Seagen vs. Daiichi Sankyo: CAFC Opinion

Seagen vs. Daiichi Sankyo marks a major setback for broad biologics genus claims as the Federal Circuit overturned Seagen’s $41.8M jury verdict, invalidating US10808039B2 for lack of written description and enablement under the post Amgen v. Sanofi standard.

Litigation
UPDATED: December 4, 2025

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The Federal Circuit’s Dec. 2, 2025 opinion in Seagen vs. Daiichi Sankyo is a major warning shot for biologics patentees relying on broad genus claims in fast-moving biologics fields like antibody-drug conjugates (ADCs). The court reversed a $41.8 million Texas jury verdict, holding Seagen’s US10808039B2 invalid for lack of written description and lack of enablement, and wiping out findings of willful infringement against Daiichi and AstraZeneca over blockbuster cancer drug Enhertu (Trastuzumab Deruxtecan).

Seagen vs. Daiichi Sankyo – Case Background:

Plaintiff-Appellee: Seagen Inc
Defendants-Appellants: Daiichi Sankyo Company, Ltd., Astrazeneca Pharmaceuticals LP, Astrazeneca UK Ltd.,
Appeal Number: 2023-2424, 2024-1176
Decide on: Dec. 02, 2025
Opinion Link: Click Here
Appeals from the United States District Court for the Eastern District of Texas in No. 2:20-cv-00337-JRG, Judge J. Rodney Gilstrap
Patent Number & Claims: US10808039B2 (Claims 1 – 5, 9, and 10)
Drug: Enhertu (Trastuzumab Deruxtecan)

The dispute centered on ADC technology, which links a targeting antibody to a cytotoxic payload via a linker designed to release the drug inside cancer cells while sparing healthy tissue. Seagen’s US’039 patent claimed ADCs featuring a specific linker architecture, in which the peptide unit (“Ww”) is a tetrapeptide made only of glycine and phenylalanine, and the drug moiety is intracellularly cleaved in the patient.

Seagen sued Daiichi and later AstraZeneca in the Eastern District of Texas shortly after the US’039 patent issued, alleging that Enhertu’s Gly Gly Phe Gly linker sequence infringed and securing a jury verdict of no invalidity, willful infringement, a lump-sum royalty of over $41 million and an 8% running royalty. In parallel, Daiichi and AstraZeneca pursued post grant review before the PTAB, which also found the asserted claims unpatentable; Seagen appealed that Board loss as a companion case.

Seagen vs. Daiichi Sankyo – Genus vs. Subgenus:

The crux of the appeal was whether the US’039 patent could claim priority back to a 2004 Seagen application that described generic ADC linkers but never expressly disclosed a “Gly/Phe only” tetrapeptide. The 2004 filing broadly allowed peptide units ranging from di to dodecapeptides and listed 39 amino acids (83 when stereoisomers were counted) as possible building blocks producing over 47 million possible tetrapeptide species.

By contrast, the US’039 patent’s asserted claims zeroed in on a much narrower subgenus of 81 tetrapeptides made only from glycine and phenylalanine, which conveniently matched Enhertu’s publicly disclosed linker architecture first seen in 2015. The Federal Circuit emphasized that merely “encompassing” a tiny subgenus within a gigantic disclosed genus is not enough; the earlier specification must contain “reasonably specific” disclosure or “blaze marks” that direct the skilled person to that particular tree in the forest.

Seagen vs. Daiichi Sankyo – Written description: no blaze marks to Gly/Phe-only linkers:

On written description, the court held that no reasonable jury could find the 2004 application showed possession of the 81-member Gly/Phe-only tetrapeptide subgenus. Both sides agreed the earlier application covered tens of millions of tetrapeptides, and the claimed group was just an infinitesimal fraction of that space; nothing in the text singled out “Gly/Phe-only” units or even hinted at a genus limited to just those two amino acids.

The court contrasted this disclosure with older CCPA authority like In re Driscoll, where a small, listed set of alternatives made the later-claimed subgenus “clearly discernible,” and instead likened Seagen’s position to In re Ruschig, where a single species buried in a massive combinatorial disclosure was held not described. That conclusion was reinforced by inventor testimony: Seagen’s own scientists admitted they had not conceived of an ADC with a Gly/Phe-only tetrapeptide in 2004 and first saw such a linker in Daiichi’s Enhertu a decade later, undercutting any claim of possession at the earlier priority date.

Seagen vs. Daiichi Sankyo – Enablement: Amgen v. Sanofi applied to ADCs:

Even assuming priority, the Federal Circuit independently found the claims invalid for lack of enablement, using the Supreme Court’s 2023 Amgen v. Sanofi decision as a template. The district court had construed “D is a drug moiety” to cover any drug, and the claims required that the drug moiety be “intracellularly cleaved” in a patient two broad functional limitations that effectively swept in vast numbers of possible ADCs across drug classes.

The record showed that ADC behavior is highly unpredictable, and Seagen’s own witnesses agreed that whether a given ADC would be successfully cleaved intracellularly could only be determined by running assays. Without disclosing a unifying quality shared by all functional embodiments, the patent essentially instructed skilled artisans to engage in open-ended trial-and-error screening precisely the kind of undue experimentation Amgen condemned for functionally defined genuses.

Seagen vs. Daiichi Sankyo – Key Takeaways:

The Federal Circuit reversed the denial of judgment as a matter of law, held all asserted claims of the US’039 patent invalid for lack of written description and enablement, and vacated the willfulness and damages verdict; it also dismissed Seagen’s PTAB appeal as moot in a companion decision.

Seagen vs. Daiichi Sankyo – Key Takeaways:

Enormous combinatorial disclosures will not rescue later, litigation driven narrowing claims unless the original filing contains clear blaze marks pointing to the specific subgenus.
Functional genus claims (like “any drug moiety that is intracellularly cleaved”) face intense enablement scrutiny after Amgen, especially in unpredictable sciences where each candidate must be empirically tested.
Priority strategy now demands early, concrete disclosure of the precise structural and functional features that may later prove commercially important, rather than relying on broad generic formulae to catch future competitor products.

Citation:

CAFC Opinion – 2023-2424, 2024-1176

Supreme Court’s 2023 Amgen v. Sanofi decision

Disclaimer:

This article is published for general educational and informational purposes only and does not constitute legal, patent, financial, or professional advice. The content reflects publicly available case information and judicial opinions as of the date of publication. Readers should not act or rely on this information without seeking independent professional advice. The author and publisher disclaim any liability for decisions taken based on this content.