Gilead Sciences Inc has reached a step closer to the approval of the first drug to fight all forms of hepatitis C. The drug maker's experimental combination has shown high rates of effectiveness in four late-stage studies.

During the trials, a once-daily, fixed-dose combination of the drugmaker's approved hep C blockbuster, Sovaldi, was evaluated with experimental NS5A inhibitor velpatasvir.

The combination is the drug maker's second single-tablet treatment for the viral infection. Gilead's approved drug Harvoni targets the most common form genotype 1 infections.

The key goal of the trials that tested the experimental combination was to achieve a cure 12 weeks after the therapy completes. In all, 1,053 patients were treated with the drug for 12 weeks in three trials, of which 98% were cured.

In the fourth trial, more advanced and weaker patients were treated. Data from this trial showed higher cure rates when patients also received an older antiviral ribavirin. The company said that nine patients died in the trial because of treatment-related side effects.

The twin blockbuster hep C offerings by Gilead, Sovaldi and Harvoni, brought in a total sales of around $4.9 billion past quarter.

JP Morgan's Cory Casimov said that Harvoni will probably remain the treatment of choice in genotype 1, given it is taken for 56 days. Gilead's Sovaldi/velpatasvir combination showed significant cure rates after 84 days.

William Blair's John Sonnier said that the combination along with Gilead's GS-9857 might reduce the treatment to 42 days while maintaining pan-genotypic potency.