The molecular basis of the action of disulfiram as a modulator of the multidrug resistance-linked ATP binding cassette transporters MDR1 (ABCB1) and MRP1 (ABCC1).

The overexpression of multidrug resistance protein 1 (MDR1) and multidrug resistance protein 1 (MRP1) gene products is a major cause of multidrug resistance in cancer cells. A recent study suggested that disulfiram, a drug used to treat alcoholism, might act as a modulator of P-glycoprotein. In this study, we investigated the molecular and chemical basis of disulfiram as a multidrug resistance modulator. We demonstrate that in intact cells, disulfiram reverses either MDR1- or MRP1-mediated efflux of fluorescent drug substrates. Disulfiram inhibits ATP hydrolysis and the binding of [alpha-32P]8-azidoATP to P-glycoprotein and MRP1, with inhibition curves comparable with those of N-ethylmaleimide, a cysteine-modifying agent. However, if the ATP sites are protected with excess ATP, disulfiram stimulates ATP hydrolysis by both transporters in a concentration-dependent manner. Thus, in addition to modifying cysteines at the ATP sites, disulfiram may interact with the drug-substrate binding site. We demonstrate that disulfiram, but not N-ethylmaleimide, inhibits in a concentration-dependent manner the photoaffinity labeling of the multidrug transporter with 125I-iodoarylazidoprazosin and [3H]azidopine. This suggests that the interaction of disulfiram with the drug-binding site is independent of its role as a cysteine-modifying agent. Finally, we have exploited MRP4 (ABCC4) to demonstrate that disulfiram can inhibit ATP binding by forming disulfide bonds between cysteines located in the vicinity of, although not in, the active site. Taken together, our results suggest that disulfiram has unique molecular interactions with both the ATP and/or drug-substrate binding sites of multiple ATP binding cassette transporters, which are associated with drug resistance, and it is potentially an attractive agent to combat multidrug resistance.