Modulation of VP-16 cytotoxicity by carboplatin and 41.8 °C hyperthermia

Purpose: To study in vitro the effect of carboplatin and/or hyperthermia in relation to etoposide (VP-16) cytotoxicity in L929 cells. Methodology/results: Cell survival assays demonstrated that the addition of 41.8 degrees C (x60 min) hyperthermia and carboplatin to VP-16 produced an antagonistic effect relative to VP-16 cytotoxicity in L929 cells; administering carboplatin and hyperthermia 24 h before VP-16 reduced this drug resistance; administering carboplatin and hyperthermia 48 h before VP-16, however, produced a supra-additive cytotoxicity. In order to gain insight into the molecular basis for these observations, we investigated the effect of hyperthermia and/or carboplatin on the stress protein GRP78, which is known to affect VP-16 cytotoxicity. Results obtained were consistent with the hypothesis that carboplatin and hyperthermia perturbation of NAD(+) pools results in down-regulation of GRP78 with subsequent modulation of VP-16 cytotoxicity. To further explicate these results we studied G-361 as a control cell line that had significantly higher pretreatment NAD(+) levels, which were not affected by carboplatin and/or hyperthermia. This cell line did not exhibit a down-regulation of GRP78 or modulation of VP-16 cytotoxicity as a function of carboplatin and hyperthermia. Conclusions: These data taken collectively, demonstrate a sequence effect (regarding the aforementioned antineoplastic agents), and provide a framework for future studies directed at the therapeutic optimization of the sequential application of carboplatin, hyperthermia, and VP-16.