Sofosbuvir (brand names Sovaldi and Virunon) is a drug used for hepatitis C, with a high cure rate.

On 6th December 2013, the U.S. Food and Drug Administration approved Sovaldi for the treatment of chronic hepatitis C
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Mechanism of Action:Sofosbuvir is a prodrug that is metabolized to the active antiviral agent 2'-deoxy-2'-α-fluoro-β-C-methyluridine-5'-monophosphate. Sofosbuvir is a nucleotide analogue inhibitor of the hepatitis C virus (HCV) polymerase. The HCV polymerase or NS5B protein is a RNA-dependent RNA polymerase critical for the viral cycle.

Clinical studies:
Sofosbuvir is being studied in combination with pegylated interferon and ribavirin, with ribavirin alone, and with other direct-acting antiviral agents. It has shown excellent clinical efficacy when used either with pegylated interferon/ribavirin or in interferon-free combinations. In particular, combinations of sofosbuvir with NS5A inhibitors, such as daclatasvir or GS-5885, have shown sustained virological response rates of up to 100% in people infected with HCV.
Data from the ELECTRON trial showed that a dual interferon-free regimen of sofosbuvir plus ribavirin produced a 24-week post-treatment sustained virological response (SVR24) rate of 100% for previously untreated patients with HCV genotypes 2 or 3.
Data presented at the 20th Conference on Retroviruses and Opportunistic Infections in March 2013 showed that a triple regimen of sofosbuvir, ledipasvir, and ribavirin produced a 12-week post-treatment sustained virological response (SVR12) rate of 100% for both treatment-naive patients and prior non-responders with HCV genotype 1. Gilead has developed a sofosbuvir + ledipasvir coformulation that is being tested with and without ribavirin.

Sofosbuvir and Ribavirin Succeed in Hard-to-Treat Hep C Group
Between half and two-thirds of a group of people with hepatitis C virus (HCV) who had various attributes weighing against their success with treatment, including liver damage, were cured of the virus with a regimen of Gilead’s sofosbuvir and ribavirin, HIVandHepatitis.com reports. Publishing their findings in JAMA, The Journal of the American Medical Association, researchers studied 60 treatment-naive participants with hep C who all had the hard-to-treat genotype 1 of the virus.

In the spring, the U.S. Food and Drug Administration (FDA) granted priority review status to sofosbuvir, a nucleotide analog hep C polymerase inhibitor. The agency’s decision on the drug’s approval is expected in December.

The Phase II, open-label study had two parts. In the first, which was a proof-of-concept study, 10 people who had liver fibrosis that ranged from absent to moderate in severity received sofosbuvir plus weight-based ribavirin. In the second part, 50 participants who had all the various stages of liver disease were randomly and evenly divided into two groups. Both received sofosbuvir, while one received weight-based ribavirin and the other received a lower dose of 600 milligrams per day of ribavirin, irrespective of their weight.

Seventy percent had the hep C genotype subtype 1a, which is more difficult to treat. More than 80 percent were African American, a racial group that tends not to respond well to interferon-based therapy when compared with whites.

Following 24 weeks of treatment for all study participants, nine out of the 10 participants in the first group completed the trial and all of them (90 percent) achieved a sustained virologic response 24 weeks after completing therapy (SVR24, considered a cure). In the second group, 68 percent of those in the weight-based ribavirin subgroup achieved SVR24, while 48 percent were cured in the low-dose ribavirin arm, although this difference was not statistically significant, meaning it could have occurred by chance.

Sofosbuvir proved generally safe and well tolerated. The most common side effects included headache, anemia, fatigue and nausea, primarily mild to moderate in severity. There were seven incidents of severe side effects, including anemia, neutropenia (diminished levels of certain key white blood cells), nausea, elevated blood phosphate, gallstones and pancreatitis.