Sanofi is taking Merck & Co to court claiming that the firm has infringed 10 patents covering its best-selling insulin glargine products Lantus and Lantus SoloStar.

The French drugmaker said it has filed a patent infringement suit against Merck in the US District Court for the District of Delaware, triggered by Merck filing a New Drug Application for its biosimilar insulin glargine drug product MK-1293.

Merck’s submission included a paragraph IV certification challenging all of the ten Sanofi patents listed in the FDA Orange Book for Sanofi’s Lantus and Lantus SoloStar products, the firm noted.

Sanofi has already fought hard to protect its intellectual property on Lantus; the group filed an infringement suit against Lilly back in 2014, but subsequently settled the case with a deal that sees Lilly delay the launch of its Lantus biosimilar to December 2016 and pay royalties to the French drugmaker.