MRK Merck and Co Inc

PARIS—Sanofi SA said it filed a lawsuit against Merck & Co. for alleged patent infringements to prevent the U.S. drugmaker from launching a rival version of the French pharmaceutical giant's best-selling diabetes treatment Lantus.

In the filing in the U.S. District Court of Delaware, Sanofi said on Monday it claims that Merck Sharp & Dohme Corp., Merck & Co.'s international division, violated as many as 10 patents held by the French company, including ones for its insulin Lantus and its insulin delivery device soloSTAR.

The Paris-based drugs company said it started the legal proceedings against Merck after the U.S. firm's filing for new drugs applications with the U.S. Food & Drug Administration.

A spokeswoman for Merck didn't immediately respond to a request for comment.

Sanofi shares were 1.3% higher at €69.91 in midday trading.

The French drugmaker's all-important diabetes business is under siege, as a flurry of pharmaceutical companies seek to sell knockoffs of its blockbuster insulin Lantus in the U.S. The expected launch of lower-cost copies of Lantus and growing pricing pressure on diabetes drugs in the U.S. is rapidly eroding earnings at Sanofi's diabetes division, which accounts for about 20% of the firm's total revenue.

In the first six months of the year, diabetes revenue fell by 6% to €2.9 billion ($3.2 billion), hit by a 15% drop in Lantus sales to €2.38 billion. The company has said it expects revenue from diabetes drugs to continue to decline this as competition among insulin makers intensifies.

In January 2014, Sanofi filed a suit against Eli Lilly & Co. to defend its patents on Lantus. It had reached a deal with the U.S. drugmaker nearly two years later, under which Lilly agreed to delay the launch of its insulin to December 2016 and pay royalties to Sanofi.

In a bid to replenish its new drugs pipeline and revive growth, Sanofi for months had pursued U.S. biotech Medivation—a Nasdaq-listed company that focuses on hard-to-treat cancers, markets one prostate-cancer therapy, Xtandi, and has two other oncology assets in clinical development.

But U.S. pharma giant Pfizer Inc. beat out Sanofi grabbing Medivation for $14 billion in August.

Write to Noemie Bisserbe at noemie.bisserbe@wsj.com and Inti Landauro at inti.landauro@wsj.com

(END) Dow Jones Newswires

September 19, 2016 08:15 ET (12:15 GMT)

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