AnaptysBio shares tumble after legal fight with GSK over cancer drug license

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(Reuters) -Shares of AnaptysBio fell 14% in premarket trading on Friday after the U.S. drug developer and Tesaro, a unit of Britain’s GSK, filed lawsuits against each other, with both claiming a breach in a partnership deal for a cancer drug.

The dispute centers on a 2014 agreement between AnaptysBio and Tesaro, GSK’s oncology unit, which gave Tesaro rights to develop and sell dostarlimab, marketed as Jemperli.

Tesaro filed a lawsuit against AnaptysBio in a Delaware court on Thursday, accusing it of breaching the agreement and claiming the alleged breach allowed it to terminate the deal, halve royalty and milestone payments and secure a permanent license for the drug.

A day later, AnaptysBio filed its own complaint, accusing Tesaro of violating exclusivity terms by taking part in trials with rival PD-1 drugs, including Merck’s Keytruda, and failing to maximize Jemperli’s commercial potential.

It also alleged GSK interfered with the agreement by favoring its antibody-drug conjugate programs over Jemperli.

Both companies have asked the Delaware Chancery Court for an expedited schedule, with trial expected in July 2026. Royalty and milestone payments to AnaptysBio will continue during the proceedings.

In 2020, AnaptysBio sued GSK over similar exclusivity concerns, which resulted in a settlement including higher royalty rates.

Under the current agreement, AnaptysBio receives royalties ranging from 8% to 25% of Jemperli sales, depending on revenue levels. The company expects to receive a one-time payment of $75 million once Jemperli reaches $1 billion in worldwide sales, which is anticipated in the fourth quarter of 2025.

Jemperli is approved in more than 35 countries for certain endometrial cancers and is being tested in other tumor types.

(Reporting by Kamal Choudhury in Bengaluru; Editing by Anil D’Silva)