GSK and AnaptysBio File Dueling Lawsuits Over Jemperli Royalty Agreement as Cancer Drug Nears Blockbuster Status
Key Insights
GSK and AnaptysBio (search) are engaged in dueling lawsuits over their 11-year collaboration agreement covering the cancer (search) immunotherapy Jemperli, with GSK seeking to terminate the deal and halve royalty payments.
The dispute centers on GSK's alleged testing of experimental drugs with competitor PD-1 (search) inhibitors like Keytruda rather than with Jemperli, which AnaptysBio (search) claims violates exclusivity obligations.
Jemperli recorded $785 million in sales through the first nine months of 2025 and is on track to become a $1 billion blockbuster, triggering a $75 million milestone payment to AnaptysBio (search).
GSK and AnaptysBio (search) have filed competing lawsuits in Delaware Chancery Court over their decade-old licensing agreement for the cancer (search) immunotherapy Jemperli, just as the drug approaches blockbuster status with $785 million in sales through the first nine months of 2025.
The legal battle began when GSK's Tesaro subsidiary filed suit claiming AnaptysBio (search) was in "material breach of the existing license agreement." GSK is seeking to terminate the agreement, obtain a permanent license to Jemperli, and halve the royalties due to AnaptysBio. The following day, AnaptysBio countersued, alleging that GSK violated the terms of their license by testing experimental cancer (search) drugs with competitors like Merck's Keytruda rather than with Jemperli.
Background of the Collaboration
The partnership traces back to a 2014 deal between AnaptysBio (search) and Tesaro for Jemperli, then an experimental PD-1 (search) inhibitor that works similarly to established drugs like Keytruda and Bristol Myers Squibb (search)'s Opdivo. GSK acquired Tesaro in 2019 for $5.1 billion, primarily for the ovarian cancer (search) drug Zejula, though executives also highlighted Jemperli's therapeutic promise at the time.
This marks the second major dispute between the companies. In 2020, AnaptysBio (search) previously sued GSK over plans to study Zejula in combination with Keytruda. That matter was resolved with GSK paying $60 million and significantly increasing Jemperli royalties payable to AnaptysBio from 4-8% to 8-25%, while gaining rights to test Zejula with third-party drugs.
Current Dispute Details
AnaptysBio (search) alleges that GSK has continued to engage in trials involving Keytruda, violating what it describes as GSK's "exclusivity obligations." One cited example is a phase 1 solid tumor (search) trial GSK initiated last year combining the B7-H3 (search)-targeting ADC GSK5764227 with various molecules, including Keytruda, Tecentriq, and Imfinzi.
"While Anaptys had approached Tesaro to engage in good faith discussions to potentially resolve these claims, on Nov. 20, 2025, Tesaro, without notice, initiated a lawsuit against Anaptys," the company stated in a release.
GSK's lawsuit does not specify what behavior constitutes AnaptysBio (search)'s alleged material breach, but the company claims this entitles it to terminate the Jemperli agreement and reduce royalty obligations.
Financial Stakes
The financial implications are substantial for both companies. Jemperli recorded sales of 600 million pounds (approximately $785 million) through the first nine months of 2025, representing 17% quarter-on-quarter growth and positioning the drug to become a $1 billion seller by year-end.
Under the current agreement, AnaptysBio (search) receives 8% royalties on sales up to $1 billion and 12% on sales between $1 billion and $1.5 billion. The company is set to collect a $75 million milestone payment once Jemperli's annual sales exceed $1 billion, expected in the current quarter.
If GSK's lawsuit succeeds, AnaptysBio (search)'s annual royalties could be reduced from an expected $390 million to $195 million once Jemperli reaches peak sales projections of $2.7 billion. Only $165 million in milestones remain outstanding, including the imminent $75 million payment.
Broader Business Implications
The dispute comes at a critical time for AnaptysBio (search), which plans to split off its royalty business into a separate entity by the end of 2026. The company also owes up to $675 million to Sagard Healthcare Royalty Partners (search) as part of a 2021 royalty monetization deal, with expectations to pay off these obligations sometime in 2027 or 2028.
Leerink Partners (search) analyst David Risinger noted that while the dueling lawsuits create uncertainty around AnaptysBio (search)'s business plans, his team still sees potential for the company to "not only retain, but potentially realize even greater economics." He questioned whether "GSK/Tesaro are trying to use leverage ahead of [AnaptysBio's] proposed separation of its royalty business into a separate entity in 2026."
The case is expected to proceed to trial in July 2026, with court documents currently under seal but expected to become available with redactions within the coming week.