Shares in Idenix Pharmaceuticals Inc., based in Cambridge, fell as much as 35 percent after a trial of the company’s hepatitis C drug was placed on hold because of heart failure concerns raised in a competitor’s study of a similar medicine.

The U.S. Food and Drug Administration is looking at other compounds from the same class of medicines called nucleotide polymerase inhibitors, Idenix Chief Executive Officer Ron Renaud said on a conference call today. Gilead Sciences Inc., Bristol-Myers Squibb Co., and Vertex Pharmaceuticals Inc. are developing similar treatments.

Idenix declined to $6 at 10:38 a.m. New York time after sinking to $5.40, its biggest intraday drop in almost two years. The experimental drug is Cambridge, Massachusetts-based Idenix’s main pipeline prospect. Bristol-Myers said Aug. 1 that it halted trials of its treatment after a patient developed heart failure.

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“This event is likely to raise concerns about the risks of nuke programs across the spectrum, but we believe it is important to understand the difference among the assets,” said Geoffrey Porges, an analyst with Sanford C. Bernstein & Co. in New York. The Vertex and Gilead drugs have different structures and may not raise the same worries, he said in a note to clients.

Gilead fell 1.3 percent to $57.20. Vertex rose less than 1 percent to $54.89.

The U.S. Food and Drug Administration gave a “verbal notice” that a “partial clinical hold” has been placed on the Idenix trial while the agency reviews the medicine’s safety, Idenix said. The company said today it plans to give the FDA data on the treatment, which isn’t being given to any patients right now.

“As a result of the recent occurrence of a serious cardiac-related adverse event encountered with a competitor’s nucleotide polymerase inhibitor for the treatment of HCV, the FDA has expressed an interest in further reviewing the safety of IDX184 and has placed IDX184 on partial clinical hold,” Idenix said in a statement.

“We believe this will be temporary, unfortunately we can’t give any concrete answer on our timeline,” Renaud said on the conference call.

Idenix, Gilead, Vertex and Bristol-Myers experimental drugs stop the hepatitis C virus from replicating, and could be used in combination with other pills to replace the current regimen of side-effect heavy injections.

Idenix is trying to track down 67 patients who took its drug so they can perform cardiac tests on them. All of the patients have been off the medicine for at least a month, Chief Scientific Officer Douglas Mayers said on the conference call.

Renaud said he didn’t know whether the FDA would look at all of the related drugs that are in clinical trials, or just some. “They did mention they’d be looking at, to start, NS5B polymerase inhibitors,” he said of the company’s discussions with regulators.

Cara Miller, a spokeswoman with Foster City, California- based Gilead, and Dawn Kalmar, a spokeswoman for Cambridge, Massachusetts-based Vertex, didn’t immediately respond to questions via phone and e-mail asking whether they’d been contacted by the FDA as well.