Cellular uptake study
The cellular accumulation of OXA was evaluated according to a previously reported method with minor modifications (Ma & Yang et al., 2017). The cells were seeded in 12-well plates (for mock and transporter transfected cells) or 6-well plates (for DRG cells) (Corning Costar, NY, USA) and cultured to ~90% confluence. MDCK-hOCTN1, MDCK-hOCT2, MDCK-hOCTN2, mock, and primary DRG cells were pre-incubated with HBSS at 37 °C for 5 min, while MDCK-hMRP2 cells were incubated for 30 min. Next, 0.5 mL of HBSS containing 40 μM OXA with or without L-THP was added to initiate cellular uptake. The experiment was performed in a humidified air/CO2 incubator (5% v/v) at 37 °C for 5 min (for uptake transporters) or 1 h (for efflux transporter MRP2) and terminated by removing the incubation buffer rapidly. Next, the cells were quickly rinsed three times with ice-cold HBSS and lysed using 100 μL of 0.01% SDS.