Both Tesaro (a subsidiary of GSK) and AnaptysBio have filed suits against one another in the Delaware Chancery Court, claiming breaches and violations in the companies’ original partnership agreement signed back in 2014.

Tesaro’s lawsuit

Tesaro confirmed its litigation against AnaptysBio in the Delaware Chancery Court in a press release on its website, contending that recent conduct by AnaptysBio is in material breach of Tesaro’s existing license agreement with AnaptysBio regarding oncology treatment Jemperli (dostarlimab).¹

Tesaro’s decision to file its suit against AnaptysBio follows allegations made by AnaptysBio that Tesaro reportedly failed to fulfill certain requirements apart of the license agreement both companies entered back in March 2014.¹ In response to Tesaro’s suit, AnaptysBio is expected to revoke Tesaro’s license for Jemperli.

According to the press release, the breach in contract grants Tesaro the option to terminate the current license agreement while also obtaining a perpetual and irrevocable license to Jemperli. The breach also entitles Tesaro the ability to reduce both royalty and milestone payments to AnaptysBio by 50%.

As stated in the press release, “Both GSK and Tesaro are firmly of the view that these allegations are entirely without merit.”

AnaptysBio’s response

Just a day after Tesaro filed its suit against AnaptysBio, the company responded back with a suit of its own, filing a verified complaint in the Delaware Chancery Court, requesting a court declaration that Tesaro has materially breached the parties’ collaboration and exclusive license agreement, along with claiming that GSK, Tesaro’s parent company, has tortiously interfered with the collaboration agreement made back in 2014.² Following the filing, Anaptys requested that the court declare that Anaptys will be entitledto all rights and remedies originally listed within the agreement.²

Anaptys’ suit claims that Tesaro breached, with GSK inducing Tesaro to breach, multiple material collaboration agreement terms, which includes Tesaro agreement that it would not conduct or participate in research, development, manufacturing or commercialization of any PD-1 antagonist other than those licensed by Anaptys to Tesaro. ²

Anaptys’suit also contends that Tesaro violated its exclusivity obligations, participating in past, ongoing, and upcoming governed clinical trials with PD-1 antagonists competing against Jemperli, including Keytruda, while also failing to use commercially reasonable efforts to obtain the “optimum commercial return” for Jemperli.² The suit continues to claim among other things that Tesaro participated in a course of conduct with GSK, favoring GSK’s antibody-drug conjugate (ADC) product candidates, including by pairing those ADCs with PD-1 antagonists that directly compete with Jemperli.²

What was the original agreement?

Back in March 2014, both AnaptysBio and Tesaro entered into a collaboration agreement, granting Tesaro rights to develop Jemperli as a monotherapy, and in combination with additional therapies, for various solid tumor indications.² The royalty terms under the collaboration extends through the expiration of composition of matter coverage on the molecule, which is set to expire in 2035 in the U.S. and in 2036 in the EU respectfully.

Sources

1. TESARO, a GSK subsidiary, initiates litigation against AnaptysBio, Inc. GSK November 20, 2025 https://www.gsk.com/en-gb/media/press-releases/tesaro-a-gsk-subsidiary-initiates-litigation-against-anaptysbio-inc/
2. Anaptys Initiates Litigation Against Tesaro, a GSK Subsidiary AnaptysBio Novmeber 21, 2025 https://www.globenewswire.com/news-release/2025/11/21/3192556/0/en/Anaptys-Initiates-Litigation-Against-Tesaro-a-GSK-Subsidiary.html