Sanofi has sued US Merck, alleging infringement of two patents related to an insulin drug. The French drugmaker said the action was triggered by a notification received from Merck in late June.

In the notification, the New Jersey-based company said it had filed a new drug application (NDA) with the US Food and Drug Administration (FDA) for an insulin glargine vial drug product. At the same time it announced it was challenging all of the relevant patents for Sanofi's Lantus and Lantus SoloStar products.

Merck has developed a cheaper follow-on biologic version of Lantus, called Lusduna Nexvue. This is expected to eat into the $6.6 billion sales of the original drug, which accounted for more than a sixth of Sanofi's 2016 total sales.

Eli Lilly also has a Lantus follow-on biologic, Basaglar, which won tentative FDA approval in August 2014. However, the US drugmaker could not launch the drug until December 2016, when it agreed to pay royalties to Sanofi.

Novo Nordisk meanwhile has warned it may have to drop prices of its insulin products in the US next year, as Sanofi will be cutting Lantus prices to thwart biosimilar competition from Lilly as well as Germany’s Boehringer Ingelheim. The Danish pharmaceutical producer said it expects average prices after rebates to be lower than in 2017, as prices of basal insulin are falling.