On Friday, shares of Vertex Pharmaceuticals (NASDAQ:VRTX) were up nearly 18% to more than $40 a share on the release of more positive data from some of the company's clinical trials testing its protease inhibitor, VX-950 (now named Telaprevir) as a treatment for genotype 1 hepatitis C (HCV).

The most exciting part of the additional data Vertex will present at the American Association for the Study of Liver Diseases this week is that five out of six patients who received Telaprevir plus another drug, pegylated interferon, in a phase 1b clinical trial had undetectable levels of hepatitis C 12 weeks after completing their treatment.

This event is significant because in order to gain approval, drugs to treat HCV need to show that they are effective in keeping HCV levels at an undetectable level for six months (24 weeks) after cessation of treatment. This is called a sustained virologic response (SVR) and is important because it is a good prognosticator of long-term control and sometimes "curing" HCV. Multiple meta-analyses have consistently showed that SVRs for various therapies to treat genotype 1 HCV have not been above 46%. Therefore, more than half of patients with this genotype (around 75% of all cases of HCV in the U.S.) are not effectively being treated for their disease.

To put this in perspective, it is estimated by the World Health Organization that 1.8% of the 300 million people in the U.S. have hepatitis C. If 75% of these people have genotype 1 and 54% are not effectively being treated for their disease, then that means more than 2 million HCV sufferers in the U.S. alone need a new treatment to help their disease. That's a huge market opportunity for Telaprevir.

These Telaprevir results are obviously from a very small trial, and are only halfway to that important point in time when an SVR can be claimed. But if the 24-week results from this study are anything close to the 83% of patients who showed undetectable levels of HCV through week 12, then Telaprevir plus pegylated interferon would most likely become the dominant treatment for genotype 1 HCV patients. It could even possibly shorten the time that HCV patients need to be treated.

Vertex is rapidly progressing Telaprevir through several phase 2 trials, and if strong results like these can continue to hold up, then the company should file for drug approval in 2008. Until that time, investors in Vertex just have to sit back and continue to monitor its clinical trial results to make sure that they continue to hold up in these later trials. If future results remain as solid as these, there will be many more 18%-plus days ahead for Vertex shareholders.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy.