U.S. drug reviewers flagged several safety concerns, including possible cancer risks, related to Sanofi and Isis Pharmaceuticals's lipid disorder drug Kynamro, sending Isis's shares down as much as 21%.

In briefing documents posted on the regulator's website on Tuesday, U.S. Food and Drug Administration reviewers said patients taking the drug had a high drop-out rate, related to adverse events like heart disorders and liver damage, and a higher incidence of cancers in the treatment group.

"This higher incidence of cancer raises the specter that the FDA may require another trial for Kynamro's approval," said ThinkEquity analyst Kimberly Lee in a note on rival Aegerion Pharmaceuticals, which is also developing a drug for the disorder.

The safety concerns also cast a shadow on Isis's antisense technology, which seeks to prevent the production of proteins involved in disease processes.

However, the reviewers, who found the drug to be highly effective in reducing so-called bad cholesterol, said it was difficult to conclude that Kynamro was playing a dominant role in the imbalance of cancer incidents between the drug and the placebo groups.

The high discontinuation rate could pose problems for a therapy that needs to be taken chronically, the FDA staff said, adding that a risk mitigation strategy was needed to ensure the benefits of the drug outweighed the potential risks.

Kynamro, co-developed by Isis and Sanofi unit Genzyme, is intended to reduce bad cholesterol in patients with a lipid disorder, who are already on a stable regimen of lipid-lowering therapies but require additional treatment.

The disorder, called familial hypercholesterolemia, is a genetic disease that results in elevated bad cholesterol levels and increased risk of premature heart disease.

The FDA reviewers said Kynamro was highly effective in reducing bad cholesterol over 26 weeks according to both their analyses and data submitted by the companies.

The review of Kynamro, or mipomersen sodium, comes ahead of an advisory meeting of independent experts on Oct. 18 to discuss the drug. The panel will discuss Aegerion's drug on Wednesday.

The FDA, which will take into account the independent panel's recommendations, is expected to make a final decision on Kynamro by Jan. 29, 2013.

Shares of Aegerion, which stands to benefit from a stumble at Isis, were up 6% to $18.58