London, April 27, 2026, 14:01 BST
Tesaro, GSK plc’s U.S. oncology arm, saw a setback in its Delaware case over Jemperli—part of its claim against AnaptysBio was tossed. The court dismissed Tesaro’s anticipatory breach argument. GSK, though, noted that the main dispute over the contract is still in play, and Tesaro’s request for declaratory judgment remains.
Timing’s key here. GSK is set to deliver first-quarter 2026 numbers on Wednesday, April 29, with investors zeroed in on whether oncology will keep doing the heavy lifting as older segments of the portfolio come under strain.
Back in February, GSK reported a 43% jump in oncology sales for 2025, hitting £2.0 billion as the drugmaker leaned harder into its specialty medicines strategy. Among five key late-stage trial results slated for 2026, the company flagged a Jemperli readout in rectal cancer. These pivotal studies typically underpin approval applications.
Jemperli—dostarlimab—functions as a PD-1 inhibitor, a type of cancer immunotherapy designed to help the immune system find and target tumor cells. AnaptysBio reported Jemperli pulled in $1.128 billion (861 million pounds) in sales for 2025, pointing to an annual run rate near $1.4 billion during the fourth quarter.
The conflict dates to a 2014 deal granting Tesaro the rights to develop and market Jemperli. Back in January, Reuters reported that Tesaro claimed AnaptysBio had breached the agreement and pushed to cut both royalty and milestone payments by half. AnaptysBio fired back, alleging Tesaro broke exclusivity by joining trials that involved competing PD-1 therapies, among them Merck’s Keytruda.
The Delaware Court of Chancery found that AnaptysBio’s October letter wasn’t repudiation, pointing out it was linked to the contract’s dispute-resolution procedure rather than being an outright refusal to perform. The court also determined that Tesaro’s anticipatory breach allegation didn’t state a valid claim.
AnaptysBio called the ruling a win for its royalties, emphasizing that it keeps existing contracted rates intact and denies Tesaro’s push for lower payments. “The Court’s decision affirms what we have maintained from the beginning,” said CEO Dan Faga, who noted the company is now gearing up for a July trial. AnaptysBio, Inc.
The tougher question hasn’t gone away. July’s trial stands to shake up the economics around Jemperli, and could determine if AnaptysBio gets a shot at reversion rights. Back in November, H.C. Wainwright’s Emily Bodnar pointed out that a win for AnaptysBio might translate into juicier royalties or even limits on GSK running other PD-1 drugs through trials. A loss, though, would ratchet up pressure on AnaptysBio and put GSK in a stronger position if the terms end up back on the table.
The takeaway for GSK: this latest order trims the case, though it’s not a clean slate. GSK and Tesaro maintain they see no merit in AnaptysBio’s allegations, and say they’re concentrating on challenging the last claim at trial.
It’s a messy backdrop for the stock. GSK finished Friday at 2,020 pence, down 2.7%, trailing the FTSE 100, which was also in the red, ahead of Monday’s litigation development.
So, two key dates on the calendar for investors: GSK reports first-quarter results April 29, then comes the Delaware trial set for July 14-17. April’s update should reveal if Jemperli’s momentum continues to deliver. As for July, that’s when we’ll see what GSK gets to hold onto—and what it might have to give up.
