Arsenic trioxide potentiates the effectiveness of etoposide in Ewing sarcomas

Ewing sarcomas (ES) are rare mesenchymal tumours, most commonly diagnosed in children and adolescents. Arsenic trioxide (ATO) has been shown to efficiently and selectively target leukaemic blasts as well as solid tumour cells. Since multidrug resistance often occurs in recurrent and metastatic ES, we tested potential additive effects of ATO in combination with the cytostatic drugs etoposide and doxorubicin. The Ewing sarcoma cell lines A673, RD-ES and SK-N-MC as well as mesenchymal stem cells (MSC) for control were treated with ATO, etoposide and doxorubicin in single and combined application. Viability and proliferation (MTS assay, colony formation, 3D spheroid culture) as well as cell death induction (western blot analysis, flow cytometry) were analysed. In the MTS viability assays ATO treatment significantly reduced the metabolic activity of all three ES cell lines (A673, RD-ES and SK-N-MC) examined. Moreover, all ES cell lines were sensitive to etoposide, whereas MSC remained unaffected by the drug concentrations used. With the exception of ATO in RD-ES cells, all drugs induced apoptosis in the ES cell lines, indicated by caspase-3 and PARP cleavage. Combination of the agents potentiated the reduction of viability as well as the inhibitory effect on clonal growth. In addition, cell death induction was obviously enhanced in RD-ES and SK-N-MC cells by a combination of ATO and etoposide compared to single application. Summarised, the combination of low dose, physiologically easily tolerable ATO with commonly used etoposide and doxorubicin concentrations efficiently and selectively suppressed viability and colony formation in ES cell lines, whereas a combination of ATO and etoposide was favourable for cell death induction. In addition to an increase of the effectiveness of the cytostatic drugs and prevention of potential drug resistance, this approach may also reduce toxicity effects, since the individual doses can be reduced.