Resistance to cisplatin (cDDP) is a major limitation to its clinical effectiveness. Review of literature data indicates that cDDP resistance is a multifactorial phenomenon. This provides an explanation why attempts to reverse or circumvent resistance using cDDP-analogues or combination therapy with modulators of specific resistance mechanisms have had limited success so far. It therefore provides a rationale to use hyperthermia, an agent with pleiotropic effects on cells, in trying to modulate cDDP resistance. In this review the effects of hyperthermia on cDDP cytotoxicity and resistance as well as underlying mechanisms are discussed. Hyperthermia is found to be a powerful modulator of cDDP cytotoxicity, both in sensitive and resistant cells. Relatively high heat doses (60 min 43 degrees C) seem to specifically interfere with cDDP resistance. The mechanism of interaction has not been fully elucidated so far, but seems to consist of multiple (simultaneous) effects on drug accumulation, adduct-formation and -repair. This may explain why hyperthermia seems to be so effective in increasing cDDP cytotoxicity, irrespective of the presence of resistance mechanisms. Therefore, the combination of hyperthermia and cDDP deserves further attention.

Summary In this study, the mechanism(s) by which heat increases cis-diamminedichloroplatinum (cisplatin, cDDP) sensitivity in cDDPsensitive and -resistant cell lines of murine as well as human origin were investigated. Heating cells at 430C during cDDP exposure was found to increase drug accumulation significantly in the cDDP-resistant cell lines but had little effect on drug accumulation in the cDDP-sensitive cell lines. DNA adduct formation, however, was significantly increased in all cell lines studied. Furthermore, ongoing formation of platinum (Pt)-DNA adducts after the end of cDDP treatment was enhanced and/or adduct removal was decreased in heated cells, resulting in relatively more DNA damage remaining at 24 h after the end of cDDP exposure. Correlation plots with survival revealed weak correlations with cellular Pt accumulation (r2 = 0.59) and initial Pt-DNA adduct formation (r2 = 0.64). Strong correlations, however, were found with Pt-DNA adducts at 6 h (r2 = 0.97) and 24 h (r2 = 0.89) after the incubation with the drug. In conclusion, the mechanism by which heat sensitizes cells for cDDP action seems to be the sum of multiple factors, which comprise heat effects on accumulation, adduct formation and adduct processing. This mechanism did not seem to differ between cDDP-sensitive and -resistant cells, emphasizing the potential of hyperthermia to reduce cDDP resistance.Keywords: thermochemosensitization; cisplatin resistance; cisplatin accumulation; cisplatin-DNA adducts; adduct repair Hyperthermia can strongly potentiate the cytotoxic action of cisdiamminedichloroplatinum (cisplatin, cDDP) both in vitro and in vivo (reviewed by Engelhardt, 1987). In addition, relatively high heat doses (above 42°C) can (partly) reverse in vitro acquired cDDP resistance (Wallner et al, 1986;Herman et al, 1988;Mansouri et al, 1989;Konings et al, 1993;Hettinga et al, 1994). Resistance to cDDP is a major problem in the clinic and limits the success of this drug. Thus, the combined use of heat and cDDP appears to be an interesting possibility to minimize this problem. The mechanism by which cells become resistant to cDDP has been extensively studied by numerous groups and has been found to be multifactorial (reviewed by Andrews and Howell, 1990), including (combinations of) decreased drug accumulation, increased detoxification of the drug via glutathione (GSH) metabolism or metallothioneins, decreased drug-DNA adduct formation and increased repair of the drug-induced DNA damage. The mechanism(s) by which hyperthermia sensitizes cells to cDDP is (are) less clear. Altered platinum (Pt) accumulation, total Pt-DNA adduct and DNA cross-link formation and/or repair of DNA damage have been reported as a result of combined heat and cDDP treatments (Meyn et al, 1980;Wallner et al, 1986;Herman et al, 1988Herman et al, , 1989Herman et al, , 1990Mann et al, 1988;Mansouri et al, 1989;Eichholtz-Wirth and Hietel, 1990;Los et al, 1993;Takahashi et al, 1993;Ohno et al, 1994). However, the published data are rather contradictory, and in most ...

It has been suggested that the expression of certain heatshock proteins (HSPs) may be prognostic markers in several tumor types. Since HSPs may be involved in determining cellular sensitivity to chemotherapeutic drugs, the possible relation between HSP expression and cisplatin (cDDP) sensitivity was studied. Three human germ-cell tumor cell lines, I human small-cell lung carcinoma (SCLC) cell line and 3 human colon carcinoma cell lines were used as a model for differences in intrinsic cDDP sensitivity. The constitutive expression of a panel of HSPs was studied by immunoblotting. No correlation was found between expression of HSP90, HSP73, HSP72, HSP60 and HSP27 and the extent of intrinsic cDDP sensitivity when all cell lines studied were considered. However, for the 3 cell lines derived from germ-cell carcinomas, HSP27 expression was inversely related t o cDDP sensitivity; i.e. decreased HSP27 levels were associated with decreased sensitivity. Constitutive HSP expression was also studied in 2 sets of human cell lines with in vitro acquired cDDP resistance. In both resistant cell lines, decreased expression of HSP27 (as determined by Western blotting) was found as compared to the sensitive parent cell lines. Thus, acquired resistance to cDDP was also accompanied by decreased HSP27 expression. Interestingly, when basal HSP27 mRNA levels were measured in the SCLC cell line (GLC,) and its subline with acquired resistance (GLC4-cDDP), no significant differences were detected. Continuous cDDP incubation increased HSP27 levels and induced HSP27 phosphorylation in GLC4 cells, but not in the resistant subline. Thus, although no general relationships between HSP expression and cDDP sensitivity are apparent, high HSP27 expression in vitro relates to high sensitivity to cDDP treatment in some tumor types. This is in accordance with reported clinical data on high HSP27 levels in tumors correlating with good prognosis.o 1996 Wiley-Liss, Inc.

A human small cell lung carcinoma cell line (GLC4) and its subline with in vitro acquired cisplatin (cDDP) resistance (GLC4-cDDP) were used to study the applicability of hyperthermia to interfere with acquired cDDP resistance. GLC4 and GLC4-cDDP did not differ in heat sensitivity (clonogenic ability). Both cell lines could be sensitized to cisplatin to a considerable extent, both at 42 and 43 degrees C. For 42 degrees C hyperthermia treatments up to 90 min no differences in TER between the cell lines were observed. Only prolonged (> or = 45 min) exposures to 43 degrees C hyperthermia sensitized the resistant cell line to a greater extent than the parent cell line, resulting in a reduction of the resistance factor from 3.6 (at 37 degrees C) to 1.7 (60 min 43 degrees C). The finding in this human system that for treatments up to 90 min, 43 degrees C heat is more suitable than 42 degrees C heat to reduce cDDP resistance, is in accordance with earlier findings with murine cells (Konings et al. 1993). Effects of heat, cisplatin and combined treatments on cell killing were not only measured with the clonogenic assay, but also with the microculture tetrazolium method (MTT assay), an assay of potential use in the clinic for rapid screening of cells obtained from patients. The data with the latter assay were comparable to those obtained with the clonogenic assay. However, its applicability to measure thermo-chemosensitization is limited due to its inability to measure more than one log of cell killing.