Poor Idenix Pharmaceuticals (Nasdaq: IDIX). Less than two weeks after the Food and Drug Administration put its hepatitis C drug IDX184 on a clinical hold, the agency was back for more, stopping development of its next-generation hepatitis C drug, IDX19368.

Being a next-generation drug, IDX19368 is further behind in the develop process. In fact, IDX19368 hasn't even been tested in humans.

Idenix isn't particularly less valuable if IDX19368 can't be developed; most of its potential is wrapped up in IDX184. But putting another drug on clinical hold is a further sign at how serious the FDA is taking BMS-986094's side effect issues. IDX184 and IDX19368 are both related to BMS-986094, which Bristol-Myers Squibb (NYSE: BMY) discontinued development of last week after nine patients taking BMS-986094 in a clinical trial were hospitalized.

The biotech thinks its drugs are different enough from BMS-986094 that they shouldn't have the same side-effect profile. But the only way to definitively prove that is by testing the drug in humans. For now, the FDA won't let Idenix do that directly -- thus the clinical hold -- so the company will have to use preclinical laboratory experiments to explain why BMS-986094 causes side effects and then prove its drugs won't have the same problem.

Interestingly, Bristol has offered its help in determining what's wrong with BMS-986094, which might make things a little easier, but investors shouldn't expect this to be a cakewalk.

If Idenix is able to emerge on the other side of the clinical-hold black hole, maybe Bristol would be interested in partnering with Idenix. Gilead Sciences (Nasdaq: GILD) certainly doesn't seem interested, and there aren't too many other drugs in the "nuc" class of drugs that BMS-986094 and IDX184 are in. Vertex Pharmaceuticals (Nasdaq: VRTX) has one further back in the clinic, although it could catch up if this clinical hold goes on for a while -- assuming, of course, that the FDA doesn't put a hold on Vertex's drug too.

Ah, the joy of drug development.

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