Bayer's patent covering its best-selling blood thinner Xarelto is invalid, London's High Court ruled on Friday in a blow to the German drugmaker.

The company's blockbuster Xarelto drug generated nearly $4.1 billion in revenue in 2023, according to research firm Statista.

Bayer's attempts to replace Xarelto were thrown into doubt last year when it aborted a large late-stage trial testing a new anti-clotting drug due to lack of efficacy.

Its European patent over once-daily rivaroxaban, with a brand name of Xarelto, has now been declared invalid following legal challenges brought by Sandoz and other rival manufacturers.

Judge Richard Hacon said Bayer's patent was invalid because of the lack of an "inventive step" in developing the drug.

Friday's decision paves the way for other drug makers to launch generic versions of rivaroxaban, though Bayer said it would be seeking to block such attempts pending any appeal.

"We are seeking permission to appeal this decision and seeking interim injunctive relief to prevent the launch of 10mg, 15mg and 20mg generic rivaroxaban products," a Bayer spokesperson said in a statement.

"We believe in the validity of our patent as confirmed by the decision of the European Patent Office in 2021 and we continue to be committed to protecting our intellectual property."

Sandoz did not immediately respond to a request for comment.

(Reporting by Sam Tobin; editing by David Evans)