It's usually pretty easy to tell when a drugmaker announces positive clinical trial results by the rise in its shares after that data is released. Shares of Pharmasset (NASDAQ:VRUS) were up 15% yesterday after it announced the first efficacy data on its hepatitis C virus (HCV) drug candidate R7128.

The first viral load reduction results for R7128 were fairly impressive. Pharmasset tested the drug in eight patients with genotype 1 HCV (the most common subtype in the U.S.) who had previously failed other therapies. [If you're a Fool who revels in detail, more about the study results can be found at the end of this article.]

Based on the results, Pharmasset and partner Roche plan to expedite the development of R7128. Rather than waiting for the second quarter of 2008 -- as previously planned -- to initiate combination studies of R7128 with Roche's Pegasys and Ribavirin, the partners will commence these combo studies in the next couple of weeks. Look for R7128 to perform even better on the efficacy side of things due to the combination treatment with Pegasys, longer 28-day treatment duration, and use in patients who have never been treated with an antiviral treatment. 

Other potent HCV oral antivirals have been derailed in the past due to problems with their safety. If R7128 can keep away from these safety issues it looks like Pharmasset has a potential hit on its hands.

I recently highlighted Pharmasset as a biotech bet that investors should keep their eyes on due to the potential of R7128 and its other HCV and HIV drugs in development. Investors should watch closely the expanded data set from this phase 1 study that Pharmasset presents at the upcoming American Association for the Study of Liver Diseases medical conference in the first week of November.

As promised, a feast of details
After 14 days of treatment, patients who received the highest dose of R7128 experienced a 2.7 log reduction in the amount of hepatitis C virus in their bloodstream. In comparison, Vertex Pharmaceuticals' (NASDAQ:VRTX) exciting Telaprevir produced a 4.4 log viral load reduction after 14 days in phase 1 testing, and ViroPharma's (NASDAQ:VPHM) non-nucleoside polymerase inhibitor (which ultimately failed due to safety concerns) produced between 2.6 and 3.2 log viral load reductions in combination with pegylated interferon for genotype 1 patients after 14 days of treatment.

Vertex Pharmaceuticals is an active pick of our market-beating Rule Breakers newsletter. You can check out all our recommendations with a 30-day free trial.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool's disclosure policy is an asset.