UPDATED: Idenix plunges after FDA throws up a roadblock for lead hep C 'nuc'

Idenix Pharmaceuticals ($IDIX) has encountered another big roadblock in the race to develop oral drugs for hepatitis C virus. The FDA has hit the biotech group with a request for more preclinical data on IDX20963, which is Idenix's lead drug in the closely watched class of uridine nucleotide prodrugs, sometimes called "nucs" for short.

The company's shares tumbled 38% in after-hours trading--dropping to $3.18--as analysts started to discount the chances of success for this program, a bitter setback for Idenix as it struggles to get back on track.

The request delays the start of human testing of the compound and follows previous troubles with the FDA over Indenix's experimental hep C drugs. In February the company ditched development of two nucleotide polymerase inhibitors known as IDX184 and IDX19368, opting to ice the programs after the FDA placed them on hold because of dire safety problems with a similar compound from Bristol-Myers Squibb ($BMY) called BMS-986094.

Bristol's compound triggered heart failure in one patient and led to serious side effects in others, prompting the company to write off most of the $2.5 billion it spent to acquire the program from Inhibitex less than a year before the problems occurred.

Idenix has stayed in the hep C race with a midstage study of an NS5A inhibitor called IDX719. Still, nucs have proven to provide potent attacks on the liver-damaging virus without requiring injections of interferon, which causes flu-like side effects. Gilead Sciences ($GILD) could have its lead nuc drug called sofosbuvir on the U.S. market by next year, giving the company the first interferon-free treatment for hep C.

The all-oral treatments for the disease are expected to surpass $20 billion in annual sales, but it could take Idenix much longer to grab a piece of the action.

- here's the release