The suit was triggered by a notification received from Merck in late June, in which Merck stated that it had filed an NDA with the FDA for an insulin glargine vial drug product. Merck also stated that its NDA included a paragraph IV certification challenging all of the Sanofi patents then listed in the FDA Orange Book for Sanofi's Lantus and Lantus SoloStar products.

Merck's cheaper follow-on biologic version of Lantus, called Lusduna Nexvue, could chip away at the original drug's sales, which reached $6.6 billion and represented over a sixth of Sanofi's total sales last year.

Eli Lilly & Co's Lantus follow-on biologic Basaglar won tentative FDA approval in August 2014 but litigation brought by Sanofi delayed final approval to December 2016. Lilly agreed to pay Sanofi royalties as part of the settlement.