While Sanofi-Aventis is busily gathering data on patient use of diet drug Acomplia in Europe to track side effects that may emerge during its initial months of sale, the company denied Oct. 6 that FDA has asked for more safety data as it weighs approving the diet pill for sale in the United States.

Doug Greene, Sanofi's chief medical officer, provided the strongest hint to date that FDA -- where Acomplia has been on hold for seven months -- remains concerned about possible safety issues associated with a novel weight-loss drug that acts on receptors in the brain.

In clinical trials, psychiatric side effects such as anxiety or depression accounted for about half of all drop outs attributed to adverse experiences with the 20 mg dose of rimonabant -- the higher trial dose that is now being prescribed to patients in Europe.

A concern of some researchers, possibly including some at the FDA, is that while submissions for Acomplia's approval were based on trial data involving 13,000 patients, further safety issues may emerge as many times larger numbers of obese and overweight individuals begin taking the drug.

"The most vulnerable period for any new drug is the 12 months after approval,'' Greene told Bloomberg news at a forum on drug research and development in Berlin. "Rare events are difficult to discern during a clinical trial period.''

A Sanofi spokesperson later in the day confirmed that the company was "collecting data in Europe on the secondary effects of Acomplia," but formally denied that the FDA had asked for the data as part of the current approval process.

"We have not been asked by the FDA for this data," the spokesperson said.

But Greene noted that "the FDA has become very cautious" as a result of a number of experiences in recent years -- including with the diet drug combination Fen-phen -- where drugs it approved were subsequently found to be dangerous for some patients.

"We're working very closely with them and helping them in their interpretation of the data," Greene said. "We think when it completes its review, Acomplia will be found to be safe and effective.''

Observers of the FDA generally found Greene's remarks on the continuing review of Acomplia at the FDA more realistic than Friday's latest reiteration of the Sanofi corporate line that Acomplia will be on the market in the U.S. before the end of 2006.

As one analyst noted: "The clock is ticking on that guidance.''