Experimental AIDS vaccine doesn't work; study halted

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    Experimental AIDS vaccine doesn't work; study halted

    Experimental AIDS vaccine doesn't work; study halted

    Sabin Russell, Chronicle Medical Writer
    Saturday, September 22, 2007
    A major international study of an experimental AIDS vaccine was halted by drugmaker Merck & Co. on Friday after an early peek at the results showed that it was not working.


    It was a notable failure for the first of a new class of experimental vaccines that were meant to prevent or sharply limit HIV infection by training the body's white blood cells to attack other cells that have been invaded by the virus.


    "This study represented an important test of a fundamental concept in this field, and unfortunately the results were not what we had hoped," said Dr. Mark Feinberg, vice president of medical affairs at Merck's vaccine unit.
    More than 3,000 uninfected volunteers had taken at least one of the three shots in the immunization regimen before Merck called a halt to the study. Half were assigned the real vaccine, the rest a placebo for comparison purposes.


    The decision came after an independent board, which monitors the safety of clinical trials, took a look at the results halfway through the study. They found, surprisingly, slightly more HIV infections - 24 - among those who had received the actual vaccine than the 21 infections among volunteers who were given a placebo.


    Among a smaller subgroup that had participated longer and received at least two injections, the difference was more stark - 19 infections among the vaccinated, compared with 11 who received the placebo. Although the differences could be the result of bad luck, they hint that the vaccine might be making volunteers more vulnerable to infection.
    "This is sobering, if not disturbing, news," said Dr. Tony Fauci, director of the National Institute of Allergy and Infectious Diseases, which helped pay for the trial.


    Fauci said the results will have to be studied carefully before a green light can be given to another study set to begin in January.


    That study will test a vaccine that is somewhat similar to Merck's. The newer vaccine will also attempt to stir up an immune response among white blood cells known as T-cells, but in addition will try to stimulate antibodies against the virus - a kind of one-two punch that the body often delivers against foreign invaders.


    Because of the findings, a newly launched study that was evaluating the same Merck vaccine using South African volunteers has been temporarily suspended.


    "This is a huge disappointment for all of us who have been involved in the search for an HIV vaccine," said Dr. Glenda Gray, principal investigator for the so-called Phambili trial.


    One factor that might make a difference in the South African trial is that it is being conducted against a different subtype of HIV that is predominant in that country.


    The initial Merck trial that turned up trouble this week began enrolling patients in 2004 and recruited half of its volunteers from the United States, the rest from Canada, the Caribbean and South America. Virtually all those infected were men who had sex with men.


    In South Africa, the HIV epidemic is primarily striking heterosexuals, and young women are disproportionately infected. Researchers believe that it is possible that the vaccine would still be more effective in protecting against heterosexual transmission, but the interrupted Merck study results may make it too risky to test that theory.


    Dr. Keith Gottesdiener, vice president and head of the infectious disease vaccine group at Merck Research Labs, said there had been high hopes for the vaccine because, in smaller initial trials, it generated a "robust" immune response in volunteers.


    That same response likely happened in the larger trial, although further examination of data will be needed to confirm it. That raises a puzzling question: why would a vaccine that stirs up a strong immune response fail to protect patients - and why did those who received the vaccine actually have a higher infection rate?


    Mitchell Warren, executive director of the AIDS Vaccine Advocacy Coalition, acknowledged the study results are disheartening. "It wasn't, by any stretch, our only hope, but it was certainly our leading candidate at this time," he said.


    Warren said that, in another sense, the study was a success - because the goal of research is to find definitive answers, even if they are disappointing ones.


    Because prior efforts to use classical vaccination techniques - stirring up antibodies that prevent infection - had failed with HIV, researchers were hoping that a shot that stimulated white blood cells to fight the virus might work instead. At the very least, such a vaccine might help an infected person keep the virus in check, so he or she would never be ill.
    Unfortunately, the initial study results show that in this case, those who were infected showed no unusual ability to control their infections. The fact that infection rates were higher in the vaccinated group also suggests that activating white blood cells might in fact raise the risk of infection, because HIV is uniquely able to attack white blood cells that have been be aroused to fight viral invaders.


    Mitchell said there is still hope that the immune system can be trained to fight HIV. "All this trial tells us is that this candidate doesn't do it," he said. "This is not the death knell for AIDS vaccines or for cell-mediated immunity."
     
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