The effect of hyperthermia in combination with melphalan on drug-sensitive and drug-resistant CHO cells in vitro

Abstract

Summary The effect of temperature on the cytotoxicity of melphalan in a pleiotropic drug-resistant mutant CHO cell line (CHR C5) and in its drug-sensitive parent (Aux BI) was studied in vitro using a clonogenic assay. The cytotoxicity of melphalan was significantly enhanced at elevated but non-lethal temperatures (39-41 'C) and hyperthermia potentiated the effect of melphalan in the lethal temperature range (43-44 'C) in both cell lines. The effect of temperature on membrane permeability to melphalan was studied to determine whether the increase in cytotoxicity was associated with increased intracellular drug levels. The uptake of '4C-labelled melphalan during 5 min increased with increasing temperature. Drug efflux, however, also increased at elevated temperatures. Intracellular drug levels at equilibrium were increased at elevated temperatures but the magnitude of this effect was small in comparison with the much larger increases in cytotoxicity.The failure of chemotherapy to eradicate tumours which initially respond to treatment may be due to the selection of drug-resistant clones of tumour cells (Goldie & Coldman, 1979). It has been shown that the acquisition of resistance to one cytotoxic drug may confer cross resistance to several other chemotherapeutic agents and that this pleiotropic drug resistance is frequently associated with increased expression of a 170,000 dalton glycoprotein (p-glycoprotein) (Center, 1983;Louie et al., 1986;Riordan & Ling, 1979). There is good evidence that p-glycoprotein is part of a cellular export system which increases drug efflux by an energy dependent mechanism (Center, 1983;Dano, 1973;Gerlach et al., 1986;Inaba et al., 1979;Skovsgaard, 1978), although reduction in intracellular drug levels alone is insufficient to account for the level of resistance observed in some systems (Bates et al., 1985a). It has previously been shown that influx of several chemotherapeutic agents is increased at elevated temperatures (Bates & Mackillop, 1986;Hahn, 1979;Nagaoka et al., 1986) and we have therefore explored the possibility that hyperthermia might be capable of overcoming the type of pleiotropic drug resistance which is associated with increased drug efflux.The combination of hyperthermia with certain chemotherapeutic agents has exhibited potentiation of effect in experimental systems (Barlogie et al., 1980; Bates et al., 1987b;Bates & Mackillop, 1986;Hahn, 1979;Herman et al., 1982;Nagaoka et al., 1986). Regional hyperthermia, therefore, has the potential to increase the cytotoxic effects of a systemically administered agent within a defined target region and may thus be of clinical value. Thermo-chemotherapy has not been extensively tested in human tumours but for some time melphalan has been used with hyperthermia in the treatment of human melanoma by a limb perfusion (Rege et al., 1983;Stehlin, 1980;Storm et al., 1982). Studies of the interaction between hyperthermia and melphalan at the cellular level have, until now, been limited (Bates et al., 1987b). We have therefore studied the...

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