Results. Sulfasalazine interacted at the drug-substrate site(s) of human ABCG2. Curcumin inhibited ABCG2 activity at nanomolar concentrations at the rat blood-brain barrier in the ex vivo assay. Based on studies in wild type and abcg2−/− mice, we observed that oral curcumin increased Cmax and relative bioavailability of sulfasalazine by selectively inhibiting ABCG2 function.