GSK AnaptysBio Lawsuit Puts Jemperli Royalties at Risk

The GSK AnaptysBio lawsuit began when Tesaro, a GSK plc (GSK) subsidiary, sued AnaptysBio (ANAB) in Delaware Chancery Court on Nov. 20, 2025. Tesaro seeks to end the Jemperli license and reduce royalty and milestone payments. AnaptysBio filed its own complaint the next day. Both parties have requested an expedited trial scheduled for July 2026.

Litigation Claims and Schedule

Tesaro alleges that AnaptysBio materially breached their 2014 collaboration and exclusive license agreement for Jemperli (dostarlimab), seeking to terminate the license, obtain a perpetual and irrevocable license, and cut royalty and milestone payments by 50%. AnaptysBio counters that Tesaro and GSK breached exclusivity and commercialization obligations by participating in clinical trials with competing PD-1 antagonists, such as Keytruda, and by favoring GSK antibody-drug conjugate candidates. AnaptysBio has asked the court to declare it entitled to all rights and remedies under the agreement. Both parties deny the other's claims and have requested an expedited schedule for trial in July 2026.

Royalties, Milestones, and Monetization

Jemperli, approved in over 35 countries for certain endometrial cancers, is central to the dispute. Under the agreement, AnaptysBio is eligible for tiered royalties: 8% on net sales below $1 billion, 12% on $1 billion to $1.5 billion, 20% on $1.5 billion to $2.5 billion, and 25% above $2.5 billion. Royalties and milestone payments will continue during litigation and run through the expiration of the composition-of-matter patents in the U.S. (2035) and EU (2036).

AnaptysBio expects to accrue a $75 million commercial milestone in the fourth quarter of 2025 if Jemperli reaches $1 billion in worldwide net sales. Current Jemperli receivables are payable to Sagard under a capped non-recourse agreement that ends after Sagard receives $600 million by March 31, 2031, or $675 million thereafter. AnaptysBio estimates $250 million will be accrued by year-end 2025 and projects the capped payout will be reached between the second quarter of 2027 and the second quarter of 2028.

The original collaboration and exclusive license agreement, signed in March 2014, granted Tesaro exclusive rights to develop and commercialize Jemperli. GSK acquired Tesaro in 2019. A prior dispute over trial conduct led AnaptysBio to notify GSK and Tesaro in 2020; the parties settled in October 2020 with increased royalties for AnaptysBio.

The litigation directly threatens AnaptysBio’s royalty and milestone cash flows and the timing of receivable monetization with Sagard, potentially affecting the company’s balance sheet and cash-flow planning tied to those payments.

"Anaptys has requested that the court declare that Anaptys is entitled to all rights and remedies under the Collaboration Agreement." — AnaptysBio press release