Some four years into Jemperli’s commercial life span, a rift has emerged between GSK subsidiary Tesaro and the potential cancer blockbuster’s original developer, AnaptysBio.
Tesaro has filed a lawsuit against Anaptys in Delaware over claims that the antibody-focused biotech is in “material breach” of an ongoing licensing deal for PD-1 drug Jemperli (dostarlimab), GSK said in a Thursday press release.
Anaptys followed up Friday morning by announcing its own complaint in Delaware Chancery Court, making the case that it is in fact Tesaro that has “materially breached the parties' collaboration and exclusive license agreement.” The biotech went on to allege that GSK “has tortiously interfered with” the deal.
The brouhaha stems back to a licensing deal from 2014, under which Tesaro paid Anaptys $17 million upfront—with the promise of tens of millions more in potential milestone payments—for exclusive rights to antibody programs going after the targets PD-1, TIM-3 and LAG-3. More recently, GSK acquired Tesaro for roughly $5.1 billion in 2018.
Subsequently, Jemperli in 2021 won its first FDA nod to be used in patients with previously treated advanced or recurrent mismatch repair-deficient (dMMR) endometrial cancer. The medication has since gone on to significantly expand its dMMR reach and pick up a green light in certain other solid tumors, as well.
Despite Tesaro announcing its legal case first, the move came in response to Anaptys' argument that the GSK subsidiary was failing to live up to the license agreement, GSK explained in its release.
“GSK and Tesaro are firmly of the view that these allegations are entirely without merit,” the press release states.
For its part, Anaptys contends not only that Tesaro breached the terms of the Jemperli licensing agreement but that GSK “induced” its subsidiary to do so. The biotech specifically took issue with the fact that Tesaro is engaging in clinical trials that include rival PD-1 antagonists to Jemperli, like Merck & Co.’s megablockbuster Keytruda.
Further, Anaptys argued that Tesaro “failed to use commercially reasonable efforts” to secure an “‘optimum commercial return’” for Jemperli. Instead, the GSK unit purportedly took part in “a course of conduct with GSK to favor GSK’s antibody-drug conjugate (ADC) product candidates, including by pairing those ADCs with PD-1 antagonists that directly compete with Jemperli,” according to Anaptys.
This isn’t the first time the partners have butted heads. Anaptys noted that it filed a similar complaint against GSK in 2020 alleging breaches of the same agreement. In that instance, the biotech’s concerns related to an experimental combination of the GSK ovarian cancer med Zejula with Merck’s Keytruda, arguing that the decision to wed those rival drugs was done “without the consent of Anaptys.”
“This complaint resulted in a settlement that included a cash payment, a royalty on Zejula and a substantial increase in royalties on sales of Jemperli due to Anaptys,” the biotech pointed out.
Anaptys stressed that it attempted to “engage in good faith discussions” to resolve its concerns with Tesaro but had no choice other than to retaliate in court after Tesaro filed its lawsuit “without notice” on Thursday.
“Anaptys contends that Tesaro’s claims are entirely without merit and intends to pursue all appropriate legal remedies against Tesaro, including dismissal of such claims at the earliest opportunity and the potential for additional litigation,” the biotech said in its release.
While the court case is proceeding, GSK and Tesaro are still required to honor milestone and royalty payment obligations on Jemperli, Anaptys noted.
GSK has previously estimated that Jemperli sales could reach $1.4 billion to $2.8 billion at peak. In 2025’s third quarter, the drug brought home (PDF) sales of 230 million pounds sterling (around $301 million), growing nearly 80% at constant currencies over the sum it generated during the period in 2024.
Meanwhile, Anaptys in late September unveiled a plan to split into two separate, publicly traded companies—one focused on the biopharma's clinical-stage pipeline and the other concerned exclusively with managing its royalty-based agreements, including that for Jemperli.
The split is expected to be finalized by the end of 2026, Anaptys said earlier this year.