AnaptysBio Inc. (ANAB) is pushing back hard against accusations that it violated a decade-old licensing deal tied to the cancer drug Jemperli. The clinical-stage biotech wants a Delaware court to dismiss what it's calling an improper lawsuit filed by Tesaro while ongoing royalty disputes were still being hashed out.

Here's the background: AnaptysBio argues it was simply exercising its contractual rights and now needs the court to settle things as litigation with Tesaro and its parent company, GSK Plc (GSK), heads toward trial.

The fight revolves around a Collaboration and Exclusive License Agreement that guarantees AnaptysBio royalties on sales of Jemperli, an immunotherapy used to treat certain solid tumors. The original deal dates back to March 2014, when AnaptysBio partnered with Tesaro, then a standalone oncology biotech before GSK acquired it.

Under the arrangement, Tesaro develops Jemperli both as a standalone treatment and in combination with other therapies. AnaptysBio receives tiered royalties ranging from 8% to 25% of net sales, with those payments continuing through U.S. patent expiration in 2035 and European Union protection through 2036. That's a potentially lucrative stream of revenue.

According to court documents, AnaptysBio previously reached out to Tesaro hoping to have good-faith discussions about what it believed were breaches of the agreement by Tesaro and GSK. But while those talks were still happening, Tesaro filed suit in November 2025 without any heads-up, claiming that AnaptysBio had repudiated the contract. Tesaro wanted a court declaration that it hadn't breached the agreement.

AnaptysBio responded by filing its own complaint in Delaware Chancery Court, asking the court to declare that Tesaro materially breached the contract and that GSK tortiously interfered with the agreement.

Things escalated in December 2025 when AnaptysBio filed a partial motion to dismiss Tesaro's anticipatory breach of contract claim. The motion, unsealed in January 2026, argued that AnaptysBio never repudiated the agreement and was merely trying to enforce its contractual rights. Under Delaware law, the company contends, asserting those rights can't possibly constitute repudiation.

AnaptysBio also leaned on Delaware's anti-SLAPP statute, framing Tesaro's lawsuit as a strategic move designed to discourage legitimate legal claims. Anti-SLAPP laws exist specifically to prevent litigation that's aimed at chilling people from exercising their legal rights in good faith.

Tesaro and GSK have countered that AnaptysBio's motion to dismiss should pause discovery. AnaptysBio disagrees, especially considering the parties are gearing up for a trial currently scheduled for July 14-17. The court was expected to hear AnaptysBio's motion by early March, consistent with the timing requirements built into Delaware's anti-SLAPP law.

In other company news, AnaptysBio discontinued a Phase 2 trial of rosnilimab for moderate-to-severe ulcerative colitis in November 2025 after it failed to meet the primary endpoint of mean change from baseline in the modified Mayo Score or the key secondary endpoints of clinical response and clinical remission at week 12.

ANAB Price Action: AnaptysBio stock was up 4.02% at $45.56 at publication on Friday.