WASHINGTON, April 2 (AFP) - The US pharmaceutical company Merck and Co. Inc. has begun testing on humans an experimental HIV vaccine based on deoxyribonucleic acid (DNA), the company said Monday.
Merck had not officially announced the beginning of the clinical trials which were launched in February because it wanted to "temper the level of enthusiasm that people have because it is just very early on," said company spokesman Gregory Reaves.
For now, "the vaccine is testing for immuno-geneticity," a phase which could last up to three years. The company wants to be sure the vaccine does not pose any health problems to humans. The vaccine's clinical effectiveness will only be assessed in a second phase, Reaves added.
Merck research teams have been working for several years on a new approach for a vaccine against the HIV virus that causes AIDS, one which would allow patients to avoid the currently avaialable but expensive three-drug anti-retroviral combination treatments.
While anti-retroviral drugs are effective at suppressing HIV replication in infected individuals, they do not eliminate the infection and have toxic side effects that impede their long-term use.
Merck's idea is to reinforce the patient's own immune system and prevent the virus from multiplying by injecting a vaccine carrying pieces of the virus' DNA.
Laboratory tests, first in test tubes then on monkeys, have shown that although the vaccine does not prevent initial infection it manages to prevent the virus from multiplying in infected monkeys before full-blown AIDS develops.
The researchers also are working on stimulating the production of a specific kind of white blood cells, known as CD8 or "killer cells" because of their ability to destroy cells infected by the virus.
About two years ago, the researchers realized that the production of these so-called "killer cells" was stimulated by the presence of proteins made by three of the HIV genes, including one deemed the most effective and dubbed Gag.
They then developed a vaccine containing bits of these genes in a technique called "naked DNA," and began injecting monkeys. Results of these first experiments, published in the October 2000 issue of Science, showed that among vaccinated monkeys the virus remained at indetectable levels in the blood.
Researchers then used a new vector for their vaccine, stitching bits of the virus' DNA to an adenovirus, a microbe that can cause the flu but had been rendered harmless first.
The first results were presented at the Keystone Symposium, "AIDS Vaccines in the New Millennium" on March 30 in Keystone, Colorado, and were considered encouraging.
Six monkeys were vaccinated then infected with SIV, the monkey form of HIV. Eight months later they were all still healthy. The vaccine did not stop the initial infection but did manage to lessen the viral load and stop the multiplication of the virus. Four of six other monkeys who were not vaccinated died.
"We're seeing a more significant level of suppression of the HIV virus," Reaves said.
Tests on humans, which are underway in several medical centers across the United States, are aimed at discovering whether this type of DNA-based vaccine attached to an adenovirus could result in a similar immune response.
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