This has been a productive week for drug developer Vertex Pharmaceuticals (NASDAQ:VRTX). On Monday, it reported positive results from a phase 1 clinical trial for its potential leukemia treatment VX-680, which is partnered with Merck (NYSE:MRK); the drug will now move into phase 2 clinical development. Yesterday, Vertex announced interim results from a phase 2 trial for telaprevir, its developmental candidate for hepatitis C.

The telaprevir results are much more significant to Vertex, since the drug is much further along in development, and being tested in a potentially much larger patient population. Initially, when the clinical trial results were announced, shares dropped nearly 8%. That was surprising, considering that the drug's efficacy in this phase 2 trial was consistent with its performance in earlier but much smaller trials of the drug.

In this interim statistical analysis of one of its pivotal phase 2 trials, telaprevir plus standard care after 12 weeks of treatment left 88% of patients with undetectable levels of the hepatitis C (HCV) virus. The control arm of the trial only saw 52% of patients achieve this result. If these results hold up in the long run (which they usually do -- the pivotal twelfth week of treatment is a great predictor of a longer-term cure), telaprevir would be significantly more efficacious than the approximate 50% long-term cure rates seen for other genotype 1 HCV treatments.

Importantly, this early response in such a large group of patients also holds the potential for those who take telaprevir to endure debilitating HCV drugs for a much shorter amount of time than the 48 weeks of treatment that most patients for genotype 1 HCV must undergo. This alone will provide a huge potential market for telaprevir.

The reason for the negative initial market reaction to the trial results was probably related to the higher dropout rates and adverse events seen in the telaprevir arm of the trial, compared to the placebo arm. A reported 3% of patients taking telaprevir experienced serious adverse events, compared to only 1% of patients in the placebo arm. But with so few patients in the placebo arm, it's way too early to call these results meaningful.

The higher level of adverse events seen in those taking telaprevir is obviously not good news, but given the significantly higher efficacy levels for the drug, they're not even close to stopping telaprevir from becoming one of the dominant forms of treatment for patients with genotype 1 HCV. Considering that as much as 25% of people infected with chronic HCV develop liver cirrhosis or liver cancer within 20 years of being infected, provided the disease progresses unchecked, few patients should have a problem taking their chances with telaprevir.

There's still plenty of time left in the various arms of the telaprevir trials for further light to be shed on the drug's efficacy and safety. If telaprevir continues to show anything like the 80%-90% rate of undetectable HCV virus at end of its trials (and after the important six-month follow-up, after which patients can be considered cured), Vertex shareholders won't be dealing with too many more down days like yesterday.

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Merck is an Income Investor recommendation.

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Brian Lawlerdoes not own shares of any company mentioned in this article. The Fool has adisclosure policy.