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AnaptysBio deploys Anti-SLAPP defense in $2B GSK royalty feud

Written by Liezl GambePublished Jan. 9 2026Last Updated Jan. 9 2026

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AnaptysBio (NASDAQ:ANAB) moved to dismantle a key legal pillar of GSK’s (NYSE:GSK) defense in the Delaware Chancery Court on Thursday, filing a motion to dismiss claims that the biotech "repudiated" its lucrative royalty agreement for the blockbuster-intent cancer drug Jemperli.

The filing is the latest salvo in a deteriorating relationship between San Diego-based Anaptys and GSK’s subsidiary, Tesaro.

At the heart of the conflict is a 2014 collaboration agreement that grants Anaptys royalties on Jemperli sales that scale as high as 25%.

Anaptys alleges that GSK has materially breached this contract and engaged in tortious interference to suppress the biotech's rightful earnings as the drug expands into broader oncology markets.

The legal skirmish turned hostile in November 2025 when Tesaro, according to Anaptys, preemptively sued the biotech while settlement talks were still active.

Anaptys’ new motion, unsealed Jan. 8, argues that Tesaro’s lawsuit is a strategic attempt to silence the company’s legitimate contract claims—a move Anaptys contends violates Delaware’s anti-SLAPP law designed to prevent "strategic lawsuits against public participation."

"Anaptys has never repudiated the Collaboration Agreement; we have only sought to vindicate our contract rights," the company stated in the filing.

The financial stakes are immense.

Jemperli (dostarlimab) has emerged as a powerhouse in the PD-1 inhibitor class, showing a 100% complete response rate in certain rectal cancer trials and gaining rapid market share in endometrial cancer.

The current royalty structure entitles Anaptys to 8% of sales below $1 billion, jumping to 25% for sales exceeding $2.5 billion—thresholds that analysts believe are within reach as GSK pursues dozens of new indications.

A major point of contention remains the "exclusivity" of the deal.

Anaptys claims GSK violated the pact by participating in clinical trials involving rival PD-1 blockers, such as Merck & Co.’s Keytruda, rather than prioritizing Jemperli.

GSK and Tesaro have countered by seeking to reduce their royalty obligations by 50%, claiming Anaptys’ aggressive legal posturing constitutes a material breach that would allow them to trigger a "perpetual and irrevocable" license at a discount.

The court is expected to hear the motion to dismiss by early March.

While GSK has argued that the motion should stay all discovery, Anaptys is pushing to keep the case on track for a four-day trial scheduled to begin on July 14, 2026.