Cold atmospheric helium plasma causes synergistic enhancement in cell death with hyperthermia and an additive enhancement with radiation

Cold atmospheric plasmas (CAPs) have been proposed as a novel therapeutic method for its anti-cancer potential. However, its biological effects in combination with other physical modalities remain elusive. Therefore, this study examined the effects of cold atmospheric helium plasma (He-CAP) in combination with hyperthermia (HT) 42 °C or radiation 5 Gy. Synergistic enhancement in the cell death with HT and an additive enhancement with radiation were observed following He-CAP treatment. The synergistic effects were accompanied by increased intracellular reactive oxygen species (ROS) production. Hydrogen peroxide (H₂O₂) and superoxide (O₂ ^•-) generation was increased immediately after He-CAP treatment, but fails to initiate cell death process. Interestingly, at late hour's He-CAP-induced O₂ ^•- generation subsides, however the combined treatment showed sustained increased intracellular O₂ ^•- level, and enhanced cell death than either treatment alone. He-CAP caused marked induction of ROS in the aqueous medium, but He-CAP-induced ROS seems insufficient or not completely incorporated intra-cellularly to activate cell death machinery. The observed synergistic effects were due to the HT effects on membrane fluidity which facilitate the incorporation of He-CAP-induced ROS into the cells, thus results in the enhanced cancer cell death following combined treatment. These findings would be helpful when establishing a therapeutic strategy for CAP in combination with HT or radiation.