Potentiation of radiation lethality in mouse melanoma cells by mild hyperthermia and chloroquine

To test the hypothesis that radiosensitization by combined mild hyperthermia and chloroquine may be increased by the presence of melanin in treated cells, Cloudman melanotic mouse melanoma S91/6 cells, and the amelanotic S91/amel cells were incubated during a 3 h post-irradiation period with 0.03 mM chloroquine at 41ºC. A considerable increase in radiation lethality was observed (radiation potentiation factor >1.6) in both cases. Addition of 0.1 mM isobutyl-methyl xanthine (IBMX), a promoter of melanin synthesis, to the growth medium of S91/6 cells 10 days before irradiation, did not further increase the lethality of radiation followed by combined heat and chloroquine treatment. Under these conditions, toxicity to unirradiated cells was slight. On the other hand, 10 μM chloroquine showed similar toxicity to unirradiated B-16 mouse melanoma cells, but did not increase radiation lethality. Factors other than melanin content therefore play a role in the potentiation of radiation lethality by mild hyperthermia and chloroquine.