NEW YORK – Schering-Plough Corp Wednesday said it has begun two late-stage trials of its experimental boceprevir treatment for hepatitis C, lifting its shares and bringing down those of rival drugmaker Vertex Pharmaceuticals Inc.

The new medicine works by blocking the so-called protease protein that the hepatitis C virus requires to reproduce itself, the same mechanism used by Vertex' experimental drug telaprevir which is further along in development.

Schering-Plough and Swiss drugmaker Roche Holding AG sell similar pairs of standard treatments for the liver-harming disease – an injectable form of interferon and an antiviral pill called ribavirin.

But protease inhibitors hold the promise of eliminating the virus from the bloodstream in perhaps half the time of current treatments, which now take 48 weeks and produce hard-to-tolerate flu-like symptoms.

Schering-Plough said its planned Phase III trials will study boceprevir in combination with PegIntron – the company's widely used interferon – and ribavirin. That triple-drug combination will be tested against PegIntron and ribavirin alone.

The trials will involve a total of more than 1,400 patients infected with the genotype 1 strain of hepatitis C – the most common and most difficult to treat form.

Vertex began its own Phase III trials of telaprevir in March, and data are expected to be available by mid-2010.

Shares of Schering-Plough were up 56 cents or 2.9 percent to $19.82 in morning activity on the New York Stock Exchange. Vertex shares were down $2.03, or 7.4 percent, to $25.45 on the Nasdaq.

(Reporting by Ransdell Pierson, editing by Gerald E. McCormick)