Figure 2.

Competitive and non-competitive mechanisms of resistance to coreceptor inhibitors. In a competitive mechanism of resistance (A), HIV acquires the ability to bind unoccupied coreceptors more efficiently, and in this case there is an IC₅₀ shift in a dose response curve relative to a sensitive virus. Alternatively, in a non-competitive mechanism of resistance (B), the virus adapts to enter using a drug-bound conformation of the coreceptor, resulting in a dose-response curve where no further inhibition of virus entry occurs beyond a certain drug concentration (“plateau effect”) and no shift in IC₅₀ is observed.