom[at]stocktwits[dot]com.

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