AnaptysBio sues Tesaro and GSK in Delaware Chancery Court over Jemperli collaboration

AnaptysBio, Inc. said in a Form 8-K filed Nov 21, 2025, that it filed a Verified Complaint on Nov 20, 2025, in the Delaware Court of Chancery. The defendants are TESARO, Inc. and GlaxoSmithKline, which the filing calls Tesaro's corporate parent. The complaint asks the court to declare that Tesaro materially breached the parties' Collaboration and Exclusive License Agreement, and that GSK tortiously interfered with it. AnaptysBio asks the court to declare that it is entitled to all rights and remedies under that agreement. Tesaro filed its own suit against AnaptysBio the same day.

These are AnaptysBio's allegations. The filing does not include a court ruling, a response from Tesaro or GSK, or a case number for either action.

What AnaptysBio alleges

The complaint says Tesaro agreed not to research, develop, manufacture or commercialize any PD-1 antagonist other than those AnaptysBio licensed to it. It also says Tesaro must use commercially reasonable efforts to obtain the optimum commercial return for Jemperli in all major markets. AnaptysBio contends that Tesaro violated the exclusivity terms by taking part in past, ongoing and planned clinical trials of PD-1 antagonists that compete with Jemperli, including Keytruda. It further contends that Tesaro failed to use commercially reasonable efforts by joining a course of conduct with GSK to favor GSK's antibody-drug conjugate candidates, including by pairing them with competing PD-1 antagonists.

The filing says GSK induced those breaches. It does not describe what evidence supports the GSK claim beyond the allegations in the complaint.

The 2020 dispute and Tesaro's suit

According to the filing, AnaptysBio filed a similar complaint against GSK in 2020. That dispute concerned a planned GSK trial of Zejula, another GSK oncology product, combined with Keytruda, without AnaptysBio's consent. The filing says that complaint settled with a cash payment, a royalty on Zejula and a substantial increase in royalties on Jemperli sales payable to AnaptysBio.

AnaptysBio says it had approached Tesaro to discuss resolving the current claims. It says Tesaro sued it on Nov 20 without notice. According to AnaptysBio, Tesaro's suit seeks injunctive relief and claims AnaptysBio breached the agreement by alleging that Tesaro had breached it. AnaptysBio says Tesaro is trying to restrain it from using its legal rights, in violation of Delaware's anti-SLAPP statute and other protections. The filing does not describe Tesaro's complaint in its own words or say which court has it.

Schedule and money

AnaptysBio says the parties have agreed to ask the Chancery Court for an expedited schedule, with trial anticipated in July 2026. The schedule is a request to the court, not an order, and the filing gives no date for a ruling on the claims. The filing says AnaptysBio intends to seek dismissal of Tesaro's claims at the earliest opportunity.

The company says milestone and royalty payments under the collaboration remain due during the proceedings. That matters because the filing's forward-looking statements list the potential termination of the agreement and the return of Jemperli to the company as possible outcomes. The filing does not say what either outcome would mean for those payments.

AnaptysBio's latest annual report on file is its 10-K for 2024. It reports cash, cash equivalents and investments of $420.8 million at Dec 31, 2024, compared with $417.9 million a year earlier. The company reported a net loss of $145.2 million for 2024 and used $135.3 million in operating activities. It also says the company's existing resources will fund its operating plan for at least the next twelve months from the report's issuance.

The 10-K also shows how AnaptysBio has funded itself. In 2024, AnaptysBio raised $94.4 million from public offerings, net of underwriters' fees, and $50.0 million from the sale of future royalties. Those royalty sales are a financing that the company has already used, and the filing does not put a dollar figure on the value of the Jemperli rights in dispute.

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