Temperature dependence of melphalan efflux kinetics in Chinese hamster ovary cells

Sensitization to the cytotoxicity of melphalan by ethacrynic acid and hyperthermia in drug-sensitive and multidrug-resistant Chinese hamster ovary cells

The ability of physical and pharmacological modulators to increase the cytotoxicity of melphalan was investigated in Chinese hamster ovary cells using a clonogenic cell survival assay. Hyperthermia has potential for use in cancer treatment, particularly as an adjuvant to chemotherapy or radiotherapy. Ethacrynic acid is a glutathione S-transferase inhibitor and also undergoes conjugation with glutathione. Interactions between hyperthermia (41-43 degrees C), ethacrynic acid and melphalan were evaluated in multidrug-resistant (CH(R)C5) cells with overexpression of P-glycoprotein (33.69-fold), and in drug-sensitive (AuxB1) cells. GST alpha was expressed at a higher level (3.65-fold) in CH(R)C5 cells than in sensitive cells, whereas levels of isoforms pi and mu were the same. GST pi was the most highly expressed isoform in the two cell populations. Ethacrynic acid was cytotoxic at elevated temperatures, while it caused little or no cytotoxicity at 37 degrees C. This effect occurred in drug-resistant and drug-sensitive cells, and attributes thermosensitizing properties to ethacrynic acid. Ethacrynic acid (20 microM) alone did not alter the cytotoxicity of melphalan at 37 degrees C. Hyperthermia potentiated drug cytotoxicity in cells, both with and without ethacrynic acid treatment. Ethacrynic acid could be useful in cancer treatment by acting as a thermosensitizer when combined with heat and by enhancing the cytotoxicity of melphalan at elevated temperatures. A major advantage arising from the use of regional hyperthermia is the ability to target drug cytotoxicity to the tumor volume. A useful finding is that ethacrynic acid, heat and/or melphalan are also effective against multidrug-resistant cells with overexpression of P-glycoprotein.

Down-regulation of CD98 in melphalan-resistant myeloma cells with reduced drug uptake

Although melphalan has been used as a therapeutic reagent for multiple myeloma, many patients become refractory. To elucidate the mechanism of resistance to melphalan, we generated a melphalan-resistant myeloma cell line, KHM-11(EMS), by treating a parental line, KHM-11, with a mutagen, ethylmethanesulfonate. KHM-11(EMS) is 55 times more resistant to melphalan. gamma-Glutamylcysteine synthetase, P-glycoprotein, multidrug-resistance-associated protein, lung-resistance-related protein and the Bcl-2 family of proteins were not responsible for the drug resistance in KHM-11(EMS). Intracellular incorporation of melphalan to myeloma cells was determined by using [(14)C]-labeled melphalan. Accumulation of melphalan in KHM-11(EMS) was 43% of KHM-11, while the efflux rates were comparable in both cell lines. The uptake of melphalan was inhibited by the addition of L-phenylalanine, indicating that melphalan is incorporated through the L-phenylalanine transporter as reported previously. Expression of CD98, which was recently cloned as an L-phenylalanine transporter, was 6-fold decreased in KHM-11(EMS), suggesting that CD98 may be correlated with the incorporation of melphalan. CD98 expression and incorporation of melphalan were analyzed in fresh purified myeloma cells from 5 patients. All myeloma cells from 4 cases expressed CD98 at a high level and incorporated melphalan. However, tumor cells from 1 case expressed CD98 at low levels and did not incorporate melphalan. Taken together, reduced melphalan uptake could be responsible for the drug resistance in KHM-11(EMS), and down-regulation of CD98 may be related to this phenomenon. Further investigation of the correlation between impaired drug uptake and down-regulation of CD98 in myeloma cells should be important to understand the mechanism of resistance to melphalan.

Melphalan resistance and photoaffinity labelling of P-glycoprotein in multidrug-resistant Chinese hamster ovary cells: reversal of resistance by cyclosporin A and hyperthermia

The multidrug resistance phenotype is often associated with overexpression of P-glycoprotein, an energy-dependent efflux pump responsible for decreased intracellular accumulation of chemotherapeutic agents. The role of P-glycoprotein in the mechanism of cross-resistance to melphalan in multidrug-resistant Chinese hamster ovary cells (CH(R)C5) was investigated by photoaffinity labelling of P-glycoprotein using [3H]azidopine. We investigated whether the chemosensitiser cyclosporin A and hyperthermia, either used alone or combined, could reverse melphalan resistance and alter transport processes for [14C]melphalan in CH(R)C5 cells. Melphalan inhibited azidopine photolabelling of P-glycoprotein, implicating drug efflux mediated by P-glycoprotein in the mechanism of melphalan resistance in CH(R)C5 cells. Azidopine photolabelling also was inhibited by the chemosensitiser cyclosporin A, which binds to P-glycoprotein. Cyclosporin A alone reversed melphalan resistance in CH(R)C5 cells, but had no effect in drug-sensitive AuxB1 cells. Hyperthermia (40-45 degrees) alone increased melphalan cytotoxicity in both cell lines. When hyperthermia was combined with cyclosporin A, a large increase in melphalan cytotoxicity occurred, but only in CH(R)C5 cells. This effect increased with temperature and exposure time. Sensitisation to melphalan cytotoxicity by heat and cyclosporin A in CH(R)C5 cells appeared to be explained by altered drug transport processes. Lower accumulation of melphalan occurred in CH(R)C5 cells than in drug-sensitive cells. At 37 degrees, cyclosporin A increased drug accumulation in CH(R)C5 cells, but not in AuxB1 cells, by slowing drug efflux from cells. Heat alone increased both melphalan uptake and drug efflux for both cell lines. Our findings suggest that the combination of cyclosporin A and hyperthermia could be very useful in overcoming melphalan resistance by increasing intracellular drug accumulation in multidrug-resistant cells.

Hyperthermia, cyclosporine A and melphalan cytotoxicity and transport in multidrug resistant cells

The ability of hyperthermia and cyclosporine A to modulate melphalan cytotoxicity and transport processes was investigated in a pleiotropic MDR Chinese hamster ovary cell line (CH(R)C5) and in the drug-sensitive parent line (AuxB1). Cyclosporine A increased cytotoxicity of melphalan in MDR cells, but not in drug-sensitive cells. In MDR cells, hyperthermia caused marked enhancement of melphalan cytotoxicity when cyclosporine A was present. The increased melphalan cytotoxicity in MDR cells was accompanied by changes in membrane permeability to the drug. Cyclosporine A caused an increase in melphalan uptake in MDR cells and a decrease in melphalan efflux out of cells, leading to an overall increase in intracellular drug accumulation. Drug transport processes were not affected by cyclosporine A in drug-sensitive cells.