Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Achillion Pharmaceuticals (NASDAQ:ACHN), a developer of therapies targeting the hepatitis-C virus and other infectious diseases, plummeted as much as 60% following disappointing news from the Food and Drug Administration regarding sovaprevir, its lead hepatitis-C compound.

So what: According to an after-the-bell Friday press release, Achillion noted that the FDA had decided to keep its clinical hold on sovaprevir despite meeting all requirements as laid out in the FDA's clinical hold notification three months ago. The clinical hold stems from an early-stage drug-to-drug interaction trial between sovaprevir and atazanavir in which higher than expected ALT liver enzymes were detected in patients. Achillion noted its disappointment in the FDA's decision but also pointed to three additional clinical-stage drugs to which it can turn to in the meantime.

So what: It's not that hard to understand why this clinical hold is so devastating to shareholders. Achillion represents one of the very few non-nucleotide development programs, yet it can't seem to get its most promising candidate off the ground. Meanwhile, Gilead Sciences' (NASDAQ:GILD) sofosbuvir and AbbVie's (NYSE:ABBV) direct-acting antiviral combo are progressing exceptionally well, with sofosbuvir currently under FDA review and the DAA combo receiving the highly coveted breakthrough designation and currently in phase 3 trials. In other words, even if Achillion gets sovaprevir back, it's still going to make it to market a good two or three years behind these oral and highly effective drugs from Gilead and AbbVie. Achillion's window of opportunity is closing rapidly and shareholders can see that. I would suggest keeping your distance until there's something tangibly positive from this whole situation.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

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