The Vertex drug had the more impressive results, the data showed.

The results from midstage trials may raise questions about a notification last month from U.S. regulators and Merck that use of the company's Victrelis drug in such "co-infected" patients could lessen the effectiveness of some widely used medicines for human immunodeficiency virus, or HIV.

Both Victrelis, also known as boceprevir, and Vertex's Incivek, or telaprevir, are protease inhibitors designed to block an enzyme that the hepatitis C virus requires to replicate.

Vertex said 74 percent of trial patients treated with Incivek followed by the standard regimen of interferon and ribavirin were free of the hepatitis C virus, or HCV, 12 weeks after ending treatment, compared with 45 percent of patients given interferon and ribavirin alone.

There were no instances of a rebound of HIV for patients in the Incivek trial. Side effects seen more frequently with that drug were itching, headache, nausea, rash, fever and depression. No cases of severe rash were reported.

Merck's mid-stage trial found that 63.9 percent of patients treated with Victrelis and the standard hepatitis C therapy were free of HCV at 48 weeks of treatment, compared with 29.4 percent of patients treated only with interferon and ribavirin.

Two patients on Victrelis and three in the control group had an increase in HIV.

The Victrelis numbers will be updated to reflect the 12-week post-treatment mark at a presentation in Seattle at the Conference on Retroviruses and Opportunistic Infections later on Tuesday.

"The drugs that are used to treat HIV have certain metabolic effects," said Eliav Barr, Merck's vice president, infectious diseases. "You have to be careful not to mess with those drug levels."

Barr estimated that between 15 percent and 20 percent of U.S. HIV patients are also infected with HCV, a liver-destroying virus which has come to be a leading cause of death for HIV patients.

In the first half of this year, Merck plans to start a larger, pivotal trial of Victrelis in a broader range of HIV patients, with those results expected a couple of years later, Barr said. The company is also conducting a number of drug interaction studies.

Vertex said it was currently enrolling patients in a late-stage study of Incivek combination regimens in people also infected with HCV and HIV.

The company also said laboratory studies of Incivek and HIV protease inhibitors had found no harmful effects on antiviral activity when combined with HIV medicines Agenerase (amprenavir) from GlaxoSmithKline Plc, Prezista (darunavir) from Johnson & Johnson's Janssen unit and lopinavir. Slight antagonistic effects were observed on the antiviral activity of Reyataz (atazanavir) from Bristol-Myers Squibb.

Merck said last month that a study among healthy volunteers showed Victrelis as lessening the effect of a combination of HIV drug Norvir (ritonavir) from Abbott Laboratories with one of three other anti-HIV pills: Reyataz, Prezista and Abbott's Kaletra (lopinavir/ritonavir).

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