Overexpression of the multidrug resistance-associated protein (MRP1) gene encoding a human GS-X pump in cultured cells resulted in increased cellular resistance to antitumor agents, including doxorubicin (Dox) and daunomycin (Dau), as well as certain heavy metals. However, studies with membrane vesicles prepared from the resistant cells revealed that Dox and Dau are poor substrates for the transport mediated by MRP/GS-X pump, suggesting that metabolic modifications of these drugs might be required for the transport. To test this hypothesis, we prepared four glutathione conjugates by linking the cysteine residue of GSH to Dox and Dau at eitehr the C-7 or C-14 position. The affinity of the synthesized conjugates toward MRP/GS-X pump was examined in the LTC4 transport assay using membrane vesicles prepared from an MRP1 gene-overexpressing cell line, SR3A. Unconjugated Dox and Dau failed to inhibit the transport of LTC4, whereas 30 microM GS-Dox or GS-Dau conjugates completely inhibited the transport. Kinetic analyses revealed that the inhibition by these GS-conjugates is competitive with Ki values ranging from 60 to 200 nM, suggesting that these compounds have high affinities toward MRP/GS-X pump and share the common binding site(s) with LTC4. Our present results support the hypothesis that glutathionation can facilitate the transport of anthracyclines by the MRP/GS-X pump.