A two-drug combination being tested by Merck & Co to treat hepatitis C cured 98 percent of previously untreated patients without cirrhosis in a midstage clinical trial, providing the latest evidence that the US drugmaker will be highly competitive in the fast evolving field.

Results of the study called C-Worthy were presented on Thursday at the annual meeting of the European Association for the Study of the Liver (EASL) in London. Researchers are due on Friday to present results of how the Merck pills fared in more difficult to treat patients, such has those who failed to be helped by prior treatments and those with more advanced liver disease.

In the arm of the Phase II study presented on Thursday, 43 of 44 patients treated with 100 milligrams of MK-5172 and 50 mg of MK-8742 once a day for 12 weeks achieved sustained virologic response (SVR), which is considered cured. One patient relapsed, researchers said.

Those who have no detectable levels of the hepatitis C virus in their blood 12 weeks after completing the 12 weeks of treatment are deemed to have achieved SVR.

Current standard treatments take 24 or 48 weeks, cure about 75 percent of patients and involve miserable side effects that have led thousands of patients to put off treatment and wait for highly touted new drugs to become available.

The study also included results of patients treated with the two Merck drugs plus the older drug ribavirin for both 12 weeks and eight weeks. But all eyes will be on the ribavirin-free results as several companies race to produce all-oral treatment regimens that include neither ribavirin nor interferon, which are both used in current treatments and cause flu-like symptoms, anemia and other side effects.

Gilead Sciences, AbbVie and Bristol-Myers Squibb Co are also developing a new generation of all-oral hepatitis C treatments that in previous trials have demonstrated cure rates in excess of 90 percent, while cutting treatment duration to 12 weeks with few side effects.

Gilead, which later this year could have a one pill, once a day two-drug regimen approved in the United States, is widely perceived by Wall Street to be the best of the bunch with some analysts forecasting annual sales of $9 billion or more.

Merck is a bit behind the other three companies in its development timeline, but could prove to be the one that gives Gilead a run for its money as it aims to also produce a one pill, once a day regimen. The AbbVie and Bristol-Myers programs involve more pills and more drugs, but equally impressive cure rates so far in clinical testing.

"Merck has begun a Phase III trial in (previously untreated patients) using one pill, once per day. This should increase everyone's confidence that Merck really has a regimen competitive with Gilead's," ISI Group analyst Mark Schoenebaum said in a research note.

The Merck anti-viral medicines, which hamper the virus' ability to replicate in different ways, were tested in patients who had the genotype 1 form of the virus - the most prevalent and considered the most difficult to treat.

The most common side effects seen with the Merck drugs were fatigue, headache and nausea.

An estimated 170 million people are believed to be infected with hepatitis C worldwide. If left untreated, the progressive disease can lead to cirrhosis, liver cancer or the need for a liver transplant. -Reuters