US biotech Enanta Pharmaceuticals has launched a dual-front legal battle against pharmaceutical giant Pfizer, filing patent infringement lawsuits in both US and European courts over the COVID-19 antiviral Paxlovid. The company claims Pfizer's blockbuster drug infringes its patents covering coronavirus 3CL protease inhibitors, potentially positioning Enanta for a share of Paxlovid's massive revenue stream.
Legal Claims Span Two Continents
The Massachusetts-based biotech filed its initial lawsuit in a Massachusetts district court, alleging that Paxlovid (nirmatrelvir and ritonavir) infringes its 11,358,953 patent, which covers "compounds and pharmaceutically acceptable salts which inhibit coronavirus replication activity, and methods of treating a coronavirus infection." The patent was awarded earlier this month.
On August 20, 2025, Enanta expanded its legal challenge by filing a second patent infringement lawsuit in the Unified Patent Court of the European Union. This suit seeks to determine liability for the use and infringement of Enanta's European Patent No. EP 4 051 265 in relation to Paxlovid across 18 EU countries, following the recent grant of the patent by the European Patent Office.
Financial Stakes and Market Impact
The financial implications are substantial, with Paxlovid expected to generate $22 billion in sales this year after its December 2021 launch. If Enanta's complaints are upheld, the company could be entitled to a portion of these revenues through damages and compensation.
Enanta's lawsuit requests judgment that Pfizer has infringed the patent, plus damages to compensate for that infringement and costs. The company is seeking trebled damages, claiming Pfizer's infringement was willful. However, Enanta has stated it "recognizes the importance of Paxlovid's availability to patients and does not intend to seek an injunction or take other action in this litigation that would impede the production, sale or distribution of Paxlovid."
Enanta's Antiviral Heritage and Pipeline
Enanta brings significant expertise in antiviral development to this dispute. The company has a long heritage in developing antiviral compounds, having worked with AbbVie on therapies for hepatitis C virus (HCV) infection, including Viekira Pak (paritaprevir/ritonavir/ombitasvir/dasabuvir) and Mavyret/Maviret (glecaprevir/pibrentasvir).
The biotech is also advancing its own therapeutic pipeline for respiratory syncytial virus (RSV), human metapneumovirus (hMPV), and chronic hepatitis B virus (HBV). Notably, Enanta began work on COVID-19 early in the pandemic, focusing on the same 3CL protease target as nirmatrelvir, the active component of Paxlovid.
Competing Development Programs
Enanta has advanced its own lead COVID-19 candidate, EDP-235, into phase 1 clinical testing and secured fast-track designation from the FDA, according to court documents. This parallel development program underscores the competitive dynamics in coronavirus therapeutic development and the strategic importance of intellectual property protection in this space.
Potential Complications and Timeline
The legal landscape may be further complicated by additional patent claims. According to SVB Securities analyst Rosanna Ruiz, Pardes Biosciences filed a patent on its 3CL protease inhibitor PBI-0451 earlier than Enanta, potentially creating a three-way dispute over intellectual property rights.
The legal challenge carries inherent risks for Enanta, as it could impact the development of the company's own drug candidate if Pfizer can demonstrate it was first to develop a drug targeting the protease. Legal experts anticipate the proceedings will likely take years to resolve, creating ongoing uncertainty for all parties involved.
The outcome of these cases could significantly impact Enanta's market position and financial performance, while also influencing broader patent protection strategies in the rapidly evolving antiviral therapeutics sector.
