TRENTON, N.J. — Two HIV drugs approved last year for patients who have developed resistance to older drugs also work well in new patients, and with fewer troubling side effects than a widely used regimen, according to the drugs' makers.

Late-stage, company-funded studies on the drugs, aimed at winning approval to also market them for previously untreated patients, were presented Sunday at a conference of infectious disease specialists in Washington, D.C.

"There was a desperate unmet medical need for those patients who had failed other therapies" until recent years, said Dr. Robin Isaacs, executive director for infectious disease clinical research at Merck & Co.

He said Merck's Isentress and Pfizer Inc.'s Selzentry, both approved last year, helped address that need, along with three other new drugs: Boehringer Ingelheim's Aptivus and Johnson & Johnson's Prezista and Intelence, made by its Tibotec Therapeutics unit.

"They have all these different options now, which they didn't before, to build new successful regimens," Isaacs said.

Prezista, which had been approved only for previously treated patients, got Food and Drug Administration approval Wednesday to also be sold to new patients as part of a multidrug "cocktail."

The latest research likewise is meant to win such approval:

—Merck plans later this year to seek first-line approval for Isentress, the only integrase inhibitor on the market. It works by blocking the integrase enzyme, which takes DNA from the HIV virus and puts it inside human cells, which then churn out more copies of the virus.

In Merck's study of 563 previously untreated patients, half got Isentress and half got Bristol-Myers Squibb Co.'s widely used Sustiva, with both groups also taking a widely used combination drug called Truvada. After 48 weeks, 86 percent of those getting Isentress and 82 percent on Sustiva had virus levels reduced to undetectable.

Meanwhile, 44 percent getting Isentress and 77 percent taking Sustiva had serious side effects; eight Isentress patients, compared with 18 on Sustiva, stopped taking the drug because of side effects.

—Pfizer presented a new analysis of older data from a 48-week study comparing its Selzentry with Sustiva in a total of 417 patients also getting an HIV drug combination called Combivir. In both groups, 68 percent of patients had the HIV virus reduced to undetectable levels.

Meanwhile, 4.2 percent of those who got Selzentry and 14.2 percent taking Sustiva stopped because of side effects.

This analysis included only patients identified by a newly available test as likely to respond to treatment with Selzentry because they have a specific virus strain found in 50-80 percent of patients.

Selzentry works by preventing the HIV virus from infecting immune system cells via a "door" called the CCR5 co-receptor.

—That same receptor is the target of an experimental drug called vicriviroc from Schering-Plough Corp., which presented a study of 205 people with advanced HIV infection who had received a regimen of other HIV drugs. The patients all got vicriviroc — there was no comparison group — and were followed for up to four years.

The company said the drug showed sustained effect in suppressing the HIV virus and improving counts of crucial CD4 immune cells, and less than 5 percent developed infections. But 14 patients developed AIDS complications and 13 developed various types of cancer.