The latest data from an ongoing trial show that a cocktail treatment using Merck's experimental drug MK-0518 achieved a reduction in viral load comparable to Bristol-Myers Squibb's efavirenz combo in HIV-infected patients.

Combined with antiretroviral drugs commonly used in AIDS cocktails, MK-0518 controls HIV well, the company reported.

Details of the latest trials of MK-0518 will be presented at an international AIDS conference in Toronto that begins this weekend.

The enzyme integrase is one of three avenues traveled by HIV during its cycles of replication. Up to now, there has been no drug that targets the integrase path.

Scientists compared results using two different cocktail treatments in 190 HIV-infected patients over a 24-week period. One group took MK-0518 in combination with Gilead's tenofovir, sold as Viread, and GlaxoSmithKline's lamivudine, sold as Epivir.

The other group took a similar cocktail, with Bristol Myer Squibb's efavirenz, sold under the brand names Sustiva and Stocrin, in place of the MK-0518.

Both cocktails acted as strong HIV suppressants, the scientists said, but the combination that included MK-0518 seemed to work faster in some patients.

"This early study showed a rapid and significant reduction in viral load up to 24 weeks with MK-0518," said study leader, Dr. Martin Markowitz, clinical director and staff investigator for the Aaron Diamond AIDS Research Center in New York. "This study should lend further insight into the potential of HIV integrase inhibitors as a new and exciting class of antiretroviral agents."

Scientists are enthusiastic about the drug's potential because HIV patients become resistant to therapy over time. That happens because HIV does not always make perfect replications of itself. With billions of viruses being created every day, many small, random mutations occur. Mutations can change the parts of the virus that a particular drug targets, rendering it ineffective. The mutated virus continues to replicate with abandon and eventually takes over, rendering the patient drug resistant.

That is why cocktails using different drugs in different combinations have been effective in treating many HIV patients. However, these treatment advances mean they are now living longer than ever, and new approaches are needed for patients who have become resistant to all the available drug regimens

Side-effects of MK-0518 include diarrhea, headaches and insomnia. One patient left the study because of negative effects.

Phase 3 trials involving 700 participants are planned. Merck has not indicated when the experimental drug might reach the market.