Pfizer Faces Patent Infringement Lawsuit from Enanta over Paxlovid Treatment Pill

Enanta Pharmaceuticals has sued Pfizer in Europe over patent infringement for Covid-19 treatment pill Paxlovid. Enanta claims Pfizer designed the pill via unlawful means, seeking a determination of liability in the EU. Pfizer is confident in its intellectual property and will respond in court. This lawsuit comes after a US federal judge invalidated a US patent related to Paxlovid, with Enanta planning to appeal.

Enanta Pharmaceuticals, Inc. (NASDAQ: ENTA), a clinical-stage biotechnology company focused on small molecule drugs for viral infections and immunological diseases, has filed a patent infringement suit in the Unified Patent Court (UPC) of the European Union against Pfizer Inc. and certain of its subsidiaries. The lawsuit, filed on August 20, 2025, seeks a determination of liability for the use and infringement of European Patent No. EP 4 051 265 (the '265 Patent) in the manufacture, use, and sale of Pfizer's COVID-19 antiviral, Paxlovid™ (nirmatrelvir tablets; ritonavir tablets) [1].

Enanta Pharmaceuticals claims that Pfizer designed the pill via unlawful means and seeks a determination of liability in the EU. The company's patent, which covers coronavirus protease inhibitors, was granted by the European Patent Office and published in the European Patent Bulletin on August 20, 2025. The '265 Patent is the European counterpart of U.S. Patent No. 11,358,953 (the '953 Patent), which is involved in ongoing U.S. litigation between Enanta and Pfizer [1].

Pfizer, confident in its intellectual property, is expected to respond in court. The UPC's published 12-month target for a hearing on the infringement action is expected to be met, with a decision rendered within weeks thereafter. If the UPC determines there has been infringement, subsequent proceedings would be required to determine damages. All timelines remain subject to potential rights of appeal and other customary proceedings in European patent litigation [1].

Enanta Pharmaceuticals' clinical programs are focused on respiratory syncytial virus (RSV) and its earlier-stage immunology pipeline aims to develop treatments for inflammatory diseases by targeting key drivers of the type 2 immune response, including KIT and STAT6 inhibition. The company's protease inhibitor, glecaprevir, is part of one of the leading treatment regimens for curing hepatitis C virus (HCV) infection and is sold by AbbVie in numerous countries under the tradenames MAVYRET® (U.S.) and MAVIRET® (ex-U.S.) (glecaprevir/pibrentasvir) [1].

This lawsuit comes after a US federal judge invalidated a US patent related to Paxlovid, with Enanta planning to appeal. The impact of this suit on Enanta's financials and stock price remains to be seen, but investors should be aware of the potential risks and uncertainties involved in intellectual property litigation [1].

References:
[1] https://www.marketscreener.com/news/enanta-pharmaceuticals-files-patent-infringement-suit-against-pfizer-in-the-unified-patent-court-of-ce7c51d2dd8df22c
[2] https://ir.enanta.com/news-releases/news-release-details/enanta-pharmaceuticals-files-patent-infringement-suit-against-0
[3] https://www.morningstar.com/news/business-wire/20250820282912/enanta-pharmaceuticals-files-patent-infringement-suit-against-pfizer-in-the-unified-patent-court-of-the-european-union