"KP-1212 can replace cytidine when the viral enzyme reverse transcriptase is building a new copy of HIV, and pair normally with guanosine. It does not terminate the DNA chain. But KP-1212 was chemically designed to be a flexible molecule, such that it can also look like thymidine and then pair with adenosine. This introduces an error that then is locked into the viral DNA.
These errors happen at random, anywhere in the virus; and when they do not kill the virus outright, they accumulate over generations in the DNA of the viral population. The result is eventually an error catastrophe that can wipe out the entire quasispecies, at least in laboratory tests. If you then take the drug away, the virus does not come back. And the cells on which the virus grew are still alive -- cured of the infection.
AZT and the other approved nucleoside analogs terminate the growth of the DNA chain, killing the copy of virus being built. But that copy is easily replaced by other copies that do not have an abnormal error accumulation, so the population as a whole is not damaged. In contrast, KP-1212 continues to add new errors to the population, in addition to the errors that are already there due to the very high normal mutation rate of HIV."