AnaptysBio Fires Back at GSK in Billion-Dollar Cancer Drug Dispute

SAN DIEGO, CA – January 08, 2026 – The legal conflict between AnaptysBio and pharmaceutical giant GSK has intensified, as the San Diego-based biotech company filed a motion to dismiss a key claim in the ongoing litigation over its lucrative royalty agreement for the cancer drug Jemperli. The move, unsealed today, represents the latest salvo in a high-stakes battle that could determine the flow of billions of dollars in future revenue and impact the strategic direction of both companies.

At the heart of the dispute is a 2014 Collaboration and Exclusive License Agreement between Anaptys and Tesaro, a GSK subsidiary. AnaptysBio, which discovered the antibody that became Jemperli (dostarlimab), is entitled to a significant share of the drug's sales. However, the partnership soured, culminating in a flurry of legal actions in the Delaware Chancery Court, with a full trial scheduled for July 2026.

Anaptys has now formally asked the court to throw out Tesaro’s claim of “anticipatory breach of contract,” arguing the claim is a strategic lawsuit intended to intimidate Anaptys from exercising its contractual rights. This legal showdown puts a spotlight on the often-fraught relationships between small biotech innovators and the large pharmaceutical partners that bring their discoveries to market.

A Partnership Soured Over a Blockbuster Drug

The conflict erupted into public view on November 20, 2025, when Tesaro filed a lawsuit against AnaptysBio. Tesaro alleged that Anaptys had repudiated their agreement and sought a court declaration that it had not breached the contract. The lawsuit was filed without notice while the two parties were purportedly in discussions to resolve their differences.

Anaptys responded with its own complaint, alleging that it was Tesaro that materially breached the agreement and that its corporate parent, GSK, tortiously interfered with the contract. Anaptys claims that Tesaro and GSK failed to uphold exclusivity provisions by testing other GSK experimental drugs with Jemperli’s direct competitors, such as Merck’s Keytruda. Furthermore, Anaptys alleges that its partners failed to use the necessary effort to “obtain the optimum commercial return” for Jemperli on a global scale.

This is not the first time the partners have clashed. A previous dispute in 2020, also related to exclusivity concerns, was settled with GSK making a cash payment, granting Anaptys a royalty on another drug, Zejula, and significantly increasing the royalty rates for Jemperli—the very rates now at the center of this new conflict.

Billions on the Line: The Financial Stakes

The financial stakes of this legal war are immense. Jemperli has become a cornerstone of GSK’s oncology portfolio, with sales demonstrating explosive growth. In the first nine months of 2025, the drug generated approximately $785 million (£600 million), putting it on a trajectory to surpass the $1 billion annual sales mark. Analyst consensus forecasts project Jemperli’s global sales could reach $2.56 billion by 2030.

Under the terms of the agreement, Anaptys is entitled to a tiered royalty structure that becomes increasingly lucrative as sales grow:

• 8% on net sales below $1.0 billion
• 12% on net sales between $1.0 billion and $1.5 billion
• 20% on net sales between $1.5 billion and $2.5 billion
• 25% on net sales above $2.5 billion

This royalty stream, which extends through patent expirations in 2035 in the U.S. and 2036 in Europe, represents a foundational financial asset for Anaptys. In fact, the company has already monetized a portion of this future income. A deal with Sagard Healthcare Royalty Partners entitles Sagard to the first $600 million in royalties. Anaptys estimates that Sagard will have collected $250 million of that by the end of 2025, with the full amount expected to be repaid between mid-2027 and the second quarter of 2028. Tesaro's lawsuit, which seeks to cut the royalty payments in half, directly threatens this entire financial structure.

A High-Stakes Legal Gambit in Delaware

AnaptysBio’s January 8th motion to dismiss is a bold legal maneuver. The company argues that Tesaro’s claim of anticipatory breach is baseless because Anaptys was simply seeking to “vindicate its contract rights,” which cannot legally be considered a repudiation of the contract.

Crucially, Anaptys invokes Delaware’s anti-SLAPP (Strategic Lawsuit Against Public Participation) law. The company contends that Tesaro's lawsuit is a classic SLAPP suit, designed to deter Anaptys from pursuing its legitimate claims through costly and intimidating litigation. While the anti-SLAPP statute is intended to prevent such tactics, its application in a complex commercial contract dispute, rather than its more typical use in land-use cases, could set a new precedent in the influential Delaware court.

Adding another layer of contention, Tesaro and GSK argue that Anaptys’s motion should halt all pre-trial discovery, a position Anaptys “strongly opposes” as the July 2026 trial date approaches. The court is expected to hear arguments on the motion to dismiss by early March, a hearing that will be closely watched for early indications of the court’s direction.

A Corporate Split Hangs in the Balance

The litigation casts a long shadow over AnaptysBio's most significant strategic initiative: a plan to separate its business into two distinct public companies by the end of 2026. The proposed split would create a “Biopharma Co” focused on developing its pipeline of immunology therapeutics and a “Royalty Management Co” designed to manage and maximize the value of its royalty assets, with the Jemperli stream as its crown jewel.

The viability and valuation of the Royalty Management Co. are almost entirely dependent on the integrity of the Jemperli royalty agreement. An adverse legal outcome, such as the 50% reduction in royalties sought by Tesaro, would severely damage the financial foundation of the new entity and could derail the entire separation plan. The uncertainty created by the lawsuit diverts management attention and resources and presents a major obstacle to unlocking the shareholder value the separation was intended to create.

With a trial date set for mid-2026 and a critical hearing on the motion to dismiss expected by March, the battle for Jemperli’s billion-dollar future has officially entered a decisive new phase.