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The only other anti-HIV drugs approved in the U.S. -- DDI and DDC -- are variations on the AZT theme. Researchers have begun examining other types, however. One variety targets the gene that codes for another enzyme, protease, that is crucial to the manufacturing of viral proteins. The research looks promising, but a breakthrough is not expected anytime soon.
The same adaptability that makes HIV so troublesome to drug designers threatens to stymie vaccine development as well. Researchers are not at all confident that they can devise a simple series of shots that would give a person lifetime protection against AIDS. To do that, a vaccine would have to ward off all of HIV's current strains as well as any future mutants.
Neutralizing HIV is especially tough because its coat is laced with sugar molecules that shield it from the human immune system. Some viruses, such as the one that causes polio, have no sugar in their protein coat. Others, like flu viruses, have only a little. It is no coincidence that the most effective vaccines have been made to fight these kinds of viruses. Never before have scientists tried to devise a vaccine against a pathogen as well protected as HIV.
Undaunted, researchers are testing about a dozen experimental vaccines. After the trials have been thoroughly evaluated, the most promising prototypes will be chosen -- probably in the next two years -- for testing to determine if they can stimulate the immune system to produce antibodies capable of blocking HIV infection. The trouble is that scientists can only guess at what constitutes an effective collection of AIDS antibodies. No one has ever survived the disease to provide researchers with any clues. Even if the experiments go well, a preventive vaccine will probably not be available before the end of this century.
In the meantime, Dr. Robert Redfield of the Walter Reed Army Institute of Research in Washington and his colleagues are trying to develop a vaccine that helps people who are already infected. By injecting a slightly modified form of the virus' protein coat, the Army researchers hope to kick-start the patients' immune systems into mounting an effective counterattack. Redfield thinks that his version of the viral coat may share enough characteristics with all the known mutant strains of HIV to overcome the variability problem. Said Redfield, a rare, unabashed optimist at the Amsterdam meeting: "I believe HIV is very simple, very straightforward, and it's going to be solved."
THE EVOLVING EPIDEMIC
One of the most baffling enigmas of AIDS is the fact that the disease spread primarily among homosexual and bisexual men and intravenous drug abusers in the U.S. and Europe but became a largely heterosexual infection in Africa. Researchers announced last week that they may have an answer. Based on a study of the newly emerging epidemic in Thailand, they concluded that HIV has shown predilections for different human host cells in different parts of the world.