AnaptysBio Inc., trading on NASDAQ under the symbol ANAB, has responded assertively to legal action initiated by Tesaro and its parent company, GSK Plc (NYSE:GSK), concerning royalty payments related to their collaborative cancer drug, Jemperli. The Delaware Chancery Court is currently the venue for this dispute involving the interpretation and enforcement of a licensing agreement established nearly a decade ago.
The core of the disagreement lies in the Collaboration and Exclusive License Agreement formed in March 2014 between AnaptysBio and Tesaro, prior to Tesaro’s acquisition by GSK. This agreement grants AnaptysBio entitlement to tiered royalties, ranging from 8% to 25% of net sales, stemming from the commercialization of Jemperli, an immunotherapy medication targeting certain solid tumor cancers. The royalties are stipulated to continue through the expiration of U.S. patents in 2035 and extend to 2036 under European Union protections.
Under the terms of the deal, Tesaro was responsible for the development and commercialization of Jemperli both as a standalone therapy and in combination regimens. AnaptysBio, as the licensor, receives corresponding royalties based on the product's market performance.
Contention has arisen as AnaptysBio identified what it views as breaches by Tesaro and GSK, prompting the company to seek good-faith negotiations aimed at resolving these concerns. However, prior to substantive resolution, Tesaro filed a lawsuit in November 2025, accusing AnaptysBio of repudiating the collaborative agreement and seeking judicial affirmation that Tesaro itself had maintained compliance.
In defense, AnaptysBio launched its own claim alleging that Tesaro materially breached the agreement and that GSK engaged in tortious interference, complicating the contractual relationship. This countersuit seeks the court's declaration to clarify the respective rights and obligations of the parties ahead of a trial scheduled for mid-July 2026.
Further, in December 2025, AnaptysBio filed a partial motion to dismiss Tesaro’s anticipatory breach of contract claim. This motion, which became public in early 2026, advanced the argument that AnaptysBio never repudiated the contract but rather endeavored to enforce its contractual rights. The company asserted that Delaware law does not recognize the assertion of legal rights under a contract as grounds for repudiation.
AnaptysBio also invoked Delaware’s anti-SLAPP (Strategic Lawsuit Against Public Participation) statute to characterize Tesaro’s complaint as a legal strategy aimed at deterring legitimate claims. Anti-SLAPP provisions exist to protect parties defending their rightful claims without being subject to lawsuits designed to suppress legal action.
Tesaro and GSK have countered that AnaptysBio’s motion to dismiss should halt the discovery process as the litigation proceeds. AnaptysBio, however, opposed this suspension, emphasizing the need to move forward in preparation for the impending trial.
Aside from the licensing dispute, AnaptysBio announced the discontinuation of its Phase 2 trial for rosnilimab, a drug candidate for moderate-to-severe ulcerative colitis, in November 2025. The trial failed to meet primary and key secondary endpoints, specifically the mean change in the modified Mayo Score and clinical response at 12 weeks.
Reflecting investor sentiment amid these developments, AnaptysBio’s stock was trading up over 4% to $45.56 at the time of publication. The company remains under close observation given its pipeline and ongoing legal complexities.