After years of patent litigation between cancer drug developers Daiichi Sankyo and Seagen, the U.S. Patent and Trademark Office (U.S. PTO) has handed Daiichi the latest win in the saga.

For years, the companies have clashed over Seagen’s so-called “‘039 patent,” which covers certain peptides conjugated to an antibody through various linkers. 

After a 2008 antibody-drug conjugate (ADC) partnership between the drugmakers ended in 2015, Daiichi Sankyo entered into a lucrative agreement with AstraZeneca to work on what became the cancer blockbuster Enhertu.

Seagen staked a claim on the AZ contract in a 2020 lawsuit, arguing that the technology used for the med was created under its previous Daiichi ADC partnership.

In 2022, a Texas jury found that Daiichi Sankyo stepped on Seagen’s patent willfully, awarding the latter company $41.8 million in royalties. The now Pfizer-owned company took its case a step further by asking for royalties from future U.S. Enhertu sales until the patent expires later this year.

Last October, the courts agreed with Seagen in an amended final judgment, ordering Daiichi Sankyo to pay Seagen an 8% royalty through the patent’s expiration date in November 2024.

It seemed like the end of the road for Daiichi Sankyo after the company’s various motions were shot down, but a petition at the U.S. PTO offered the Japanese drugmaker a sliver of hope in the legal entanglement.

Now, the U.S. PTO has invalidated all claims of the ‘039 patent, according to Daiichi Sankyo. In a statement, Daiichi Sankyo’s corporate officer and general counsel, Naoto Tsukaguchi, said the company is “pleased” with the decision.

Seagen, meanwhile, is “disappointed in the decision and will request an appeal,” a company spokesperson said in an emailed statement. 

Meanwhile, Daiichi’s appeal on the royalty judgment in Texas is pending.