GSK’s Tesaro ,Four years into the commercial life of the PD-1 inhibitor Jemperli (dostarlimab), a legal dispute has emerged between Tesaro, a subsidiary of GlaxoSmithKline (GSK), and AnaptysBio, the original developer of the potential cancer blockbuster. The dispute involves allegations of material breaches of a licensing agreement that dates back more than a decade.
According to a recent press release by GSK, Tesaro filed a lawsuit against AnaptysBio in Delaware, claiming the biotech company has materially breached the existing collaboration and licensing agreement for Jemperli. In response, AnaptysBio launched its own complaint in Delaware Chancery Court, asserting that Tesaro—and by extension GSK—have materially breached the agreement and engaged in tortious interference with the licensing deal.
The conflict traces back to a 2014 licensing agreement, under which Tesaro paid AnaptysBio $17 million upfront, with the promise of tens of millions more in potential milestone payments. The deal granted Tesaro exclusive rights to AnaptysBio’s antibody programs targeting PD-1, TIM-3, and LAG-3 pathways, key targets in cancer immunotherapy. GSK acquired Tesaro in 2018 for approximately $5.1 billion, inheriting the licensing rights and obligations related to Jemperli.
Following the acquisition, Jemperli received its first FDA approval in 2021 for treatment of patients with previously treated advanced or recurrent mismatch repair-deficient (dMMR) endometrial cancer. Since then, the drug has expanded its approval to additional dMMR cancers and select other solid tumors, making it a high-potential oncology asset for GSK and Tesaro.
The current legal battle, though initiated publicly by Tesaro, stems from AnaptysBio’s argument that the GSK subsidiary failed to meet its obligations under the licensing agreement. GSK, in its press release, categorically rejected the allegations, stating that they are “without merit.” Meanwhile, AnaptysBio claims Tesaro violated the licensing conditions by conducting clinical trials involving PD-1 antagonist competitors, such as Merck’s Keytruda, without consent.
AnaptysBio further alleges that Tesaro did not make commercially reasonable efforts to maximize Jemperli’s commercial potential, preferring instead to prioritize GSK’s other antibody-drug conjugate (ADC) candidates. This allegedly included ADCs combined with PD-1 antagonists controlled by Jemperli, raising questions about potential conflicts of interest.
This is not the first clash between the two companies. In 2020, AnaptysBio filed a similar complaint concerning alleged violations of the same agreement. That dispute involved the ovarian cancer drug Zejula and Keytruda, and it resulted in a settlement that included a cash payment and enhanced royalties on Jemperli sales.
Despite attempts at good faith negotiations, AnaptysBio stated that it had no choice but to respond with legal action after Tesaro initiated its lawsuit without prior notice. Tesaro, for its part, maintains that all claims are baseless and has pledged to pursue all legal remedies, including dismissal of the claims and potential counterclaims.
Amid the legal wrangling, Jemperli continues to generate significant revenue, with GSK projecting peak sales between $1.4 billion and $2.8 billion. In Q3 2025, the drug recorded £230 million (~$301 million) in sales, marking nearly an 80% increase over the same period in 2024 at constant currency. As part of the litigation, AnaptysBio noted that milestone and royalty payments on Jemperli remain ongoing to both GSK and Tesaro.
In parallel, AnaptysBio has announced plans to split into two publicly traded companies. One entity will focus on the company’s clinical-stage pipeline, while the other will manage its royalty-based agreements, including the Jemperli licensing deal. The company expects the split to be completed by the end of 2026, further shaping its strategic focus on both innovation and licensing revenue streams.
This high-stakes dispute highlights the challenges in biotech licensing and commercialization agreements, particularly when high-value immuno-oncology assets such as Jemperli are involved. As the court proceedings unfold, all eyes in the oncology and biotech sectors will be on the implications of this legal conflict for GSK, AnaptysBio, and the broader PD-1 inhibitor market.
Legal Battle Heats Up Between GSK’s Tesaro and AnaptysBio Over PD-1 Cancer Drug Jemperli
A high-stakes confrontation is escalating as GSK’s Tesaro enters a heated legal dispute with AnaptysBio over the development and commercial rights of the PD-1 cancer drug, Jemperli. The conflict highlights growing tensions in the immuno-oncology space, where innovative therapies and billion-dollar royalties often drive competitive and legal challenges.
At the center of the dispute, GSK’s Tesaro claims that AnaptysBio is attempting to renegotiate long-standing agreements regarding the antibody technology behind Jemperli. According to GSK’s Tesaro, the original collaboration clearly outlined royalty structures and development obligations — terms they argue must remain unchanged despite AnaptysBio’s efforts to secure a larger financial stake.
AnaptysBio, on the other hand, asserts that new clinical indications, expanded global approvals, and market growth require updated compensation terms. They argue that GSK’s Tesaro is benefiting disproportionately from commercial success, especially as Jemperli gains wider recognition in endometrial, colorectal, and solid tumor treatment lines.
The legal fight is shaping the future of PD-1 inhibitors, with industry observers noting that any ruling could set important precedents for biotech-pharma partnerships. As Jemperli becomes a central part of oncology pipelines, the resolution between GSK’s Tesaro and AnaptysBio will likely influence future collaboration models and IP agreements within the cancer drug development ecosystem.
For now, the biotech world watches closely as GSK’s Tesaro and AnaptysBio prepare for a potentially lengthy and impactful courtroom showdown.
