Daiichi Sankyo v. Seagen – The Cost of Inadequate Support in a Priority Application

If you are considering filing a continuation application with claims directed to a product developed after the parent filing, it would be prudent to ensure that the parent application provides adequate written description support for the continuation claims. The consequences of failing to do so were recently demonstrated in a legal battle between Seagen Inc., owner of US patent No. 10,808,039 (“the ’039 patent”), and Daiichi Sankyo, which Seagen accused of infringing the ’039 patent.

Summary of events

Seagen had filed a US patent application in 2004 (“the 2004 application”), covering a linker for antibody-drug conjugates (ADCs). The linker, which links the antibody to the drug, included a peptide having 2 to 12 amino acids, and the amino acids could be any one of a total of 83 amino acids (including isomers).

In 2011, Daiichi Sankyo developed an ADC later marketed under the brand name Enhertu® (fam-trastuzumab deruxtecan-nxki), used for treating several HER2-expressing cancers. The linker in this ADC included the tetrapeptide Gly-Gly-Phe-Gly. Enhertu’s structure and mechanism of action were publicly disclosed in December 2015.

In July 2019, Seagen filed a continuation of the 2004 application, which issued in 2020 as the ’039 patent, including claims specifically covering only linkers having tetrapeptides made from Gly and/or Phe. Taking into account two Phe isomers, the scope of the ’039 patent covers 81 different tetrapeptides (3^4), encompassing the Enhertu® tetrapeptide Gly-Gly-Phe-Gly.

Shortly after the grant of the ’039 patent, Seagen sued Daiichi in the Eastern District of Texas, asserting infringement of claims 1-5, 9, and 10 of the ’039 patent. AstraZeneca, which started collaborating with Daiichi, joined as a co-defendant. In parallel with the district court proceedings, Daiichi and AstraZeneca petitioned for post-grant review (PGR) of the ’039 patent claims. Because the district court jury had found that Enhertu® met each limitation of one of the asserted claims, the key issue was whether the 2004 application provided written description support for the ’039 patent claims, entitling them to the priority date of the 2004 application. If it did not, then the 2015 publication of Enhertu® would anticipate the ’039 patent claims.

The district court jury found that the 2004 application provided adequate written description support for claims directed to a Gly/Phe-only tetrapeptide, entitling the ’039 patent to the 2004 priority date, and that Daiichi and AstraZeneca willfully infringed at least one of the patent claims. Daiichi and AstraZeneca appealed the decision.

The Federal Circuit reversed the district court ruling. Analysis of the disclosure of the 2004 application revealed that it covered peptides including anywhere between two and twelve amino acids, with 47 million (83^4) different possibilities only for tetrapeptides. The Court stated that a patent’s disclosure of a “broad genus,” without more, is inadequate to satisfy the written description requirement for claims directed to a “particular subgenus or species contained therein”. Further, the Court stated that “although the 81 claimed Gly/Phe-tetrapeptides are “encompassed” within those 47 million, … they are merely an infinitesimal fraction of those peptide units generally included”. Referencing a quote from a decision in In re Ruschig, analogizing a genus and its subgenera or species to a forest and trees, the Court reiterated that “We are looking for blaze marks which single out particular trees”. Clearly, in the Court’s view, such blaze marks were missing from the 2004 application to direct a person skilled in the art to the claims of the ’039 patent.

In sum, the Court concluded that the broad disclosure of the 2004 application failed to provide written description support for the much narrower subgenus claimed in the ’039 patent.

Notably, the ’039 patent inventors also stated that they had never contemplated an ADC with a Gly/Phe-only tetrapeptide and first encountered it when Enhertu® was publicly disclosed in 2015.

The Court additionally addressed the issue of whether the 2004 disclosure was enabling for the ’039 patent claims. The Court referenced the Supreme Court’s decision in Amgen v. Sanofi, which rejected the applicant’s argument that the disclosure in the specification was sufficient to enable a skilled artisan to make and use the full scope of the claimed invention. The Supreme Court held that in view of the unpredictability of antibody science where even a minor change of structure can alter functionality, without disclosing “a quality common to every functional embodiment,” the methods were effectively telling a skilled artisan to perform “trial-and-error discovery” in order to determine whether an antibody may possess the recited function. Similarly, in the present case, the recitation “D is a drug moiety” in claim 1 of the ’039 patent was construed by the district court to encompass any type of drug moiety. The ’039 patent further recites a functional limitation that the drug moiety is intracellularly cleaved in a patient from the antibody. It follows that the ’039 patent covers an ADC containing any drug moiety with the recited function of cleaving in a patient. However, the specification does not disclose a quality common to all functional embodiments. In view of the unpredictability of the ADC science, the Court reasoned that identifying suitable drugs would necessitate undue experimentation.

Since the 2004 application was found not to provide written description support or enablement for the’039 patent claims asserted in the district court litigation, the asserted claims were found not to be entitled to the 2004 priority date. As a result, the 2015 publication of Enhertu® anticipated the asserted ’039 patent claims, rendering them invalid.

In the separate PGR proceeding, the Board reached a conclusion similar to that of the Federal Circuit, holding the asserted claims unpatentable on the same grounds asserted by Daiichi in the district court case, namely based on lack of written description support and enablement.

Take home message:

The earliest priority date to which continuation claims are entitled is the filing date of the earliest application that provides adequate written description support and enablement for those claims. This principle is particularly important in the unpredictable life sciences, especially when drafting applications directed to platform technologies. To mitigate the risk of written description or enablement challenges, the initial application should include representative subgenera and species spanning the full claimed scope, together with a meaningful set of working examples. In addition, when pursuing claims in a continuation application, careful consideration should be given to whether the earlier applications in the priority chain adequately support the claimed subject matter.