Shares in Idenix have plummeted about 40% after news US regulators have placed a partial hold on the company’s experimental hepatitis C drug IDX184 following safety concerns.

The move comes after news that a competitor’s drug for HCV, also a nucleotide polymerase inhibitor like IDX184, may be linked to a serious cardiac-related adverse event. The US Food and Drug Administration has placed a hold on Idenix’s trial to review the safety of IDX184 and has requested additional data on patients treated with the drug.

Idenix maintains “there has been no evidence to date of cardiotoxicity” in patients taking the drug in previous trials and the current Phase IIb trial.

“Patient safety is our main concern and Idenix will immediately begin work to comply with the FDA request and expects to submit these data to the FDA in the coming weeks,” the company said in a statement.

Idenix will perform cardiac tests on the 67 patients that were recently treated with the drug.

While the statement did not mention who the competitor is, reports suggest it could be Bristol-Myers Squibb, who was forced to halt trials at the start of August on its Phase II HCV drug BMS-986094 after a patient developed heart failure. The move to halt the Idenix trial results from analysis that points to a similar chemical structure between the Idenix and BMS drugs.

The other HCV drug developers, Vertex and Gilead, are not faced with the same concerns because their drugs have different structures. However, it has not been confirmed if the FDA will also review these drugs as well.

This is not the first setback for Idenix. In September 2010 the FDA halted testing on a drug when concerns were raised about the potential side effects on the liver. Meanwhile, in 2007 the FDA also halted another of the company’s HCV treatments, valopicitabine, based on its risk/benefit profile.