Daiichi Sankyo & AstraZeneca v. Seagen: ENHERTU® ADC Patent DJ Voluntarily Dismissed
Daiichi Sankyo and AstraZeneca filed a declaratory judgment action in Delaware against Seagen's U.S. Patent No. 10,808,039, asserting non-infringement over ENHERTU® (fam-trastuzumab deruxtecan). After a stay in favour of parallel Eastern District of Texas litigation, the Federal Circuit invalidated the '039 patent in December 2025 — prompting plaintiffs to dismiss all claims across 1,971 days of proceedings.
ENHERTU® ADC declaratory judgment: five years, a stay, and a Federal Circuit invalidation
On 13 November 2020, Daiichi Sankyo, Inc., Daiichi Sankyo Company, Limited, and AstraZeneca Pharmaceuticals LP filed suit in the U.S. District Court for the District of Delaware (Case No. 1:20-cv-01524) against Seagen, Inc., seeking a declaratory judgment of non-infringement of U.S. Patent No. 10,808,039. The '039 patent is held by Seagen and relates to antibody-drug conjugate technology. The product at the centre of the dispute is ENHERTU® (fam-trastuzumab deruxtecan, also marketed as fam-trastuzumab deruxtecan-nxki), an FDA-approved ADC indicated for adult patients with certain breast cancers.
Seagen did not answer the complaint or move for summary judgment. Instead, it moved to stay the Delaware action in favour of parallel litigation pending in the Eastern District of Texas concerning the same '039 patent. The Delaware court granted the stay on 28 April 2021. The case remained stayed until December 2025, when the U.S. Court of Appeals for the Federal Circuit reversed the Eastern District of Texas's rulings on invalidity and found the '039 patent invalid on written description and enablement grounds (Seagen Inc. v. Daiichi Sankyo Company Limited, Fed. Cir. Nos. 2023-2424, 2024-1176, Dec. 2, 2025). Seagen did not seek rehearing or petition for certiorari, and the deadline to do so passed. Plaintiffs then filed a Rule 41(a)(1)(A)(i) notice dismissing all claims in the Delaware case. The recorded basis of termination is voluntary dismissal; the public record does not specify whether the dismissal was with or without prejudice.
The five-year duration of this case is almost entirely attributable to the stay, not active merits litigation in Delaware — a pattern consistent with defendants using parallel proceedings to shift the dispositive battleground. The ultimate resolution was driven entirely by the Federal Circuit's invalidity ruling in the Texas-originated appeal rather than by anything decided in Delaware. What drove the parties to choose voluntary dismissal rather than a stipulated consent judgment, and whether any collateral terms were agreed, is not disclosed in the available record.
See Complete Case & Patent Analysis →Filing to Voluntary dismissal in 1971 days
1,971 days — over five years from filing to voluntary dismissal, driven largely by a court-ordered stay
US10808039B2 — antibody-drug conjugate technology underlying ENHERTU®
The '039 patent has been found invalid by the Federal Circuit on written description and enablement grounds, and Seagen did not pursue further appeal. For companies developing or commercialising antibody-drug conjugates — particularly HER2-targeting ADCs — this removes a specific enforcement risk. However, Seagen holds additional ADC-related patents that are not addressed by this record. Any company with ADC products in development or on the market should assess whether related patents in Seagen's portfolio share the same specification language or claim structures, as those may carry analogous validity risks or may be independently enforceable.
Official order — verbatim text
The Rule 41(a)(1)(A)(i) notice recites the procedural history — the stay, the Federal Circuit's December 2025 invalidity ruling in the parallel Texas appeal, and Seagen's decision not to seek further review — as the factual backdrop for the dismissal. The notice does not, on its face, specify whether the dismissal is with or without prejudice, and the available record does not resolve that question. The operative legal event disposing of the '039 patent's validity was the Federal Circuit ruling, not anything adjudicated in this Delaware proceeding.
Voluntarily dismissed: what the Rule 41 notice means for both parties
Rule 41(a)(1)(A)(i): a plaintiff-initiated exit before answer or summary judgment
Under Federal Rule of Civil Procedure 41(a)(1)(A)(i), a plaintiff may dismiss an action without a court order by filing a notice before the opposing party serves an answer or a motion for summary judgment. That precondition was met here: Seagen never answered. The dismissal is self-executing upon filing. The public record does not specify whether the dismissal is with or without prejudice; that distinction carries significant legal consequences and is not disclosed in the available record.
Rule 41(a)(1)(A)(i) noticeThe prejudice question: the public record is silent
A dismissal with prejudice permanently bars the plaintiff from refiling the same claims; a dismissal without prejudice preserves the right to refile. The recorded basis of termination is 'Voluntary dismissal' without further specification. The verdict text equally does not state which applies. Practitioners should treat this distinction as unresolved on the public record and should not assume either outcome. The specific terms are not disclosed in the available record.
Prejudice status undisclosedSeagen's '039 patent rendered invalid by Federal Circuit — not by this court
The Delaware declaratory judgment case never reached the merits. The operative blow to Seagen's patent position came from the Federal Circuit's December 2025 ruling in the parallel Texas appeal, which found the '039 patent invalid for lack of written description and enablement. Seagen's decision not to seek rehearing or certiorari allowed that invalidity finding to become final. The Delaware dismissal is a downstream consequence of that outcome, not an independent adjudication.
Patent invalidated — Fed. Cir.ENHERTU® ADC freedom-to-operate risk from the '039 patent is extinguished
With the '039 patent invalidated by the Federal Circuit and the Delaware declaratory judgment action dismissed, the specific patent risk that Daiichi Sankyo and AstraZeneca sought to neutralise through this filing appears resolved. Companies operating in the ADC oncology space should note that the invalidity finding arose on written description and enablement grounds — grounds that may inform how broadly future ADC patent claims are drafted and challenged. Other Seagen-originated ADC patents are not addressed by this record.
ADC IP risk landscape shiftedFull party and counsel information
| Role | Name | Type | Detail |
|---|---|---|---|
| Plaintiff | Daiichi Sankyo | Individual | /Search in Eureka ↗ |
| Co-Plaintiff | Daiichi Sankyo Company, Limited | Company | Search in Eureka ↗ |
| Co-Plaintiff | Astrazeneca Pharmaceuticals, LP | Company | Search in Eureka ↗ |
| Defendant | Seagen, Inc. | Company | /Search in Eureka ↗ |
| Plaintiff counsel | Andrew Colin Mayo | Attorney | Counsel for Daiichi SankyoSearch in Eureka ↗ |
| Plaintiff counsel | Ashley N. Mays-Williams | Attorney | Counsel for Daiichi SankyoSearch in Eureka ↗ |
| Plaintiff counsel | Isaac S. Ashkenazi | Attorney | Counsel for Daiichi SankyoSearch in Eureka ↗ |
| Plaintiff counsel | Jeff A. Pade | Attorney | Counsel for Daiichi SankyoSearch in Eureka ↗ |
| Plaintiff counsel | Joseph M. O'Malley | Attorney | Counsel for Daiichi SankyoSearch in Eureka ↗ |
| Plaintiff counsel | Preston K. Ratliff , II | Attorney | Counsel for Daiichi SankyoSearch in Eureka ↗ |
| Plaintiff counsel | Steven J. Balick | Attorney | Counsel for Daiichi SankyoSearch in Eureka ↗ |
| Plaintiff law firm | Ashby & Geddes PC | Law Firm | Representing Daiichi SankyoSearch in Eureka ↗ |
| Defendant counsel | Anne Shea Gaza | Attorney | Counsel for Seagen, Inc.Search in Eureka ↗ |
| Defendant counsel | Bryan Wilson | Attorney | Counsel for Seagen, Inc.Search in Eureka ↗ |
| Defendant counsel | Michael A. Jacobs | Attorney | Counsel for Seagen, Inc.Search in Eureka ↗ |
| Defendant counsel | Pieter S. de Ganon | Attorney | Counsel for Seagen, Inc.Search in Eureka ↗ |
| Defendant counsel | Samantha G. Wilson | Attorney | Counsel for Seagen, Inc.Search in Eureka ↗ |
| Defendant law firm | Young Conaway Stargatt & Taylor, LLP | Law Firm | Representing Seagen, Inc.Search in Eureka ↗ |
| Presiding judge | Judge Gregory B. Williams | Judge | Delaware District CourtSearch in Eureka ↗ |
R&D signals in the antibody-drug conjugate patent space
Forward-looking patent and innovation intelligence derived from Daiichi Sankyo, AstraZeneca, and Seagen's ADC activity — beyond the outcome of this Delaware DJ case.
Daiichi Sankyo's ADC filing activity beyond the '039 dispute
Daiichi Sankyo has built a substantial ADC patent portfolio anchored by the deruxtecan payload-linker platform that underpins ENHERTU®. Monitoring Daiichi Sankyo's recent PCT and US continuation filings in the ADC space can reveal the next generation of conjugate architectures and payload chemistries the company is protecting — key intelligence for competitors and licensing teams.
Daiichi Sankyo ADC filingsADC patent filing trends: HER2-targeting conjugates and § 112 pressure
The Federal Circuit's written description and enablement analysis of the '039 patent reflects a broader tightening of § 112 scrutiny on functionally claimed ADC patents. Tracking filing trends in HER2-targeting ADC patents can reveal how applicants are adjusting claim strategies — narrowing to specific linker-payload combinations — in response to this heightened standard.
HER2 ADC filing trendsSeagen/Pfizer's ADC patent estate post-invalidation
Following Pfizer's acquisition of Seagen, the combined ADC portfolio represents one of the most extensive in oncology. The invalidation of the '039 patent does not extinguish related family members or continuation applications. Mapping the Seagen/Pfizer ADC patent landscape can identify which patents retain enforceability risk for companies operating in the ADC space.
Seagen/Pfizer ADC estateAdjacent ADC innovation: novel payloads and next-gen linker chemistry
The contested claim space around established HER2 ADC platforms suggests R&D opportunity in adjacent areas: novel cytotoxic payloads, cleavable linker variants, and site-specific conjugation technologies that fall outside the scope of broadly invalidated claims. Identifying white space in these sub-domains can guide patenting strategy for next-generation ADC programmes.
ADC innovation white spaceSimilar ADC patent declaratory judgment cases in Delaware and EDTX
Explore related antibody-drug conjugate patent disputes filed in Delaware District Court and the Eastern District of Texas, including parallel Seagen and Daiichi Sankyo ADC litigation.
What this case signals for the ADC oncology IP landscape
A five-year stay followed by a Federal Circuit invalidity ruling raises pointed questions about ADC patent drafting standards and declaratory judgment strategy.
Staying a DJ action transfers all dispositive risk to the parallel forum
Seagen's successful motion to stay this Delaware action meant the Delaware court never ruled on non-infringement. When the Federal Circuit later invalidated the '039 patent in the Texas appeal, Daiichi Sankyo and AstraZeneca obtained the practical outcome they sought without litigating the merits here. Companies facing DJ actions should evaluate whether a stay strategy genuinely serves their interests or simply defers — and concentrates — risk in a single appellate outcome.
Written description and enablement remain high-stakes battlegrounds for ADC patents
The Federal Circuit's invalidity ruling turned on written description and enablement — the same doctrines that have challenged broad biological patent claims in other contexts. ADC patent portfolios with broad functional claims are particularly susceptible to these challenges. R&D and IP teams drafting or evaluating ADC patents should audit claim scope against the written description standard in light of this outcome.
Sankyo v Seagen — key questions answered
The case was voluntarily dismissed by plaintiffs Daiichi Sankyo, Inc., Daiichi Sankyo Company, Limited, and AstraZeneca Pharmaceuticals LP pursuant to Federal Rule of Civil Procedure 41(a)(1)(A)(i). The dismissal followed the Federal Circuit's December 2, 2025 ruling invalidating Seagen's U.S. Patent No. 10,808,039 in parallel Texas litigation. The public record does not specify whether the dismissal was with or without prejudice.
U.S. Patent No. 10,808,039 (application no. US16/507839) is a Seagen patent relating to antibody-drug conjugate technology. Daiichi Sankyo and AstraZeneca sought a declaratory judgment of non-infringement because their FDA-approved cancer drug ENHERTU® (fam-trastuzumab deruxtecan) was alleged by Seagen to fall within the scope of the patent. The Federal Circuit ultimately found the patent invalid for lack of written description and enablement.
Seagen moved to stay the Delaware declaratory judgment action in favour of parallel patent litigation pending in the U.S. District Court for the Eastern District of Texas concerning the same '039 patent. The Delaware court granted the stay on April 28, 2021. The case remained stayed while the Texas litigation proceeded to a Federal Circuit appeal, which culminated in the '039 patent being found invalid on December 2, 2025.
The U.S. Court of Appeals for the Federal Circuit reversed the Eastern District of Texas's rulings and found U.S. Patent No. 10,808,039 invalid for lack of written description and enablement (Seagen Inc. v. Daiichi Sankyo Company Limited, Fed. Cir. Nos. 2023-2424, 2024-1176). Seagen did not seek rehearing or petition for certiorari, and the deadline to do so passed. With the patent invalidated, Daiichi Sankyo and AstraZeneca filed a Rule 41 notice voluntarily dismissing all claims in the stayed Delaware case.
ENHERTU® (fam-trastuzumab deruxtecan / fam-trastuzumab deruxtecan-nxki, also known as DS-8201) is an antibody-drug conjugate developed by Daiichi Sankyo and co-commercialised with AstraZeneca. It is FDA-approved for the treatment of adult patients with certain breast cancers. As a commercially significant ADC product, it has been the subject of multiple patent disputes, including Seagen's enforcement of ADC platform patents. The Federal Circuit's invalidation of the '039 patent removes one patent enforcement vector against ENHERTU®, though other patents in the ADC space may remain relevant.
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