P-gp inhibitor 1 is a novel inhibitor reversing P-glycoprotein-mediated multidrug resistance.IC50 & Target: P-glycoproteinIn Vitro: P-gp inhibitor 1 (12k) possesses high potency (EC50=57.9±3.5 nM), low cytotoxicity, and long duration of activity in reversing doxorubicin (DOX) resistance in K562/A02 cells (1 μM, 80 minutes). P-gp inhibitor 1 also boosts the potency of other MDR-related cytotoxic agents with different structures, increases accumulation of DOX, blocks Pgp-mediated Rh123 efflux, and suppresses P-gp ATPase activity in K562/A02 MDR cells (0.1, 1, 5 μM, 1 hour).