The company, which recently paid nearly $11 billion to acquire the drug and its developer, Pharmasset, said last month that six out of 10 patients with a prior "null response" to standard hepatitis C therapy saw the virus return within four weeks of treatment with a combination of GS-7977 and the antiviral drug ribavirin.

The latest results from the mid-stage trial bring to eight the number of patients who have relapsed.

One patient has not reached the four-week point and the other showed a response to the drug, Dr. Edward Gane from New Zealand's Auckland City Hospital and the study's lead investigator said here at the Conference on Retroviruses and Opportunistic Infections.

The trial segment he updated involved patients infected with genotype 1 hepatitis C -- the most common, but also the most difficult to treat, subset of the disease.

"The majority of null responders have relapsed post treatment," Dr. Gane said, adding that such patients will likely need either a longer duration of therapy or combination treatment with other direct-acting antiviral agents.

He said the relapsed patients will be offered a "rescue protocol" in the form of another trial of GS-7977 in combination with a different experimental drug.

Shares of Gilead have dropped 18 percent since the company's announcement of the trial results, which suggested that an all-oral treatment for hepatitis C may be further away than many had hoped.

Dr. Gane said ongoing pivotal-stage trials of GS-7977 "should establish that interferon-free treatment is not a dream. It's a reality that should be here within the next five years."

Current hepatitis C drug regimens require injections of interferon, which causes severe flu-like symptoms and cannot be tolerated by some patients.

GS-7977 is designed to block an enzyme essential to the replication of the hepatitis C virus. It is one of a new class of treatments designed to be given without injections of interferon, which helps boost the body's immune system but can also cause debilitating flu-like side effects.

Gilead has said it expects to announce at the end of this month results from a mid-stage trial of GS-7977 in previously untreated hepatitis C patients.

Hepatitis C is a liver-destroying disease that affects some 170 million people worldwide. Untreated, it can lead to cirrhosis, liver cancer and the need for a liver transplant.

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