In a highly speculative sector like small-cap biotech, Wall Street may not walk softly, but it certainly carries an exceptionally big stick. And today the Street focused on two stocks with hepatitis-C candidates in development, lavishing praise on one and throwing scorn on the other. Let's jump in and take a closer look at the logic behind the upgrades and downgrades and see what price movements those calls wrought.

To make sense of the winner, let's start with today's loser, Idenix Pharmaceuticals (Nasdaq: IDIX), which plunged 10%, after Bank of America cut its price target on the stock by 25% to $6 per share while dropping its "buy" rating. What was left for B of A to do? Both of Idenix's nucleotide inhibitors, including the all-important IDX184, are stuck on clinical hold for the forseeable future.

Idenix shareholders can thank Bristol-Myers Squibb (NYSE: BMY) for their troubles. The pharma giant ran into serious safety issues with BMS-986094, including a fatality and several hospitalized patients, and discontinued the drug. The problem for Idenix is that its drug is in the same class as Bristol's, and although Idenix hasn't had any serious issues, the FDA is playing it safe until Idenix can argue otherwise. Bristol appears willing to help Idenix discover what went wrong with BMS-986094, but don't think that collaboration will lead to any sort of buyout. Bristol is still wiping plenty of egg off its face for blowing $2.5 billion on a drug that didn't even make it a year in the pipeline.

Given all that, it's no surprise that Wall Street's new darling, Achillion (Nasdaq: ACHN), was up 15% today. After fears that Achillion was left behind when the M&A in the space slowed down, the company has kept plugging away -- and now with Bristol's stumble, it could get a second look. Its lead hep-C fighter is in a different class and hasn't had any safety issues, demonstrating solid efficacy data so far. Both Bank of America and Deutsche Bank took notice, labeling it a "buy," with B of A nearly doubling its price target to $13 and Deutsche setting it at $12 per share.

Now, it's incredibly early, but if everything goes right, Achillion is developing all the parts of a treatment cocktail that could potentially compete with what Gilead (Nasdaq: GILD) is cooking up in-house. So far, Gilead's GS-7977 has aced every test with flying colors and will be the first to challenge the current standard of care Vertex's (Nasdaq: VRTX) Incivek. But Big Pharma isn't going to let Gilead keep the market to itself.

Just don't expect a buyout for Achillion anytime soon. Although Gilead appears to have succeeded in buying a blockbuster where Bristol failed, executives are generally risk-averse. As Achillion continues to advance its drugs without stumbles, a partnership with a deep-pocketed company looking to stake a claim in the space is the more likely outcome.

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