Pfizer battles another Paxlovid lawsuit from Enanta

If you don’t succeed at first, try again – in separate regions. That’s the motto Enanta Pharmaceuticals is following, at least, after disclosing it has sued Pfizer in Europe over a patent infringement relating to Covid-19 treatment pill Paxlovid (nirmatrelvir/ritonavir).

In June 2022, Enanta filed a lawsuit against Pfizer in a US district court in Massachusetts, claiming that the big pharma company infringed on a patent describing protease inhibitors invented by its scientists. Enanta has now followed that up with another filing in Europe, making the same accusation.

Since being emergency authorised in 2021, anti-viral Paxlovid has generated Pfizer more than $26bn in global revenue. This includes a staggering $18.9bn in 2022 when Covid-19 cases were still prevalent. Despite waning demand for Covid-19 treatments, the pill still brought in $1.2bn in 2024, buoyed by government orders.

However, Enanta – known for co-developing hepatitis C virus treatment glecaprevir/pibrentasvir with AbbVie – believes Pfizer designed Paxlovid via unlawful means.

The US biotech stated it is “seeking a determination of liability for 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”.

In an emailed statement to Pharmaceutical Technology, a Pfizer spokesperson said: “We are confident in our intellectual property (IP) surrounding Paxlovid and will respond in due course in court.”

The lawsuit, filed in the European Union’s (EU) Unified Patent Court (UPC), targets Pfizer’s commercial activity in the 18 countries of the EU. The company confirmed the ’265 patent in question is the European counterpart of US patent number 11,358,953 (the ’953 Patent) that is the centre of the US lawsuit.

Although it is technically ongoing, Enanta’s US lawsuit hit a major roadblock. In December 2024, a federal judge in Massachusetts sided with Pfizer, granting that the ‘953 patent is invalid. Enanta confirmed at the time it would appeal the decision, adding it “believes strongly in the merits of our case”.

Pfizer reported strong Q2 2025 results this month, bucking a tepid earnings window that gripped the wider pharma industry. Sales for the Paxlovid grew 71% while the Covid-19 vaccine Comirnaty revenue surged 95%.

However, the legal challenge posed by Enanta marks the second issue Pfizer has had to firefight this week. The big pharma company reported a Phase III trial failure for a sickle cell disease candidate purchased as part of a $5.4bn takeover of Global Blood Therapeutics in 2022.

“Pfizer battles another Paxlovid lawsuit from Enanta” was originally created and published by Pharmaceutical Technology, a GlobalData owned brand.

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