Idenix Pharmaceuticals (IDIX) shares tanked Thursday after the company said the Food and Drug Administration was putting a partial hold on human studies of the company's experimental hepatitis C treatment.

The FDA is concerned about potential heart risk in patients after a safety issue was discovered in a similar drug being developed by Bristol-Myers Squibb (BMY). Initially, Bristol-Myers' setback -- which led to the suspending of its drug trial -- seemed to be a positive for Idenix and other companies in the race for a better hepatitis treatment. 

Shares of Idenix were down over 28% to $5.99 Thursday afternoon. The stock is still up almost 29% over the past 12 months.

Idenix CEO Ronald Renaud told investors on a conference call Thursday that the company hasn't seen any evidence in studies so far that would suggest its drug, known as IDX184, poses a serious heart threat. The company will provide additional study data to the FDA as it tries to show that the drug is safe.

"We remain confident in the safety profile" of the drug, Renaud said on the call.

Evidence of heart failure is the type of risk that can defeat an experimental drug even if there are only one or two cases among 10,000 patients, Leerink Swann analyst Howard Liang says in a recent note.

So far, the other leading companies developing similar hepatitis C drugs, including Gilead Sciences (GILD) and Vertex Pharmaceuticals (VRTX) have not indicated there are any heart safety issues related to their medicines.

Gilead shares dropped more 2.5% to $56.47 Thursday afternoon, while Vertex was up slightly to $54.52.

Idenix, Gilead, Vertex, and Bristol-Myers have all been studying a class of drugs known as nucleotide polymerase inhibitors. The drugs target an enzyme needed for the hepatitis C virus to replicate. The goal of all these companies is to create an all-oral regimen for hepatitis patients that doesn't include the injected therapy interferon, which can cause side effects such as fatigue and flu-like symptoms.

The government estimates as many as 3.9 million people in the U.S. may be chronically infected with the liver-destroying virus. Worldwide, that figure is about 150 million, according to the World Health Organization.


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