Question

why is it important to look for a new class of drug for HIV rather than a new drug that acts in a known manner?

Answer 1

No known class of drug kills viruses, whereas a new class of drug might do so.

Answer 2

HIV usually adapts quickly to many known drug treatments through natural selection. Quickly, in fact. When someone takes a drug currently in the market, it kills of many of the viruses -- the ones susceptible to drugs. However, because so many are formed, some are randomly resilient to the drug. When you kill off the weak ones, the strong ones remain and reproduce.

Answer 3

^ and when the types of drug are the same, and the virus has already developed a resistance to that type of drug, you simply create more resistant virus. This explains why, when you take someone off HIV medication for a while and put them back on it, the drugs are effective again- the new virus that has multiplied since then may have mutated to a point that it is no longer resistant.