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Vertex Pharmaceuticals

(VRTX) - Get Report

shares are rebounding Thursday on speculation that prescription growth of its hepatitis C drug Incivek may not be flattening as much as previously feared.

Anadys Pharmaceuticals

( ANDS) also released Thursday new mid-stage data on its experimental Hep C drug setrobuvir, which the company hopes will re-ignite partnership talks. More on Anadys in a moment, first let's tackle Vertex and the case of the missing Incivek mail-order scripts…

Wednesday night, IMS Health issued a bulletin to subscribers explaining that a "significant portion" of mail-order prescription data for Incivek was not reported correctly for the last two weeks of September. IMS used historical mail-order data as a replacement, but the research service conceded that the Incivek prescription numbers reported as a result may not have been entirely accurate.

And those IMS numbers have shown Vertex's

Incivek prescription growth flat for September

, which had investors worried about what was happening following the drug's rocket-like launch this spring. If Incivek prescriptions remain on this flat trend-line, meeting sales expectations for the rest of 2011, and more importantly 2012, could be difficult.

This explains why Vertex's stock price has been so weak lately.

Unless, of course, IMS' prescription data for Incivek, which much of the Street reads on a weekly basis, is totally screwed up and has been under-counting what's really going on with Incivek.

IMS has fixed the problem of the missing mail-order data and will announce a detailed update on Monday, Oct. 17. It's possible that IMS is actually over-counting Incivek prescriptions but on Thursday at least, investors were hopeful and betting that Incivek's numbers are actually better than what's been previously reported.

Hence, Vertex shares are up $2.65, or 6.5%, to $42.91 Thursday.

Vertex, itself, will release actual third-quarter Incivek sales when the company reports earnings on Oct. 27.

Moving to Anadys: The new setrobuvir results from a phase IIb study announced Thursday look okay from an efficacy and safety perspective but the data aren't going to prompt investors to scream "wow" like they are when Pharmasset says anything about its experimental Hep C drugs.

Anadys shares were up just 5 cents, or 5%, to $1.09 Thursday following the data announcement, which further underscores the "meh-ness" of the setrobuvir data.

To be fair, Anadys isn't trying to top



; it just wants to generate enough data on setrobuvir to demonstrate the drug's potential as part of a future combination therapy against Hep C.

Anadys has tried unsuccessfully in the past to find a development partner for setrobuvir

but hopes Thursday's data will help resurrect some interest. The company is already working on a drug-drug interaction study of setrobuvir with an undisclosed company and its own Hep C drug. Whether this early study leads to an actual partnership remains to be seen.

Setrobuvir belongs to the "non-nuc" class of direct-acting antiviral Hep C drugs that has somewhat fallen out of favor compared to the "nuc" class of drugs exemplified by Pharmasset's PSI-7977. Setrobuvir's potency looks good although the drug appears to work more slowly than other drugs, which could cause problems with higher rates of viral breakthrough (or lower cure rates) as patients are treated for longer periods of time.

From a safety perspective, setrobuvir didn't cause as much rash in the phase IIb study compared to what was shown in earlier studies. However, rates of setrobuvir-related rash are still higher so it's not known whether that will cause problems once the drug is combined with other Hep C drugs that also cause rash.

--Written by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback;

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