March 7 (Reuters) - The U.S. Food and Drug Administration has asked Regeneron and Sanofi to assess potential neurocognitive side effects of their experimental cholesterol drug, Sanofi said in its annual report on Friday.

The regulatory filing sent shares of Regeneron down 6 percent in Nasdaq trading. U.S.-listed shares of France-based Sanofi were down 1 percent.

Their drug, alirocumab, is part of a new class known as PCSK9 inhibitors designed to block a protein that maintains "bad" LDL cholesterol in the bloodstream.

Pfizer and Amgen are also in the late stages of developing PCSK9 drugs.

Pfizer said in an emailed statement that it has not received a similar request from the FDA. "At this stage of our bococizumab development program, we are not aware of any neurocognitive safety signals," the company said.

Officials at Amgen did not immediately respond to a request for comment.

Sanofi's report echoed a filing made by Regeneron last month, in which the company said the FDA advised it was aware of adverse neurocognitive effects associated with PCSK9 inhibitors.

The FDA and Regeneron did not immediately respond to requests for comment.

The companies said they did not know how the FDA learned of the potential side effects, and they were not aware of any such side effects with alirocumab.

Rare issues such as memory loss, impaired concentration, and paranoia have been associated with the use of statins for lowering LDL cholesterol.

Statins, such as AstraZeneca's Crestor and generic forms of Pfizer's Lipitor, are the most widely used cholesterol-lowering treatments and work by blocking the liver's production of LDL cholesterol.

"While we continue to believe the PCSK9 class has multi-billion dollar potential, we note that increased speculation on adverse events may increase the probability that the FDA could require outcomes data prior to full approval," JP Morgan analyst Geoff Meacham said in a research note.

The FDA said last year that PCSK9 drugs could get regulatory approval based on their ability to lower bad cholesterol, and may not need to show that they reduce the risk of heart attack and stroke.

In their filings, Sanofi and Regeneron said that if studies detect neurocognitive or other adverse side effects, development of alirocumab could fail or be delayed.