UPDATE 2-Idenix hep C drugs on clinical hold; shares plummet

* Says has not yet received a formal letter from FDA

* Says 3 test subjects experienced serious adverse events

* Shares crash 48 pct (Adds analyst comments, conference call details; updates stock movement)

BANGALORE, Sept 7 (Reuters) - Idenix Pharmaceuticals Inc IDIX.O said U.S. health regulators halted all trials of two of its experimental hepatitis C drugs on safety concerns, putting the breaks on a potential multi-billion dollar market opportunity and shaving off half the stock's value.

The company, which focuses on treating viral diseases, said the drugs were placed on clinical hold after it detected liver function abnormalities in three healthy volunteers during an early-stage study of a combination of the compounds IDX184 and IDX320.

“The key thing to see is where this liver toxicity came from,” Wedbush Securities analyst Katherine Xu said.

“My sense is that probably it is not coming from IDX184. It was probably IDX320 or the combination that caused the problem.”

The combination therapy of the two drugs is in early-stage trials. IDX184, an oral prodrug that maximizes drug efficacy, is in mid-stage trials as a stand-alone therapy, while IDX320, a protease inhibitor that prevents viral replication, alone is in early-to-mid stage trials.

The company said it has not yet received a formal letter nor has the U.S. Food and Drug Administration reviewed the safety and efficacy data from the recently completed clinical trials.

All planned studies of the two drugs to date have been completed and there are currently no healthy volunteers or patients receiving the drugs, the company said in a statement.

Cambridge, Massachusetts-based Idenix said it will submit full data of the preclinical and clinical studies to the FDA and assess the next steps in the development of both the compounds.

On a conference call with analysts, the company said it will be screening the three patients for HIV, hepatitis, alcohol or medication as a potential reason for the adverse reactions.

It said it does not yet have an explanation for the adverse events after two patients received IDX320, but believes that the negative reaction in a patient who was on IDX184 was related to gallstones.

“It has come at the absolute worse time for Idenix because they were looking to partner around,” WBB Securities analyst Steve Brozak said.

The company will be reviewing all the data with independent experts in the external safety committee and supply all the information to the FDA to help resolve the issue by the end of this year.

Wedbush’s Xu said she expects IDX184 to be cleared by year end as there is a lot of data on it which looks “pretty clean,” but thinks clearance for IDX320 and the combination therapy could take longer.

The U.S. Centers for Disease Control and Prevention estimates about 4.1 million people in the U.S to be infected with hepatitis C, with 3.2 million of these being chronically infected.

Bristol-Myers Squibb Co BMY.N, Gilead Sciences GILD.O, GlaxoSmithKline GSK.L and smaller companies like Anadys Pharmaceuticals Inc ANDS.O and Pharmasset Inc VRUS.O are some of the other drugmakers with hepatitis C therapies.

If Idenix does not get the clinical hold removed, WBB’s Brozak expects investors to become a little wary of investing in some of the other smaller developers.

“A typical institutional investor is basically going to say: I just got burned on one product. You mean to tell me I’m going to go out now and burn more money on another product?”

Idenix, which has two other drugs in clinical trials and one hepatitis B drug in the market, does not expect the clinical hold to have any significant impact on its balance sheet.

The company’s shares, which have more than doubled in the last six months since it released promising data on IDX184, were down 46 percent to $3.12 in afternoon trade on Nasdaq. (Reporting by Shravya Jain in Bangalore; Editing by Unnikrishnan Nair, Anthony Kurian, Aradhana Aravindan)