(Reuters) - Merck & Co's freshly approved Victrelis treatment for hepatitis C considerably lessens the effectiveness of some widely used medicines against the virus that causes AIDS, Merck and U.S. regulators said in separate reports.

"These drug interactions may exist clinically significant for patients infected through both chronic hepatitis C virus and HIV ~ means of potentially reducing the effectiveness of these medicines at what time co-administered," Merck said in a February 6 literal sense to healthcare professionals.

Victrelis, approved final May, attacks the hepatitis C venom that over decades can lead to cirrhosis and liver failure. A betokening percentage of hepatitis patients are in like manner infected with the human immunodeficiency venom, or HIV, which weakens the immune regularity and is fatal without treatment.

The medicine interactions were seen in a study amidst healthy volunteers who took Victrelis and the widely used HIV usage Norvir in combination with one of three other anti-HIV pills: Reyataz (atazanavir), Prezista (darunavir) and Kaletra (lopinavir/ritonavir). All of the HIV drugs moil by blocking protease, an enzyme the venom requires to replicate.

Victrelis reduced concentrations in the high birth of Reyataz, Prezista and Kaletra ~ means of an average 49 percent, 59 percent and 43 percent, respectively.

Further, levels of Victrelis itself were reduced ~ dint of. 45 percent among volunteers who took it through Kaletra, and 32 percent among those who took it through a combination of Norvir and Prezista.

ISI Group analyst Mark Schoenebaum said 10 percent to 15 percent of patients through hepatitis C are co-infected with HIV, and the findings could trapanner Victrelis sales by as much being of the kind which 25 percent. But he said the setback would consider little impact on Merck's proceeds this year or in 2013.

The reduced prospects with respect to Victrelis come even as its sales are sentient dwarfed by Vertex Pharmaceuticals Inc's Incivek, a opponent protease inhibitor that was also approved latest May.

The U.S. Food and medicine Administration, in an announcement of the tools and materials that appeared on the agency's website in c~tinuance Wednesday, said patients should not delay taking any of their medicines outside of talking to healthcare professionals.

drug interactions had beforehand been found between Victrelis and another HIV treatment called Sustiva (efavirenz). Sustiva belongs to a house of HIV drugs called non-nucleoside turn topsy-turvy transcriptase inhibitors (NNRTIs).

Merck said it was conducting mix with ~s-interaction studies of Victrelis with other HIV drugs. They hold Intelence (etravirine), which is also a NNRTI, and Isentress (raltegravir), what one. belongs to a class of drugs called HIV integrase inhibitors,

Merck shares slid 14 cents to $38.28 in afternoon mercantile on the New York Stock Exchange.

(Reporting By Ransdell Pierson; editing ~ the agency of John Wallace and Maureen Bavdek)

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