Fig. 1

Complicated mechanisms of multidrug resistance (MDR) in tumor. The main mechanism of MDR is overexpressing ATP-binding cassette (ABC) transporters to increase drug efflux, resulting in a decrease in intracellular drug concentration. Other mechanisms of MDR are reducing drug uptake by influx transporters, boosting drug metabolism, blocking apoptotic signaling pathways, elevating adaptability by epigenetic regulation and microRNA regulation, mutation in drug targets or feedback activation of other targets and signaling pathways, and change of tumor microenvironment. ABCB1, ATP-binding cassette subfamily B member 1; ABCG2, ATP-binding cassette subfamily G member 2; ABCC1, ATP-binding cassette subfamily C member 1; CYP450, cytochrome P450