January 2026 A recent decision from the US Court of Appeals for the Federal Circuit in Seagen vs Daiichi Sankyo has sent a clear and consequential message to applicants and patentees in biotechnology and life sciences. The ruling appears to raise the risk profile for broadly claimed biologic and platform patents in the United States, extending well beyond the antibody space.
At its core, the decision reinforces a stringent US approach to patent sufficiency: if a patent specification does not clearly support and enable the full scope of the claims at the priority date, those claims are vulnerable, no matter how valuable the underlying product may eventually be.
Written Description and Enablement: Two Separate Hurdles in the US
Unlike in Europe, US patent law treats sufficiency as two distinct requirements: written description and enablement.
Written description asks whether the inventor was actually in possession of the claimed invention at the time of filing. Courts apply this test strictly, often invoking the familiar “forest and trees” analogy. Disclosing a broad forest of possibilities is not enough; the specification must include clear “blaze marks” pointing the skilled person to the specific trees that are later claimed.
Enablement, by contrast, requires that the skilled person can make and use the full scope of the claim without undue experimentation. Following the Supreme Court’s decision in Amgen vs Sanofi, this requirement has become an especially high bar for broad functional claims, particularly in unpredictable fields such as biologics.
The Seagen Case
The dispute in Seagen v. Daiichi Sankyo arose from an antibody-drug conjugate (ADC) used in the blockbuster cancer therapy Enhertu®. The asserted patent claimed an ADC linker defined as a four-amino-acid peptide composed solely of glycine and phenylalanine.
The difficulty for Seagen lay in priority. The priority application disclosed peptide linkers in extremely broad terms: lengths ranging from two to twelve amino acids, with 83 possible amino acids at each position. For tetrapeptides alone, this translated into more than 47 million possible combinations.
While the later-claimed sub-genus (81 glycine/phenylalanine tetrapeptides) was technically encompassed by that disclosure, the Federal Circuit found this insufficient. The claimed sequences represented only an infinitesimal fraction of the disclosed possibilities, and crucially, the specification contained no clear pointer directing the skilled person toward them.
Seagen argued that a skilled person could “work towards” the invention from the examples provided. The court rejected this argument outright, stating that if one must leap to the invention, it is not disclosed. As a result, the claims failed the written description requirement and were denied the benefit of the earlier priority date.
Enablement: Functional Claims
The claims also failed on enablement. The drug payload was defined purely by function, i.e., any moiety that is “intracellularly cleaved.” According to the court, this language covered a vast and unpredictable universe of compounds.
Critically, the specification did not disclose any common structural or functional feature that would allow a skilled person to identify suitable payloads without extensive trial-and-error experimentation.
Relying heavily on Amgen vs Sanofi, the Federal Circuit concluded that the claims were not enabled across their full scope.
Consequences and Broader Implications
Because the patent was held invalid, the Federal Circuit vacated the jury’s earlier finding of wilful infringement, along with a damages award exceeding $40 million and ongoing royalty obligations.
More broadly, the decision highlights a sharp and growing divergence between US and European practice. In the US, the sheer scale of experimentation required to traverse a broad functional claim can itself defeat patentability. In Europe, the focus remains on whether the skilled person can carry out the invention without inventive effort, even if extensive routine experimentation is necessary.
Practical Takeaways
The lesson from Seagen vs Daiichi Sankyo is clear. For US filings, especially those directed to biologic platforms or broad functional concepts, it is no longer safe to rely on generic genus disclosures or placeholder priority applications. If a specific sub-genus or functional scope is intended to be claimed, the application must clearly identify it and enable it from day one.
Finally, while in silico data may play an increasing role in supporting enablement across large numbers of variants, innovators should also be aware of the flip side: competitors may use such data to identify non-working embodiments in an effort to undermine broad claims.
In today’s US patent landscape, breadth without precision appears to be a liability, not an asset.
