Enanta files patent infringement suit against Pfizer over Paxlovid in EU Unified Patent Court

Enanta Pharmaceuticals said on Aug 20, 2025, that it 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 defendants are Pfizer Inc. and those subsidiaries. The Form 8-K and the press release do not name the subsidiaries. The case carries action number 35071/2025. Enanta seeks a determination of liability for use and infringement of European Patent No. EP 4 051 265, which it calls the '265 Patent. These are the company's allegations, and no court has ruled on them.

The suit targets Pfizer's manufacture, use and sale of Paxlovid, the COVID-19 antiviral made of nirmatrelvir and ritonavir tablets. Enanta says the action covers Pfizer's activities in the 18 countries of the European Union that now take part in the UPC.

The patent and the U.S. case

According to Enanta, the European Patent Office recently granted the '265 Patent to the company, and it was published in the European Patent Bulletin on Aug 20, 2025, the same day the suit was announced. The patent is based on a July 2020 Enanta patent application covering coronavirus protease inhibitors invented by Enanta scientists.

The '265 Patent is the European counterpart of U.S. Patent No. 11,358,953, which the company says is involved in ongoing U.S. litigation between Enanta and Pfizer. The filings do not describe that U.S. case, its court or its status, so the European suit cannot be read against any U.S. ruling.

The timetable and what it leaves open

Enanta says that under UPC procedures a hearing on the infringement action is expected within the court's published 12-month target, and that a decision would be rendered within weeks of the hearing. If the court finds infringement, the company says further proceedings would be needed to determine damages. It also says all timelines remain subject to rights of appeal and other customary steps in European patent litigation.

Two gaps stand out. The filings do not say when the case was filed in the UPC, when Pfizer must respond, or what Pfizer has said about the claim. They also do not say what revenue Enanta would claim for Paxlovid sales in the 18 countries, which is the amount that would determine any damages.

The risks Enanta lists are the ones common to patent disputes. It says the patents could be found invalid or unenforceable if challenged in court, that the litigation could be costly and distract senior management and scientific staff, and that it could bring unfavorable publicity that harms the company's reputation and share price.

The money behind the suit

Enanta's most recent annual report on file is its 10-K for the fiscal year ended Sept 30, 2024, filed Nov 27, 2024. That report describes the company as focused on small molecule drugs for virology and immunology. Glecaprevir, a hepatitis C protease inhibitor it discovered, is part of a regimen AbbVie sells under the collaboration. AbbVie sells glecaprevir in combination as Mavyret in the United States and Maviret outside it. Enanta's royalties from those products fund part of its operations, according to the press release.

The company's cash, cash equivalents and restricted cash fell to $41,201 thousand at Sept 30, 2024, from $89,356 thousand a year earlier. That balance fell by $48,155 thousand during the year. The 10-K reports a net loss of $116,045 thousand and net cash used in operating activities of $78,764 thousand for fiscal 2024; the cash figure above excludes marketable securities. The filing gives no estimate of what the European or U.S. litigation will cost.

Enanta's clinical programs, as its release describes them, are focused on respiratory syncytial virus (RSV). Its immunology pipeline targets inflammatory diseases by inhibiting KIT and STAT6. The filings give no figure for any payment Enanta seeks from Pfizer, and no decision is expected before the UPC hearing.

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