UPDATE 1-Once-daily Schering hepatitis C drug shows promise

* 85-87 pct have no trace of virus after 12 wks

* Side effects similar to standard 2-drug combo

* Study of once-daily drug continues up to 36 weeks longer

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By Ransdell Pierson

NEW YORK, Nov 2 (Reuters) - An experimental newer treatment for hepatitis C being developed by Schering-Plough Corp showed impressive effectiveness and was well tolerated in an ongoing mid-stage study, researchers said on Monday, citing interim data.

The pill, narlaprevir, is the first once-a-day member of an emerging new class of treatments for hepatitis C called protease inhibitors. They prevent replication of hepatitis C, a virus that can badly harm the liver for decades before symptoms develop and which is believed to infect as many as 5 million Americans.

Narlaprevir was tested among subjects with genotype 1, the most common but most difficult to treat strain of the virus, who had not previously been treated for their infections.

Before taking the new drug, patients in two arms of the study were given "lead-in" treatment with a pair of standard drugs for four weeks -- Schering-Plough's SGP.N Pegintron form of interferon and an oral antiviral medicine called ribavirin.