Hyperthermic potentiation of cisplatin by magnetic nanoparticle heaters is correlated with an increase in cell membrane fluidity

Magnetic fluid hyperthermia as a cancer treatment method is an attractive alternative to other forms of hyperthermia. It is based on the heat released by magnetic nanoparticles subjected to an alternating magnetic field. Recent studies have shown that magnetic fluid hyperthermia-treated cells respond significantly better to chemotherapeutic treatment compared with cells treated with hot water hyperthermia under the same temperature conditions. We hypothesized that this synergistic effect is due to an additional stress on the cellular membrane, independent of the thermal heat dose effect that is induced by nanoparticles exposed to an alternating magnetic field. This would result in an increase in Cis-diammine-dichloroplatinum (II) (cDDP, cisplatin) uptake via passive transport. To test this hypothesis, we exposed cDDP-treated cells to extracellular copper in order to hinder the human cell copper transporter (hCTR1)-mediated active transport of cDDP. This, in turn, can increase the passive transport of the drug through the cell membrane. Our results did not show statistically significant differences in surviving fractions for cells treated concomitantly with magnetic fluid hyperthermia and cDDP, in the presence or absence of copper. Nonetheless, significant copper-dependent variations in cell survival were observed for samples treated with combined cDDP and hot water hyperthermia. These results correlated with platinum uptake studies, which showed that cells treated with magnetic fluid hyperthermia had higher platinum uptake than cells treated with hot water hyperthermia. Changes in membrane fluidity were tested through fluorescence anisotropy measurements using trimethylamine-diphenylhexatriene. Additional uptake studies were conducted with acridine orange and measured by flow cytometry. These studies indicated that magnetic fluid hyperthermia significantly increases cell membrane fluidity relative to hot water hyperthermia and untreated cells, and hence this could be a factor contributing to the increase of cDDP uptake in magnetic fluid hyperthermia-treated cells. Overall, our data provide convincing evidence that cell membrane permeability induced by magnetic fluid hyperthermia is significantly greater than that induced by hot water hyperthermia under similar temperature conditions, and is at least one of the mechanisms responsible for potentiation of cDDP by magnetic fluid hyperthermia in Caco-2 cells.