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

AnaptysBio

21/11/2025 16:06

Updates share movement, adds analyst comment in paragraph 7-8

By Kamal  Choudhury

Nov 21 (Reuters) - Shares of AnaptysBio ANAB.O fell more than 5% on Friday after the U.S. drug developer and GSK's GSK.L oncology unit, Tesaro, filed lawsuits against each other, with both claiming a breach of a partnership deal for a cancer drug.

The dispute centers on a 2014 agreement between AnaptysBio and Tesaro that gave the latter rights to develop and sell Jemperli, a PD-1 inhibitor approved for certain endometrial cancers.

     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 countersued, accusing Tesaro of violating exclusivity terms by taking part in trials with rival PD-1 drugs, including Merck's MRK.N 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.

U.S.-listed shares of GSK were up 2%.

H.C. Wainwright analyst Emily Bodnar said a win could lead to higher royalties for AnaptysBio or restrictions on GSK from using other PD-1 drugs in trials, boosting Jemperli's prospects.

A loss, however, could strain AnaptysBio's finances and favor GSK in any revised agreement, Bodnar added.

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

AnaptysBio expects a $75 million milestone payment once global sales hit $1 billion, anticipated in late 2025.

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

 ((Kamal.Choudhury@thomsonreuters.com;))