When GSK plc (GSK) dropped $5.1 billion to acquire Tesaro back in December 2018, they were betting big on cancer treatments. Now, that bet has landed them in Delaware Chancery Court in a dispute that could reshape the economics of Jemperli, a cancer drug currently approved in over 35 countries.

Tesaro Inc., now operating as a GSK subsidiary, has filed litigation against AnaptysBio, Inc. (ANAB), alleging that the biotech company has materially breached their licensing agreement for dostarlimab, the drug marketed as Jemperli. This isn't just corporate saber-rattling. The stakes are enormous: Tesaro claims the alleged breaches entitle it to terminate the existing license, secure a perpetual and irrevocable license to dostarlimab, and slash the royalties and milestone payments owed to AnaptysBio by a whopping 50%.

The Original Deal and What's at Stake

To understand why this matters, you need to know what the companies agreed to back in March 2014. Tesaro paid AnaptysBio an upfront license fee of $17 million, plus funding for development costs. For each development program, AnaptysBio stood to collect milestone payments totaling $18 million, plus an additional $90 million tied to U.S. and international regulatory submissions and approvals across multiple indications.

In December 2014, they doubled down, expanding their collaboration to include development of a bispecific antibody targeting two undisclosed immune checkpoints. Fast forward to today, and Jemperli has become a meaningful product, approved for certain endometrial cancers, which happens to be the most common gynecologic cancer in the United States. Clinical trials are ongoing to evaluate the drug's potential in rectal, colon, and head and neck cancers.

Here's where things get messy. AnaptysBio is expecting a one-time $75 million commercial sales milestone payment once Jemperli crosses $1 billion in worldwide net sales. The company was planning to end 2025 with roughly $300 million in cash, cash equivalents, and investments, including that anticipated milestone accrual in the fourth quarter. Now? That payment is very much in question.

Who Started This Fight?

The lawsuit comes after AnaptysBio fired the first shot, alleging that Tesaro failed to fulfill certain requirements of the 2014 license agreement and threatening to revoke Tesaro's license for dostarlimab entirely. Tesaro's response was to take the fight to court, arguing that AnaptysBio's recent conduct constitutes a material breach.

GSK and Tesaro are adamant that AnaptysBio's allegations are "entirely without merit." But when you're facing a potential license revocation for a drug that's approaching $1 billion in sales, you don't wait around to see what happens. You sue.

AnaptysBio's Tough November

This lawsuit caps off a difficult month for AnaptysBio. Earlier in November, the company announced that its investigational drug rosnilimab was safe and well-tolerated but failed to meet the primary endpoint for treating ulcerative colitis. The drug didn't achieve the mean change from baseline in the modified Mayo Score or hit key secondary endpoints of clinical response and clinical remission at Week 12. AnaptysBio decided to discontinue the trial, though they're saving at least $10 million in the process.

Perhaps sensing turbulence ahead, AnaptysBio also authorized an amended stock repurchase plan of up to $100 million. That's a pretty classic move when you're expecting your stock to take a hit—and it did. Shares dropped 12.37% to $33.00 in premarket trading following the litigation news.

What Happens Next?

The case will play out in Delaware Chancery Court, which handles these kinds of corporate disputes with some regularity. If Tesaro wins, the financial implications for AnaptysBio are severe: losing half of all future royalties and milestones, plus seeing Tesaro walk away with permanent rights to a drug that could become a blockbuster across multiple cancer indications.

If AnaptysBio prevails and somehow manages to revoke the license, well, that creates a whole different set of problems for everyone involved. Either way, this is going to be expensive, contentious, and closely watched by anyone who cares about licensing deals in biotech.