Mild hyperthermia predisposes tumor cells to undergo apoptosis upon treatment with onconase

Onconase (ONC), (ranpirnase) a cytotoxic ribonuclease isolated from amphibian oocytes and early embryos targeting tumor cells in vitro and in vivo, is currently in a confirmatory Phase IIIb clinical trial for unresectable malignant mesothelioma where it demonstrates antitumor activity with relatively minor overall toxicity to patients. Since hyperthermia has been shown to be synergistic with certain antitumor modalities, the aim of the present study was to explore whether the cytotoxic effects of ONC can be enhanced under conditions of mild hyperthermia. Treatment of human lymphoblastoid TK6 cells with 2 or 5 µg/ml of ONC at 40°C for 24 or 48 h led to 64-200% enhancement in incidence of apoptosis assessed by frequency of cells showing the presence of activated (cleaved) caspase-3 or activated serine proteases, compared to treatment at 37.5°C. The incidence of apoptosis at 40°C in the absence of ONC was unchanged compared to 37.5°C, for up to 48 h. Although at 41°C in absence of ONC the incidence of apoptosis was elevated compared to 37°C the cytotoxicity of ONC was further enhanced and the overall pro-apoptotic effect was above the level of additive effects of ONC plus that of 41°C-hyperthermia. While the mechanism of the observed enhancement of ONC cytotoxicity is currently under investigation, the findings suggest that a combination of ONC and mild hyperthermia should be explored to increase effectiveness of ONC in cancer treatment.