Rethinking Idenix Pharma in Wake of Bristol's Hep C Blow Up

I initially considered Idenix Pharmaceuticals (IDIX) and its nuc IDX-184 a winner emerging from the BMS-094 blowup. Now, I'm less sure.

IDX-184 has a chemical structure that is similar to both BMS-094 and Pharmasset's PSI-938, another Hep C nuc that was killed off by toxicity problems last year. More specifically, the active moiety of IDX-184 -- the part of the molecule that makes the drug work -- appears to be similar to the active moieties of both BMS-094 and PSI-938. That could be a big problem.

Chemists often attach nonessential atoms to a drug candidate in an attempt to change the compound's overall physiological behavior. As the drug is metabolized in the body, those byproducts are cleaved and the working core of the drug -- the active moiety -- is unveiled.

The byproducts are usually innocuous, but not always. For example, it's not clear whether the side effects that killed BMS-094 were caused by the active moiety or by 1-naphthol, a toxic metabolite of the drug. Similarly, PSI-938's fatal flaw remains unclear. This worries me, and it should worry Idenix bulls.

Idenix does have a very reasonable defense. Management contends IDX-184 is nearly entirely targeted to the liver and metabolized poorly in other cell types. Further, IDX-184 seems to be more slowly metabolized by the liver than BMS-094. These factors certainly lessen the risk of toxicity. Thus far, Idenix has treated roughly 60 patients for 12 weeks with IDX-184 and observed no safety signals, including no cardiovascular side effects.

That's encouraging, but I would prefer to overpay for Idenix after there are more data on IDX-184. I would reduce or eliminate long exposure to Idenix until the drug's safety has been more completely established.

Two more Idenix red flags that make me cautious: First, Novartis (NVS), Idenix's partner for nearly a decade, ended the relationship last week. I understand that Novartis wasn't much of a collaborator so walking away might actually be a net positive for Idenix. But I still tend to view pharma-biotech breakups as a negative, although in this case a bit less so.

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