S_nary Previous studies have shown that multidrug resistance (MDR) in the doxorubicin-selected lung tumour cel lines COR-L23/R, GLC4 'ADR and MOR/R is associated with overexpression of the MRP gene.In this study we report that resistance to daunorubicin, vincristine and rhodamine 123 can be partially reversed in these cell lines by exposing the cells to buthionine sulphoximine (BSO), an inhibitor of glutathione (GSH) synthesis. This effect of BSO on drug resistance was associated with an increased intracellular accumulation of daunorubicin and rhodamine 123, owing to inhibition of the enhanced drug efflux. In contrast, the accumulation of daunorubicin was not increased by BSO treatment in a P-glycoprotein (P-gp)-mediated MDR cell line. BSO treatment (25 FM, 20 h) of the cell lines resulted in 60-80% depletion of cellular GSH levels. The effects of BSO on daunorubicin accumulation in the COR-L23/R and GLC4/ADR cells were associated with cellular GSH depletion. In addition, increase of cellular GSH levels in BSO-treated COR-L23/R and GLC4/ADR cells as a result of incubation with 5 mM GSH ethyl ester restored the accumulation deficit of daunorubicin. However, the transport of daunorubicin did not increase the GSH release in any of the cell lines. These results demonstrate that drug transport in MRP-but not in P-gp-overexpressing MDR tumour cell lines can be regulated by intracellular GSH levels.

Cells exposed to calcein acetoxymethyl ester (calcein AM) in the growth medium become fluorescent following cleavage of calcein AM by cellular esterases to produce the fluorescent derivative calcein. It has previously been shown by others that multidrug resistant cells which overexpress P-glycoprotein accumulate much less fluorescent calcein than the corresponding parental cells. We have now examined the transport of calcein in multidrug resistant cells which overexpress an alternative transporter, the multidrug resistance-associated protein (MRP). Accumulation of calcein fluorescence was greatly reduced in the MRP-overexpressing human lung cancer cell lines COR-L23/R and MOR/R compared with their parental lines. Energy depletion resulted in a considerably increased accumulation in the resistant lines. Treatment of resistant cells with buthionine sulfoximine (BSO), which depletes cellular glutathione (GSH), did not affect calcein accumulation, in marked contrast to our previous results for daunorubicin or the fluorescent probe rhodamine 123. Genistein, verapamil, cyclosporin A and ouabain were also each able to modify, to some extent, accumulation of daunorubicin, whilst having essentially no effect on calcein accumulation. However, the organic anion transport inhibitor probenecid was able to increase accumulation of both calcein and daunorubicin in the resistant cells. Genistein and verapamil treatment preferentially reduced the GSH content of resistant cells, whilst probenecid did not. However, probenecid caused a clear decrease in release of GSH from resistant cells into the medium.

No abstract

Selective protection of the normal host tissues from the toxic effects of anticancer agents would allow the use of higher, probably more effective, doses of the drugs. It has been demonstrated that delayed high-dose uridine administration after 5-fluorouracil decreases the extent of myelosuppression and causes faster regeneration of the bone marrow. We studied the biochemical consequences of the gastrointestinal toxicity caused by 5-fluorouracil and the potential of high-dose uridine treatment to influence these adverse effects. 5-Fluorouracil caused dose-related decreases in the biochemical parameters (thymidine kinase, sucrase, maltase, alkaline phosphatase) selected as early markers of the impaired metabolic activity of the intestinal mucosa. The nadir of the biochemical changes was reached between 24 h and 72 h after 5-fluorouracil treatment, and complete regeneration of the mucosa took 6-7 days. Delayed high-dose uridine administration failed to mitigate the severity of the gastrointestinal damage that ensued after 5-fluorouracil treatment, but caused significantly earlier regeneration of the mucosa.