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* Tested in genotype 2/3 patients who failed prior therapy
* Cure rate 73 pct for 16 weeks, 50 pct for 12 weeks
* Gilead seen seeking US, European approval in 2nd quarter
* Shares up 1.7 percent
Feb 19 (Reuters) - Gilead Sciences Inc said its experimental hepatitis C drug sofosbuvir, in a late-stage clinical study, was clearly superior to historical cure rates at both 12 and 16 weeks of treatment in patients who were not helped by prior therapy.
The cure rate for those with genotype 2 and 3 of the serious liver disease was significantly higher for patients who received 16 weeks of treatment: 73 percent versus 50 percent for those treated for 12 weeks, according to study results released on Tuesday.
Both rates were far higher than the comparable 25 percent historic cure rate - or sustained virologic response (SVR) - for these types of patients.
"Fusion is the fourth Phase III trial of sofosbuvir that has met its primary endpoint with no significant adverse events," Sanford Bernstein analyst Geoffrey Porges said in a research note.
"We believe these results represent the final piece of de-risking for the asset, and approval is as close to certainty as it can be," he added.
In the latest trial, genotype 2 patients fared better than those with genotype 3, which is more difficult to treat. Rates of SVR, which is considered tantamount to a cure, were also lower in patients suffering from cirrhosis, typically among the sickest of hepatitis patients.
The study tested sofosbuvir in combination with the older oral hepatitis treatment ribavirin in patients who did not respond to prior treatments with ribavirin and injectable interferon.
No patients in the study discontinued treatment due to adverse side effects.
Fatigue, headache, insomnia and nausea were the most common adverse effects, reported in less than 15 percent of patients, Gilead said.
"With positive results from all four Phase 3 trials now in hand, Gilead is on track to meet its goal of filing regulatory applications in the United States and Europe in the second quarter," Gilead Chief Scientific Officer Norbert Bischofberger said in a statement.
Earlier this month, Gilead said the drug combination also achieved the main goals set in two other late-stage trials of patients who had not received prior treatment.
Sofosbuvir also showed promise in a late-stage study called Positron in patients who were unable or unwilling to take interferon, a standard hepatitis C drug known for its unpleasant side effects that lead many patients to delay or discontinue treatment.
With analysts estimating a multibillion-dollar annual market, new interferon-free treatment options for hepatitis C are among the hottest areas of pharmaceutical research.
Several other companies, including Abbvie Inc, Bristol-Myers Squibb Co and Vertex Pharmaceuticals Inc , are developing interferon-free treatment combinations for hepatitis C, with most focusing on by far the most common strain of the virus, genotype 1.
ISI Group analyst Mark Schoenebaum said Gilead needs to find ways to improve cure rates in Genotype 3 patients, possibly by combining sofosbuvir with other experimental drugs in its pipeline.
"Competition in the Genotype 2/3 space is sparse, so even with these data, Gilead probably has many years to optimize before they will face a serious threat," he said.
The SVR rates for genotype 3 was 30 percent with 12 weeks of treatment and 62 percent after 16 weeks of therapy. That compared with SVR rates of 86 percent and 94 percent for genotype 2.
Gilead said it will present full data from the four Phase III studies at a future scientific conference.
Shares of the company were up 70 cents, or 1.7 percent, at $42.30 in midday trading on the Nasdaq on Tuesday.