Cell culture
MDCK cells stably transfected with plasmid vector pcDNA3.1(+) containing human OCT2 cDNA (MDCK-hOCTN1) (Bai & Ma et al., 2017), plasmid vector pcDNA3.1(+) containing human OCTN1 cDNA (MDCK-hOCT2) (Li & Weng et al., 2017), plasmid vector pcDNA3.1(+) containing human MRP2 cDNA (MDCK-hOCTN2) (Ma & Yang et al., 2017), and blank vector (mock cells) were all established in our laboratory and grown in DMEM medium supplemented with 10% FBS and 1% penicillin/streptomycin. SW620 and HT29 cell lines were obtained from the American Type Culture Collection (ATCC; Manassas, VA, USA) and grown in GI 1640 medium supplemented with 10% FBS and 1% penicillin/streptomycin. All cells were maintained in a humidified air/CO2 incubator (5% v/v) at 37 °C.