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Idenix: Two Steps Forward and One Step Back

It has happened before- just when things were beginning to look up, Idenix has taken another beating. This time, the experimental AIDS drug it had licensed to ViiV Healthcare, an affiliate of GSK, had been placed on clinical hold by the FDA. The compound, IDX899, a non-nucleoside reverse-transcriptase inhibitor (NNRTI) had only begun Phase IIb clinical studies in November 2010. Shares fell 25% in after hours trading.
IDX899 is one of Idenix’s key assets. It has already received $60 million in upfront and milestone payments through the ViiV deal and could potentially receive an additional $390 million along with double-digit royalties. These are now in doubt.
There was some good news announced today as well; while one candidate was put on hold, a second one was changed from a full to partial hold. The FDA decided to allow trials to continue for Idenix’s lead HCV compound, IDX-184. It will move into Phase IIb in the second half of this year in a 12-week trial in combination with pegylated interferon and ribavarin.
The initial hold occurred in September 2010 as a result of three cases of elevated liver function tests observed during a drug-drug interaction study of the combination of IDX184 and IDX320, an HCV protease inhibitor. At that time, the stock was cut in half. IDX320 has now been determined as the culprit in that study.
Returning IDX-184 to the clinic should result in a net positive for the company; it is one of the few nucleotide inhibitors in development in a future where HCV treatment will be through a combination of pills with non-overlapping activity. Unattached, it also offers potential partnership opportunities for Idenix.