PGP and cancer: Efflux of drugs caused by MDR proteins augments the elimination of drugs from target cells and leads to drug resistance.28 Many microtubule-targeting agents are substrates of P-gp, and higher doses of these drugs are required to achieve adequate intracellular concentrations in multidrug-resistant cancer cells.8 We employed Rh-123 and calcein AM, both substrates of P-gp and multidrug resistance-associated protein (MRP), as probes to detect chemical compounds interacting with MDR proteins.