HIV: The Ultimate Evolver (3 of 3)

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3. How can we control HIV’s evolution of resistance to our drugs?
HIV evolves so quickly that it evolves right out from under our treatments. When a patient begins taking an HIV drug, the drug keeps many of the viruses from reproducing, but some survive because they happen to have a certain level of resistance. Because of HIV’s speedy evolution, it responds to selection pressures quickly: viruses that happen to survive the drug are favored, and resistant virus strains evolve within the patient, sometimes in just a few weeks. However, basic evolutionary theory points out a way that this evolution of resistant viral strains can be delayed. Patients are prescribed “drug cocktails”—several different HIV drugs taken together.

When taking any single drug, it is fairly likely that some mutant virus in the patient might happen to be resistant, survive the onslaught, and spawn a resistant lineage.

But the probability that the patient hosts a mutant virus that happens to be resistant to several different drugs at the same time is much lower. Although multiple-drug-resistant HIV strains do eventually evolve, drug cocktails delay their evolution.

An evolutionary trade-off.
If a patient is already infected with a drug-resistant HIV strain, basic evolutionary theory has also pointed out a way to make the drug useful again. Studies of the evolution of resistance often show that you don’t get something for nothing. Specifically, it “costs” a pest or pathogen to be resistant to a pesticide or drug. If you place resistant and non-resistant organisms in head-to-head competition in the absence of the pesticide or drug, the non-resistant organisms generally win.

Consider a patient who takes a particular drug and winds up with viruses resistant to the drug. If the patient stops taking the drug for a while, evolutionary theory predicts that her viral load will evolve back towards a non-resistant strain. If she then takes very strong doses of the drug, it may be able to halt the replication of those non-resistant viruses and reduce her viral load to very low levels.

This therapy has shown early, promising results—it may not eliminate HIV, but it could keep patients’ virus loads low for a long time, slowing progression of the disease.

Ultimately, understanding the evolutionary history of HIV and its pattern of evolutionary change may help us control this disease.

Huntington’s Chorea