The chemical structures of select first-, second-, and third-generation P-gp inhibitors. First-generation drugs are cyclosporin-A and verapamil. Second-generation drugs are analogs of first-generation drugs, of which PSC833 is a cyclosporine analogue. Examples of 2 third-generation drugs are tariquidar (XR9576) and zosuquidar (LY335979), which are currently under evaluation in phase 1/2 clinical trials.