Context: In June 2022, Enanta Pharmaceuticals sued Pfizer in the District of Massachusetts, alleging that its COVID treatment Paxlovid infringed U.S. Patent No. 11,358,953 (“Functionalized peptides as antiviral agents”). However, in December 2024, the court granted a summary judgment motion by Pfizer, ruling the asserted patent invalid because Enanta had failed to establish its claimed priority date. The plaintiff said it planned to file an appeal in the Federal Circuit. In a separate case, following failed licensing negotiations with its German rival Merck, biotech firm Halozyme Therapeutics is seeking an injunction against the launch of its subcutaneous drug Keytruda in the District of New Jersey. The suit, filed in April, alleges the infringement of 15 of Halozyme’s patents. The United States Patent & Trademark Office has since agreed to reconsider the asserted patent, following a request filed by Merck. And, following a patent infringement dispute initiated by Incyte in the District of New Jersey over Sun Pharma’s drug Leqselvi last year, and a subsequent countersuit by its opponent, the parties settled the U.S. case in July with a patent licensing agreement (July 14, 2025 Sun Pharma press release).
What’s new: In Enanta v. Pfizer, the former has now filed a new patent infringement complaint in the Unified Patent Court (UPC) over EP4051265 (“Functionalized peptides as antiviral agents”) – the European counterpart of the patent asserted in the ongoing U.S. litigation (August 20, 2025 Enanta press release). Separately, Merck has filed a countersuit against Halozyme in the High Court of Justice for England & Wales (EWHC). And, also in the EWHC, Sun Pharma has initiated a new dispute against Incyte. The details are not yet public.
Direct impact and wider ramifications: Enanta’s new UPC complaint does not come as a surprise, given its current losing battle in the U.S., and a positive outcome in Europe could mean a major turnaround for that case. Halozyme’s Enhanze® dominates the subcutaneous market, having generated over $205.6 million in Q2 2025 royalty revenue via partnerships with Janssen, Roche, and argenx (August 5, 2025 Halozyme press release), meaning Halozyme v. Merck could set a major precedent for patent enforcement in subcutaneous drug delivery.
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Counsel
In the UPC, Enanta is being represented by Hoffmann Eitle’s Dirk Schüssler-Langeheine and Thorsten Bausch.
In the EWHC, Sun Pharma is being represented by Powell Gilbert’s Bethan Hopewell, Tess Waldron, Adam Rimmer, and Jodie Goonawardena.
And finally, also in the EWHC, Merck is being represented by a team at Hogan Lovells, while Halozyme is being represented by Quinn Emanuel.