AnaptysBio filed a verified complaint in the Delaware Chancery Court on Friday, November 21, 2025, accusing Tesaro, a subsidiary of GSK, of materially breaching the 2014 collaboration and exclusive license agreement for the oncology drug Jemperli (dostarlimab). The complaint seeks a declaration that Tesaro has breached the agreement and requests that AnaptysBio be granted a perpetual and irrevocable license to the drug, the right to terminate the license, and a 50 % reduction in the royalties and milestone payments owed by Tesaro.
The lawsuit follows Tesaro’s own filing on Thursday, November 20, 2025, in which the company alleged that AnaptysBio had breached the agreement by failing to use commercially reasonable efforts to market Jemperli and by engaging in clinical trials with rival PD‑1 antagonists. AnaptysBio counters that Tesaro has not honored its obligations to promote the drug and has prioritized its own antibody‑drug conjugate (ADC) candidates over Jemperli, thereby undermining the partnership.
Jemperli has become a key revenue driver for AnaptysBio, generating £600 million ($785 million) in the first nine months of 2025, an 89 % year‑on‑year increase. The drug is approved in more than 35 countries for endometrial cancer and is under investigation for other solid tumors. The 2014 agreement granted AnaptysBio an upfront payment of $10 million, milestone payments tied to sales milestones, and a royalty rate of 12 % on net sales. The current dispute threatens the $75 million commercial sales milestone that AnaptysBio expects to receive in Q4 2025 if Jemperli reaches $1 billion in worldwide net sales.
AnaptysBio’s financial statements show that royalty income from Jemperli accounts for roughly 30 % of its total revenue, making the outcome of the litigation a critical factor in its cash flow and future growth plans. The company has announced plans to separate its biopharma operations from its royalty assets by the end of 2026, a move that could be complicated by the legal uncertainty surrounding Jemperli’s royalties.
Market reaction to the filings was swift. Shares of AnaptysBio fell between 10 % and 14 % in pre‑market trading on November 21, reflecting investors’ concerns over the potential loss of a substantial royalty stream. In contrast, U.S.‑listed shares of GSK rose 2 %, indicating that the market viewed the dispute as less material to GSK’s broader portfolio.
Management comments underscore the stakes. AnaptysBio’s CEO stated that the company is “committed to protecting the value of its partnership and ensuring that Jemperli’s commercial potential is fully realized.” GSK’s spokesperson dismissed the allegations as “without merit,” emphasizing that the company believes AnaptysBio’s conduct constitutes a material breach that entitles Tesaro to terminate the license and reduce payments.
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