The US Court of Appeals for the Federal Circuit reversed a district court’s denial of a motion for judgment as a matter of law (JMOL), finding that the patent specification failed to provide an adequate written description to practice the full scope of the claimed invention. Seagen Inc. v. Daiichi Sankyo Co., Ltd., Case Nos. 2023-2424; 2024-1176 (Fed. Cir. Dec. 2, 2025) (Lourie, Reyna, Chen, JJ.)
In 2020, Seagen sued Daiichi for infringing its patent covering antibody-drug-conjugates (ADCs). The patent claims priority to a 2004 application that broadly describes ADCs. The application lists 83 possible amino acids and includes more than 47 million possible tetrapeptide combinations. A Texas jury sided with Seagen, finding that Daiichi willfully infringed at least one claim, and awarded more than $41 million in damages plus an 8% running royalty. The district court denied Daiichi’s post-trial motion for JMOL and entered final judgment. Daiichi appealed.
Daiichi argued that no reasonable jury could have found that the 2004 application provided an adequate written description that would enable a skilled artisan to make and use the full scope of the claimed treatment. The Federal Circuit agreed, finding that the 2004 application did not provide written description support for a narrow Gly/Phe-only tetrapeptide subgenus. The application disclosed an enormous genus (approximately 47 million potential tetrapeptides derived from 83 amino acids) without identifying or singling out any tetrapeptide composed exclusively of Gly and Phe. The Court found that the mere appearance of Gly and Phe somewhere within the large list of amino acids was insufficient. The Court determined that Seagen’s claimed 81-member subgenus was merely an infinitesimal fraction of the millions of encompassed species, and that nothing in the 2004 disclosure singled out or directed a skilled artisan toward the Gly/Phe-only species claimed in the patent. Testimony from the named inventors also confirmed that, as of the application’s priority date, they had never contemplated a treatment using only the 81 tetrapeptides later claimed in the patent.
The Federal Circuit also found that the patent did not enable a skilled artisan to make and use the full scope of the claimed ADCs without undue experimentation. The district court had construed “D is a drug moiety” to encompass any drug, and the claims required that the drug be intracellularly cleaved in a patient. Together, these elements defined an extraordinarily broad genus of ADCs: any drug moiety paired with the required intracellular cleavage.
The specification, however, did not identify any common structural features predictive of intracellular cleavage across all possible drug-linker-antibody combinations. Instead, the evidence showed that ADC behavior was highly unpredictable, and scientists had to conduct assays to determine whether a given ADC satisfied the functional limitation.
Under long-standing Federal Circuit precedent where a patent disclosure requires researchers to conduct the undue experimentation doctrine, it does not satisfy the written description requirement. Because skilled artisans would need to evaluate enormous numbers of ADC permutations to determine whether they achieved the claimed intracellular cleavage, the patent failed the enablement requirement.
