June 25 (Bloomberg) -- Sanofi-Aventis SA shares fell 4.6 percent in Paris trading after UBS AG said investors are concerned about the safety of the drugmaker’s Lantus diabetes treatment.

There have been “recent market questions on the safety of insulin analogues in undisclosed studies,” Gbola Amusa, an analyst at UBS AG in London, wrote in a report today. “In our discussions with physicians, we could not confirm any safety risk but did hear a number of long-established safety concerns, in particular that Lantus use may promote cancer.”

Sanofi is betting on Lantus, its third-biggest seller, to offset sales declines caused by generic competition to other drugs. Demand for the diabetes treatment helped first-quarter earnings beat analysts’ estimates, and Amusa said Lantus probably will make up about 33 percent of annual profit by 2013.

Data from clinical studies covering more than 70,000 patients as well as research since the drug went on the market “confirm the safety profile of Lantus,” Geoffroy Bessaud, a spokesman for Paris-based Sanofi, said by telephone. Lantus, approved for sale in the U.S. in 2000, was the first once-a-day form of insulin.

Sanofi shares dropped 2.13 euros to close at 44.45 euros in Paris, the steepest one-day decline since April 23.

‘Earthquake’ Event

Concern over Lantus was stoked by Ralph DeFronzo, a diabetes researcher at the University of Texas Health Science Center, on a June 11 conference call sponsored by Credit Suisse, Amusa said in a phone interview.

DeFronzo said he expected an “earthquake” event that might prompt doctors to not “feel so comfortable with glargine” insulin, according to the call’s transcript. Glargine is a chemical name for Lantus. Pressed for details, DeFronzo said, “Can’t tell you anything. So keep your ears open. It won’t take too long.”

Competing diabetes treatments include Byetta, marketed by Eli Lilly & Co. and Amylin Pharmaceuticals Inc. DeFronzo was an investigator on a company-sponsored study of Byetta. He didn’t return a call to his office in San Antonio.

UBS’s Amusa has a “sell” rating on Sanofi.

“We have to be careful not to taint a medicine which helps many patients, but if Sanofi have data we should see it,” said Kevin Scotcher, an analyst at Liberum Capital Ltd. in London. Sanofi’s Origin study, which is investigating the cardiovascular safety of Lantus, is likely to be completed in 2012, Bessaud said.

To contact the reporter on this story: Trista Kelley in London at tkelley2@bloomberg.netAlbertina Torsoli in Paris at atorsoli@bloomberg.net

Last Updated: June 25, 2009 11:58 EDT