--- title: "AnaptysBio awaits post-trial argument in Delaware suit against GSK and Tesaro" url: "https://aibio.space/news/anaptysbio-awaits-post-trial-argument-delaware-suit-against-gsk-tesaro/" publisher: "aibio.space (Biotech Investor Relations News)" date: "2026-09-21" section: "Legal" tickers: "NASDAQ:ANAB" source: "https://www.sec.gov/Archives/edgar/data/1370053/000119312526396696/anab-20260630.htm" summary: "AnaptysBio sued TESARO, Inc. and GSK on Nov. 20, 2025 over the dostarlimab collaboration. Trial ended July 17, 2026, and post-trial argument is set for Oct" --- By the aibio.space news desk. Sources - [AnaptysBio 10-KT for the transition period ended June 30, 2026 (SEC EDGAR)](https://www.sec.gov/Archives/edgar/data/1370053/000119312526396696/anab-20260630.htm) - [AnaptysBio 8-K filed Sept. 21, 2026, Exhibit 99.1 (SEC EDGAR)](https://www.sec.gov/Archives/edgar/data/1370053/000119312526396690/anab-ex99_1.htm) [ANAB](https://aibio.space/companies/anaptysbio/) 2026-09-21 # AnaptysBio awaits post-trial argument in Delaware suit against GSK and Tesaro Tickers [ANAB](https://aibio.space/companies/anaptysbio/) AnaptysBio sued TESARO, Inc. and GSK on Nov. 20, 2025 over the dostarlimab collaboration. Trial ended July 17, 2026, and post-trial argument is set for Oct. 20. Tesaro's claim that AnaptysBio breached its duties was dismissed April 24. ## Key facts - **Company**: AnaptysBio, Inc. (NASDAQ: ANAB) - **News type**: Legal - **Primary source**: [AnaptysBio 10-KT for the transition period ended June 30, 2026 (SEC EDGAR)](https://www.sec.gov/Archives/edgar/data/1370053/000119312526396696/anab-20260630.htm) *A supreme court building with lit columns. CC0, via rawpixel. The picture shows the field, not the companies named.* AnaptysBio, Inc. (Nasdaq: [ANAB](https://aibio.space/companies/anaptysbio/)) is waiting on post-trial briefing and an Oct. 20, 2026 argument in its Delaware Chancery Court fight with TESARO, Inc. and Tesaro's corporate parent GSK over their collaboration on the cancer drug Jemperli (dostarlimab), according to its [10-KT filed Sept 21, 2026](https://www.sec.gov/Archives/edgar/data/1370053/000119312526396696/anab-20260630.htm). The company asks the court to order Tesaro to return dostarlimab rights to it. AnaptysBio filed a Verified Complaint on Nov. 20, 2025 against Tesaro and GSK. The filing gives no case number. The defendants are TESARO, Inc. and GSK. Tesaro and its affiliate Tesaro Development, Ltd. sued AnaptysBio the same day. The filing names no individual defendants. ## The case AnaptysBio asks the court to declare that Tesaro materially breached the parties' Collaboration and Exclusive License Agreement, and that GSK tortiously interfered with it. It wants a declaration that it is entitled to all rights and remedies under the agreement. The company says it had approached Tesaro to try to resolve the claims in good faith discussions, and that Tesaro filed its own suit on Nov. 20, 2025 without notice. Tesaro's complaint asked for a declaration that it has not breached the agreement and for an injunction barring AnaptysBio from terminating it. Tesaro also alleged that AnaptysBio materially breached the agreement, based on an allegation that AnaptysBio improperly accused Tesaro of breach and so caused an anticipatory breach. ## Defendants In AnaptysBio's suit the defendants are TESARO, Inc., an oncology-focused subsidiary of GSK, and GSK, which the filing calls Tesaro's corporate parent. Tesaro's separate suit names AnaptysBio as the defendant. The filing uses "GSK" to refer to Tesaro and GSK together and does not give GSK's full legal name. ## Where it stands The Court dismissed Tesaro's claim that AnaptysBio materially breached its duties on April 24, 2026. Trial on the remaining claims was held July 14 to 17, 2026. On July 28, AnaptysBio, Tesaro and GSK submitted a joint stipulation setting the post-trial schedule, which the court granted. AnaptysBio filed its opening post-trial brief on Aug. 21, 2026. Tesaro and GSK were to file their answering brief on or before Sept. 25, 2026, and AnaptysBio will reply on or before Oct. 9, 2026. Argument is set for Oct. 20, 2026. In its brief, AnaptysBio asked the court to enforce the agreement's reversion provision and order Tesaro to return dostarlimab rights. In a results release furnished with an [8-K the same day](https://www.sec.gov/Archives/edgar/data/1370053/000119312526396690/anab-ex99_1.htm), the company said it anticipates a judgment in the fourth quarter of 2026 or the first quarter of 2027. ## Why it matters to shareholders AnaptysBio says that if the litigation ends badly it may have a materially negative effect on its rights under the GSK agreement and its financial position. While the case is pending, it says, ownership of related intellectual property rights may be uncertain, and management is spending significant time and resources. The agreement is the source of AnaptysBio's Jemperli royalties. Under an amendment, it receives 8% of net sales below $1.0 billion, rising in steps to 25% above $2.5 billion. It recognized $52.8 million in royalty revenue from GSK's sales of Jemperli and Zejula in the six months ended June 30, 2026, $51.2 million of it non-cash Jemperli revenue tied to its royalty monetization agreement with Sagard. The 10-KT gives no estimate of loss from the litigation. The release says general and administrative expenses were $23.4 million for the six months ended June 30, 2026, against $8.3 million a year earlier, mainly because of legal costs for the company's separation and the GSK and Tesaro lawsuit and non-cash stock compensation. AnaptysBio describes its business as managing its financial collaborations for Jemperli with GSK and Quimilza with Vanda Pharmaceuticals Inc., with a focus on protecting and returning the value of its royalties to shareholders. For the six months ended June 30, 2026, GSK reported $644.1 million in Jemperli sales. [Report an error](https://aibio.space/reputation/#correction) ## Companies in this story ANAB AnaptysBio, Inc. NASDAQ ยท United States Open the starter desk ## More in Legal Legal ### [Alnylam details Texas patent suit, two Cipla patent cases and a U.S. Attorney subpoena](https://aibio.space/news/alnylam-details-texas-patent-suit-two-cipla-patent-cases-u-s-attorney/) Oct 29, 2026 Legal ### [Viking and Ligand dispute VK2809 license termination in San Diego court](https://aibio.space/news/viking-ligand-dispute-vk2809-license-termination-san-diego-court/) Oct 29, 2026 Legal ### [Viking's ITC trade secret ruling against Ascletis is on appeal at Federal Circuit](https://aibio.space/news/vikings-itc-trade-secret-ruling-against-ascletis-appeal-federal/) Oct 29, 2026 --- Source page: https://aibio.space/news/anaptysbio-awaits-post-trial-argument-delaware-suit-against-gsk-tesaro/. Facts in aibio.space stories are checked against company filings and releases; opinion is labelled. Cite as: aibio.space, "AnaptysBio awaits post-trial argument in Delaware suit against GSK and Tesaro", 2026-09-21, https://aibio.space/news/anaptysbio-awaits-post-trial-argument-delaware-suit-against-gsk-tesaro/