Seagen v. Daiichi Sankyo: The Enhertu Patent That Never Showed Its Own Invention
By Prashant Kotian, PhD Researcher (Chemistry), ICT Mumbai
On December 2, 2025, the Federal Circuit reversed a jury verdict that had awarded Seagen Inc. more than 41 million dollars, plus an 8 percent running royalty, against Daiichi Sankyo and AstraZeneca over Enhertu, one of the most important cancer drugs on the market. The reversal did not turn on chemistry that changed. It turned on chemistry that was never actually shown in the first place.
Enhertu is an antibody-drug conjugate: an antibody, a cell-killing drug, and a linker protein connecting them, engineered so the linker only breaks apart once it reaches the target cancer cell. Enhertu's real linker carries a specific four-amino-acid sequence: glycine-glycine-phenylalanine-glycine, a "Gly/Phe-only tetrapeptide." Seagen's asserted patent, U.S. Patent 10,808,039, claims a peptide unit limited to exactly that combination of amino acids, a subgenus the court calculated at 81 possible species.
The patent was filed in July 2019, but as a continuation claiming priority back to a Seagen application from November 2004, fifteen years earlier. That earlier date mattered enormously: Enhertu's own structure was made public in December 2015. If the 2019 patent could reach back to 2004, it predated Enhertu and could cover it. If it could only reach back to 2019, Enhertu's own 2015 disclosure came first and would invalidate the claims outright.
The 2004 application did discuss a linker built from a peptide unit, and it did list glycine and phenylalanine among the possible amino acids that unit could use. What it never did, the Federal Circuit found, was disclose an actual tetrapeptide built from only those two amino acids, the specific narrow combination the 2019 patent went on to claim. Naming ingredients that could be combined a certain way is not the same as showing that combination. The court noted, pointedly, that it could not avoid the suspicion the 2019 patent was drafted specifically to reach Enhertu after the fact.
Why it matters: A subgenus of 81 possible molecules sounds narrow enough to feel like a real, specific invention. Written description law asks a more exacting question than how narrow something sounds: did the original filing actually show that the inventors possessed this precise combination, not just the individual pieces that happened to be capable of forming it. Confirming that a claimed structural definition genuinely covers, and was genuinely disclosed for, one particular real compound rather than a broad set of theoretical possibilities is exactly the kind of check that gets skipped when a filing leans on general language instead of an exact worked example.
With the reversal, the prior infringement judgment, the damages award, and the ongoing royalty obligation are all gone. A drug used by cancer patients worldwide never had to change; only the legal conclusion about who is allowed to make money from one part of it did.
Nothing here is a comment on any other patent's drafting or validity, and nothing above should be read as legal advice. It is a summary of a public, precedential federal court opinion, based on the opinion's own text.
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