IP Law Daily, PATENT—Fed. Cir.: Seagen’s cancer antibody patent claims invalidated; Daiichi and AstraZeneca escape $41M verdict, (Dec 2, 2025)

By Saurabh Kashyap, B.A., LL.B., LL.M.

Federal Circuit finds asserted claims invalid for lack of written description and enablement under 35 U.S.C. § 112; related PTAB appeal dismissed as moot.

In a precedential disposition, the U.S. Court of Appeals for the Federal Circuit reversed a Texas district court’s decision in favor of biotechnology company Seagen Inc., invalidating multiple claims of its antibody-drug conjugate (ADC) patent. The appellate court held that the asserted claims lacked adequate written description and enablement, thus reversing the district court’s denial of judgment as a matter of law and vacating a $41 million jury verdict against Daiichi Sankyo Company, Ltd. and AstraZeneca Pharmaceuticals LP. A related appeal from a Patent Trial and Appeal Board (PTAB) decision was dismissed as moot due to the patent’s invalidity (Seagen Inc. v. Daiichi Sankyo Co., Ltd., Nos. 23-2424, 24-1176 (Fed. Cir. Dec. 2, 2025); Seagen Inc. v. Daiichi Sankyo, Inc., No. 24-1878 (Fed. Cir. Dec. 2, 2025)).

Background. The plaintiff/appellee, Seagen Inc., is a biotechnology firm focused on antibody-drug conjugates (ADCs) for cancer therapy. The defendants/appellants, Daiichi Sankyo Company, Ltd. and AstraZeneca Pharmaceuticals LP, are global pharmaceutical entities engaged in the research, development, and commercialization of oncology drugs. Daiichi developed the ADC known as Enhertu® (DS-8201), with AstraZeneca later joining as a commercial partner. Enhertu has received approval for various cancer indications and incorporates a novel linker that served as the basis for Seagen's infringement claim.

Seagen’s asserted U.S. Patent No. 10,808,039 (the ’039 patent) was filed in July 2019. It claimed priority to a 2004 application and disclosed a specific class of ADCs characterized by a tetrapeptide linker composed exclusively of glycine and phenylalanine residues ("Gly/Phe-only tetrapeptides"). The patent claims also require that the drug moiety be cleaved intracellularly from the antibody upon administration to a patient. The asserted claims include: 1–5, 9, and 10.

Seagen filed suit in October 2020 in the Eastern District of Texas, asserting that Enhertu infringed the '039 patent. At trial, the jury found in Seagen's favor, holding the claims not invalid and awarding damages exceeding $41 million, with an 8% royalty rate on future sales. The jury also found willful infringement. The district court denied Daiichi's motion for judgment as a matter of law (JMOL) on validity and damages. Concurrently, Daiichi and AstraZeneca challenged the patent’s validity in a post-grant review (PGR) before the PTAB, which ultimately found the claims unpatentable. Seagen appealed the PTAB ruling, resulting in a companion case.

Written description. In a precedential opinion, the Federal Circuit held that the ’039 patent failed to satisfy the written description requirement under 35 U.S.C. § 112(a). Relying on Ariad Pharms., Inc. v. Eli Lilly & Co., 598 F.3d 1336 (Fed. Cir. 2010) (en banc), the court emphasized that the 2004 application disclosed a broad genus encompassing over 47 million tetrapeptide possibilities but provided no specific guidance, or “blaze marks,” toward the claimed 81-member subgenus of Gly/Phe-only tetrapeptides. Citing In re Ruschig, 379 F.2d 990 (CCPA 1967), the court explained that possession of the claimed subgenus could not be inferred from a generic disclosure that merely included glycine and phenylalanine among dozens of other amino acids.

The court noted that none of the examples in the 2004 application featured Gly/Phe-only tetrapeptides, and no skilled artisan would have been led to that narrow group based on the specification alone. This conclusion was reinforced by testimony from the inventors, who admitted they had not contemplated Gly/Phe-only tetrapeptides before the 2015 public disclosure of Enhertu. The court emphasized that the written description must demonstrate actual possession of the invention as of the filing date and cannot be reconstructed through hindsight.

Enablement. The appellate court also concluded that the patent claims were invalid for lack of enablement. Relying on Amgen Inc. v. Sanofi, 598 U.S. 594 (2023), the court found that the ’039 patent failed to teach a skilled artisan how to make and use the full scope of the claimed invention without undue experimentation. The claims, as construed, covered any ADC with a drug moiety that is intracellularly cleaved from the antibody—a functional limitation that required extensive trial-and-error testing to determine if a given drug would behave as required.

In rejecting Seagen's enablement arguments, the court analogized to Amgen, in which the Supreme Court invalidated functionally defined genus claims because they relied on trial-and-error experimentation across an unpredictable field. The '039 patent, according to the panel, similarly lacked any generalizable principle or "quality common to every functional embodiment," rendering the scope of experimentation unbounded and undue under § 112.

Expert testimony and blaze marks. Seagen’s expert, Nobel laureate Dr. Carolyn Bertozzi, testified that a skilled artisan could make substitutions and infer from disclosed examples how to arrive at the claimed Gly/Phe-only tetrapeptides. However, the court found this testimony to be self-defeating. If a “straightforward leap” was needed to arrive at the claimed invention, it indicated that the disclosure did not already teach it. Citing Lockwood v. American Airlines, Inc., 107 F.3d 1565 (Fed. Cir. 1997), and Regents of the Univ. of Minn. v. Gilead Scis., Inc., 61 F.4th 1350 (Fed. Cir. 2023), the court reiterated that the written description cannot rest on speculation or a trail of breadcrumbs requiring creative reconstruction by skilled artisans.

Dismissal of PTAB appeal. In a separate, nonprecedential decision issued concurrently, the Federal Circuit dismissed Seagen’s appeal from the PTAB’s PGR decision as moot. At oral argument, Seagen conceded that if the Federal Circuit invalidated the ’039 patent in the companion appeal, the PGR appeal would be moot. Citing XY, LLC v. Trans Ova Genetics, 890 F.3d 1282 (Fed. Cir. 2018), the court reiterated that an invalidity finding renders all co-pending challenges to the same patent moot.

The Cases are Nos. 23-2424 and 24-1176, and No. 24-1878.

Judge: Lourie, A.

Attorneys: Sara Tonnies Horton (Willkie Farr & Gallagher LLP) for Seagen Inc. Christopher Neil Sipes (Covington & Burling LLP) for Daiichi Sankyo Co., Ltd., Astrazeneca Pharmaceuticals LP and Astrazeneca UK Ltd. Shehla Wynne, Office of the Solicitor, USPTO, for John A. Squires, Under Secretary of Commerce and Director of the USPTO.

Companies: Seagen Inc.; Daiichi Sankyo Co., Ltd.; Astrazeneca Pharmaceuticals LP; Astrazeneca UK Ltd.

Cases: Patent FedCirNews TexasNews USPTO