Rival Abbott Laboratories Inc, meanwhile, said a trio of its oral medicines to treat hepatitis C produced unprecedented cure rates in a larger number of patients who had failed to benefit from standard treatment, as well as very high cure rates for newly treated patients.

Data from both companies' mid-stage trials were released Saturday at the annual meeting of the American Association for the Study of Liver Disease in Boston.

Gilead's study, dubbed Electron, examined 25 patients with genotype 1 chronic hepatitis C virus (HCV) infection who were treated for 12 weeks with a combination of three drugs: sofosbuvir, ribavirin and GS-5885.

GS-5885 is from a promising new class of drugs known as NS5A inhibitors, which prevent the hepatitis C virus from replicating.

The infection was undetectable four weeks after completing therapy in all of the patients who had never received this combination of drugs before, Gilead said.

The drugs generally were well tolerated in the study, Gilead said.

In the sofosbuvir combined with GS-5885 and ribavirin patient groups, one patient dropped out because of an adverse side effect that the company said was unrelated to the drugs.

Sofosbuvir and GS-5885 are still being studied for their safety and efficacy.

The biopharmaceutical company will present the data on Tuesday at the annual meeting of the American Association for the Study of Liver Diseases in Boston.

Mark Schoenebaum, a biotech analyst with ISI Group, said in a research note that he expects Gilead shares to rise on Monday based on these "best case" results.

UBS analyst Matthew Roden said "these data strongly support Gilead's leadership position" in the hepatitis C virus space.

Gilead recently started the first Phase 3 trial evaluating a fixed-dose combination of sofosbuvir and GS-5885 in patients with genotype 1 chronic hepatitis C virus infection who had not received these drugs before.

This study is evaluating the fixed-dose combination with and without ribavirin for 12- and 24-weeks in 800 patients, 20 percent of whom have evidence of cirrhosis, or liver scarring.

(Reporting By Debra Sherman; Editing by Vicki Allen)

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