Zosuquidar (LY335979) is a potent modulator of P-glycoprotein-mediated multidrug resistance with Ki of 60 nM. Zosuquidar binds with high affinity to P-glycoprotein and inhibits P-glycoprotein-mediated multidrug resistance (MDR). P-glycoprotein, encoded by the MDR-1 gene, is a member of the ATP-binding cassette superfamily of transmembrane transporters and prevents the intracellular accumulation of many natural product-derived cytotoxic agents. Zosuquidar is a compound of antineoplastic drug candidates currently under development.