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 degrees C. [(3)H]Vinblastine efflux by MDR1-expressing oocytes was detectable and afforded a K(m) of 145.5+/-25.4microM. [(3)H]Vinblastine (5.6+/-0.3microM) and [(3)H]digoxin (1.0+/-0.1microM) were individually injected into MDR1-expressing oocytes and their efflux monitored. Quinidine and verapamil, known MDR1 substrates/inhibitors, showed trans-inhibition on MDR1-mediated [(3)H]vinblastine and [(3)H]digoxin efflux. Conversely, doxorubicin demonstrated cis-inhibition without trans-inhibition on MDR1-mediated [(3)H]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.