Sequence of treatment is important in the modification of camptothecin induced cell killing by hyperthermia

Abstract

We investigated the modification of camptothecin (CPT)-induced cell killing by hyperthermia in a radioresistant human melanoma (Sk-Mel-3) and a human normal (AG1522) cell line. CPT, a topoisomerase (topo) I inhibitor, was given as a 1 h exposure at variable doses up to 34 μm; hyperthermia was given either before or following CPT treatment. Hyperthermia was given either as a treatment of 41d`C for 8 h (termed lower temperature hyperthermia, LTH) or 45d`C for 15 min (termed higher temperature hyperthermia, HTH). LTH preceding CPT treatment had no effect on Sk-Mel-3 but potentiated killing of AG1522 cells. HTH preceding CPT treatment, however, almost completely abrogated the toxicity of CPT to both Sk-Mel-3 and AG1522 cells. These results therefore provided evidence for a lack of enhancement of CPT toxicity towards Sk-Mel-3 cells when hyperthermia preceded treatment with CPT. There was also no potentiation of killing of both cell lines when LTH followed treatment with CPT. In contrast, the killing of Sk-Mel-3 cells was slightly potentiated, whereas that of AG1522 cells was reduced, when HTH followed CPT. These results therefore suggested a potential for enhancement of killing of Sk-Mel-3 relative to AG1522 cells when HTH, but not LTH, followed CPT treatment. In addition, we found that a preceding exposure to HTH did not affect either cellular accumulation or efflux of [³H]CPT in both cell lines. Thus the significantly reduced cytotoxicity observed under those conditions was not related simply to a modification of accumulation or efflux of CPT. We found no significant differences in the catalytic activities of topo I extracted from the nuclei of Sk-Mel-3 and AG1522 cells that were either heated under HTH conditions or that were not subjected to such treatment. These results therefore suggested that the substantial reduction of cytotoxicity seen when HTH preceded CPT treatment was also not due to an effect on topo I catalytic activity. Our results therefore demonstrate that the sequence of application of hyperthermia and CPT is very important in determining the amount and, possibly, selective potentiation of tumour relative to normal cell cytotoxicity.

Key Words:

• Camptothecin
• hyperthermia
• topoisomerase activity