(Adds details, analyst comments)

By Natalie Grover

Sept 21 (Reuters) - Gilead Sciences Inc is edging closer to the approval of the first drug to fight all forms of hepatitis C as the drugmaker's experimental combination showed high rates of effectiveness in four late-stage studies.

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

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

The main goal of each trial testing the experimental combination was to achieve a cure 12 weeks after the completion of therapy.

Of the 1,053 patients treated with the drug for 12 weeks in three trials, 98 percent were cured.

Data from the fourth trial, in more advanced and weaker patients, showed higher cure rates when patients also received ribavirin, an older antiviral.

Treatment-related side effects caused the death of nine patients in the trial, the company said.

The combination reduces the need for genotyping and potential confusion about which regimen works in which patients, UBS AG's Matthew Roden said.

Gilead's twin blockbuster hep C offerings, Sovaldi and Harvoni, brought in combined sales of about $4.9 billion last quarter.

Harvoni is expected to remain the treatment of choice in genotype 1, given it is taken for eight weeks, JP Morgan's Cory Casimov said.

Gilead's Sovaldi/velpatasvir combination demonstrated significant cure rates after 12 weeks.

William Blair's John Sonnier said the combination along with Gilead's GS-9857 could decrease treatment to six weeks while maintaining pan-genotypic potency.

In July, the FDA approved two new treatments for less common forms of hep C - AbbVie Inc's Technivie for genotype 4 and Bristol-Myers Squibb Co's Daklinza for genotype 3.

"A single, pan-genotypic approach may be particularly attractive in regions outside the United States where genotyping is less routine (or not possible)," Casimov said.

The Sovaldi/velpatasvir combination bodes well for Gilead to penetrate markets outside the United States, where non-genotype 1 hep C is more prevalent, Sonnier added.

The Foster City, California-based drugmaker plans to file marketing applications for the combination by the fourth quarter.

The company's stock, however, fell 2 percent after Hillary Clinton said she will propose a plan to take on specialty drug price gouging. S&P 500 Healthcare index was down 1 pct.

(Reporting by Natalie Grover in Bengaluru; Editing by Anil D'Silva and Don Sebastian)