Merck & Co. said today that it has won tentative FDA approval for its follow-on biologic basal insulin Lusduna™ Nexvue™ (insulin glargine injection).

The approval is tentative because Sanofi has sued Merck, contending that Lusduna infringes on patents for its marketed diabetes treatment Lantus® (insulin glargine).

Under the Hatch-Waxman Act, the filing of that lawsuit in September 2016 automatically blocks the FDA from issuing a final approval for up to 30 months, or if the U.S. District Court in Delaware rules in favor of Merck, whichever comes sooner.

The FDA also granted provisional approval for Merck to market the treatment under the trade name Lusduna Nexvue, once it reaches the market. The FDA accepted Lusduna for review in August 2016.

“The tentative approval of Lusduna Nexvue is an important milestone, bringing us closer to offering this medicine to patients,” Sam Engel, M.D., associate vp, Merck clinical research, diabetes, endocrinology, and women’s health, said in a statement.

The FDA approval comes six months after Lusduna won European Commission marketing authorization on January 4 as a biosimilar drug referencing Lantus, for treatment of diabetes in patients aged at least 2 years.

Lantus lost patent exclusivity in the U.S. and Europe in 2015. Since then, Sanofi has reported sales declines for the drug. Lantus finished the first quarter having generated €1.226 billion ($1.425 billion) in net sales, down 14% from the year-ago quarter.

For all of 2016, Lantus sales fell 12.1% year-over-year, to €4.761 billion ($5.537 billion).

In the U.S. alone, Lantus sales fell 20.9%, to €690 million (about $803 million)—reflecting lower average net price, patients switching to Sanofi’s next-generation basal insulin formulation Toujeo® (insulin glargine injection), and formulary exclusions by United Healthcare and CVS, Sanofi said in releasing Q1 results on April 28.

Lusduna was the second Lantus biosimilar to win European approval; the first was Eli Lilly’s Basaglar® (insulin glargine injection).

In the U.S., however, Lusduna Nexvue was not approved by the FDA under its biosimilar pathway, but under the agency’s 505(b)2 pathway, created by the Hatch-Waxman Amendments of 1984, in part to avoid duplication of studies performed earlier on previously approved reference or listed drugs. The 505(b)2 pathway allows the FDA to consider data that were not generated by the NDA applicant.

Lusduna Nexvue, formerly known as MK-1293, was developed by Merck with partial funding from Samsung Bioepis, a joint venture of Samsung and Biogen. Merck and Samsung Bioepis began collaborating on biosimilars in 2013, then expanded their alliance to diabetes treatments  the following year.