Xenopus laevis oocytes expressing human P-glycoprotein: Probing trans- and cis-inhibitory effects on [3H]vinblastine and [3H]digoxin efflux

P-glycoprotein (P-gp; MDR1) recognizes and actively transports many structurally diverse compounds (hydrophobic neutral and cationic). We studied MDR1-mediated drug transport using a high-throughput (96-well) oocyte expression system. MDR1-expressing oocytes contained sufficient ATP levels to conduct fundamental efflux studies; the optimal experimental temperature was 25 °C. [3H]Vinblastine efflux by MDR1-expressing oocytes was detectable and afforded a Km of 145.5 ± 25.4 μM. [3H]Vinblastine (5.6 ± 0.3 μM) and [3H]digoxin (1.0 ± 0.1 μM) were individually injected into MDR1-expressing oocytes and their efflux monitored. Quinidine and verapamil, known MDR1 substrates/inhibitors, showed trans-inhibition on MDR1-mediated [3H]vinblastine and [3H]digoxin efflux. Conversely, doxorubicin demonstrated cis-inhibition without trans-inhibition on MDR1-mediated [3H]vinblastine efflux. The MDR1-expressing oocyte system offers researchers with an alternative in vitro method to screen compounds and may allow one to probe P-gp drug–drug and/or drug–inhibitor interactions.