French drugmaker Sanofi has filed a patent infringement suit in the United States District Court for the District of Delaware charging US pharmaceutical producer Merck with infringement of ten patents for its diabetes drugs Lantus and its delivery tool Lantus SoloStar.

Sanofi said it filed the suit in reaction to a notification it received from Merck in early August stating that that the US company had filed an NDA (505(b)(2) New Drug Application) with the US Food and Drug Administration (FDA) for an insulin glargine drug product. Merck said also that its NDA included a paragraph IV certification challenging all of the ten Sanofi patents for the Lantus products listed in the FDA Orange Book.

The patent for Lantus expired in 2015 but the French company continues to fight off competition from biosimilars. The diabetes drug accounts for about 20% of Sanofi’s annual sales of €37 billion, and prices have been eroding – not least, analysts note, due to the drugmaker’s own price reductions in advance of the arrival of a biosimilar produced by Eli Lilly and Boehringer Ingelheim.

Sanofi sued the two companies and won a delay of several months in the launch of certain generic versions of the drug. A successful challenge to Merck’s filing could produce similar results.

The French drugmaker also has developed a newer diabetes drug, Toujeo, but reports say it is showing few signs of becoming such a blockbuster as Lantus, which the company picked up in the 2004 acquisition of Aventis Pharma, the merged the assets of Germany’s Hoechst and France’s Rhone-Poulenc. The diabetes drug was developed by Hoechst.