DOXORUBICIN-GLUTATHIONE AND DAUNORUBICIN-GLUTATHIONE CONJUGATES, BUT NOT UNCONJUGATED DRUGS, COMPETITIVELY INHIBIT LEUKOTRIENE C-4 TRANSPORT MEDIATED BY MRP GS-X PUMP/

Overexpression of the multidrug resistance-associated protein (MRP1) g ene 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, studie s with membrane vesicles prepared from the resistant cells revealed th at Dox and Dau are poor substrates for the transport mediated by MRP/G S-X pump, suggesting that metabolic modifications of these drugs might be required for the transport. To test this hypothesis, we prepared f our glutathione conjugates by linking the cysteine residue of GSH to D ox and Dau at either the C-7 or C-14 position. The affinity of the syn thesized conjugates toward MRP/GS-X pump was examined in the LTC4 tran sport assay using membrane vesicles prepared from an MRP1 gene-overexp ressing cell line, SR3A. Unconjugated Dox and Dau failed to inhibit th e transport of LTC4, whereas 30 mu M GS-Dox or GS-Dau conjugates compl etely inhibited the transport. Kinetic analyses revealed that the inhi bition by these GS-conjugates is competitive with Ki values ranging fr om 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. O ur present results support the hypothesis that glutathionation can fac ilitate the transport of anthracyclines by the MRP/GS-X pump. (C) 1998 Academic Press.