Genetic differences between the subtypes of HIV prevalent in Africa and the subtype dominant in the United States and Western Europe appear to amplify the effects of drug-resistant mutations in the African strains of HIV, according to a new finding published in the July 9 issue of Biochemistry.
"What we observed at the molecular level might have negative implications for the long-term efficacy of antiviral therapies in patients infected with the African subtypes," said Ernesto Freire, the Henry Walters Professor of Biology in the Krieger School of Arts and Sciences. "If this is the case, the need for second generation therapies acquires renewed urgency."
Freire stressed that the new findings do not imply that AIDS therapies should be withheld from the approximately 28 million patients infected with HIV-A and HIV-C, the African subtypes of the virus.
"On the contrary, they emphasize the need to use existing therapies aggressively to suppress or delay the emergence of resistant mutations until we can develop second-generation therapies," he said.
The most important drugs currently used to treat HIV inhibit two key enzymes in the life cycle of the virus, protease and reverse transcriptase. Mutations in these enzymes can reduce the inhibitors' abilities to form a tight association with them, leading to drug resistance.
The new report builds on a previous study by Freire's research group showing that naturally occurring genetic variations in HIV-A and HIV-C could make it harder for protease inhibitors to do their job.
For the new study, Freire's group inserted into African HIV proteases a mutation commonly found in drug-resistant forms of HIV-B, the strain of HIV prevalent in Western Europe and North America. Their analyses showed that the new, drug-resistant HIV-A and HIV-C proteases were up to 1,000 times more capable of performing their function despite the presence of four common protease inhibitor drugs.
"The genetic variations that exist in the protease of these African strains are not sufficient to cause drug resistance by themselves," Freire said, "but they amplify the effects of drug-resistant mutations and therefore may lead to a faster long-term failure of the therapy."
According to Freire, the sparse clinical data available so far on the effectiveness of antiviral drugs in C and A subtype patients suggest initial responses to the drugs are similar to those achieved in North America and Europe. However, there are also indications that the long-term effectiveness of the drugs may be diminishing more rapidly in HIV-A and HIV-C patients.
"While several reasons have been advanced to explain these results, including lack of patient adherence to the demanding regimen of highly active antiretroviral therapy, the results we're presenting also suggest that a molecular basis for a poor long-term response might be present," Freire said.
Freire has been an outspoken advocate for changing drug design paradigms to better deal with existing and potential genetic variation in disease-causing pathogens. Genetic variation has been a key contributor to the spread of HIV, but drug development work has been focused almost exclusively on HIV-B, which is dominant in some regions but a minority in the worldwide epidemic. Approximately 70 percent of existing AIDS cases are HIV-A and HIV-C cases in Africa, and these subtypes can vary genetically from HIV-B by as much as 30 percent. Freire believes that these factors, together with his research results, make including HIV-A and HIV-C in AIDS drug development efforts a priority.
Earlier this year, in a commentary in Nature: Biotechnology and at the American Chemical Society national meeting, Freire asserted that drug designers should be able to combine detailed analyses of the potential for genetic variation in drug targets with an understanding of the complex thermodynamic factors that affect a drug's ability to tightly associate with its target. The result, he said, would be a new design paradigm that produces pharmaceuticals less vulnerable to drug-resistant mutations.
"Ernesto's work is truly paradigm-shifting because it demonstrates the impact of an understanding of thermodynamics on the whole area of drug resistance," said James Cassatt, director of the Division of Cell Biology and Biophysics at the National Institutes of Health, which funded Freire's research. "The next generation of inhibitors against known targets will benefit enormously from this work."
To further study the interaction between mutation and drug resistance in proteases from A and C subtypes, Freire will soon begin collaborating with scientists in South Africa to obtain HIV genetic sequence data directly from South African patients. This will allow them to characterize different mutations that occur in HIV proteases and then make recombinant forms to test further against inhibitory drugs.
Other authors on the paper were Adrian Velazquez-Campoy, an associate research scientist at Johns Hopkins, and Sonia Vega, a visiting scientist at Johns Hopkins.