Researchers said this newest weapon against the AIDS virus -- approved by the Food and Drug Administration in June -- gives patients who had run out of options in the battle with the deadly infection a new opportunity to hold HIV at bay.

"These studies show that treatment with TMC114(the former designation for Prezista) provides sustained antiretroviral efficacy and tolerability for these treatment-experienced patients," said Sharon Walmsley, associate professor of medicine at the Toronto Hospital, University Health Network in a presentation Tuesday at the 16th International AIDS Conference in Toronto

Walmsley pooled the results of two phase 4 clinical trials, studies which were used by Tibotec of Belgium to gain licensing of Prezista in the United States.

She said that she and her colleagues enrolled 131 patients and randomly assisgned them to Prezista. Their outcomes were compared with a group of patients who were treated with the best medications available. In order to receive the best possible treatment, genotype testing was conducted to determine which drugs were likely to work against the resistant virus.

After 48 weeks of treatment, 61 percent of the patients taking Prezista had reduced the level of virus in their blood by more than 90 percent compared with 15 percent of patients who were not taking the new protease inhibitor.

In addition, Walmsley said that 46 percent of the patients on Prezista had undetectable levels of virus in their blood, compared with 10 percent of those not taking the drug

"This drug looks very good," Stefano Vella, former president of the International AIDS Society, told United Press International. "Use of darunavir with other drugs will not only allow use to 'salvage' patients who have failed everything else, but when we use it with other drugs such as Fuzeon (enfuvirtide, it think we are going to sustain these heavily-experienced patients for a long time."

Vella, director of the department of drug research and evaluation at the Instituto Superiore di Sanita, said that darunavir and other third-generation protease inhibitors give doctors the ability to construct three and four lines of effective treatment for these patients.

The U.S. Food and Drug Administration granted accelerated approval to Prezista, which is combined with a low level of the protease inhibitor ritonavir (Norvir). The strategy of adding Norvir to protease inhibitors employs the ability of Norvir to slow the metabolism of other protease inhibitors, allowing for greater concentrations of the drugs to remain in the blood where they interfere with the lifecycle of the virus.

Further studies with the drug will be required for it to receive standard FDA approval.

HIV, a retrovirus, enters immune system cells and converts them into factories to produce more virus. By preventing the immune cells from functioning to protect the host from invaders, HIV infection allows for subsequent opportunistic infections that are the hallmark of AIDS, and are the infections that can be fatal.

Prezista and recent other third-generation protease inhibitors are designed to specifically attack virus that has been able to mutate into strains that are resistant to other drugs. All the patients in the study that Walmsley presented harbored viruses with multiple mutations, including major mutations that eliminate at least one of the classes of drugs used to fight HIV.

The studies reported by Walmsley also indicated that Prezista has a similar side effect profile as the patients who were taking the best optimized treatment available without Prezista.

While Prezista was effective in reducing the level of virus in most patients, those individuals who were the most heavily pretreated -- that is, those who have been under treatment the longest and had the most resistant strains of virus -- did poorer than those with fewer mutations. "Overall, the more experienced in treatment the patients were, the less well they did with Prezista," Walmsley said.

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