GL331, a Topoisomerase II Inhibitor, Induces Radiosensitization of Human Glioma Cells

Abstract

Background: GL331 is a topoisomerase II inhibitor and its effects on cultured human T98G glioma cells were investigated in this study. The effects of combined GL331 and radiation treatment on human T98G glioma cells were also examined. Materials and Methods: The glioma cells were treated with GL331 and the clonogenic assay was used to study its cytotoxicity effects. A combination treatment, using either concomitant irradiation at the beginning or end of the GL331 treatment (designated as the RT-GL331 and GL331-RT protocols, respectively), was used to investigate the radiosensitization effects of GL331. The levels of radiosensitization of various protocols were evaluated by the dose-enhancement ratio and Isobologram analysis. The cell cycle distribution of the glioma cells treated with various protocols was explored using flow-activated cell sorter (FACS) analysis. Results: The clonogenic assay demonstrated that GL331 exerts cytotoxic effects on the glioma cells in a concentration- and time-dependent manner (p<0.001). The combination treatments, RT-GL331 and GL331-RT, increased the dose-enhancement ratio, with a higher ratio for the RT-GL331 protocol. Isobologram analysis revealed sub-additive glioma cell cytotoxicity for the GL331-RT protocol; in contrast, mainly supra-additive effects were demonstrated for the RT-GL331 protocol. FACS analysis revealed that GL331 and radiation caused the glioma cells to accumulate in the G2/M-phase of the cell cycle and combination of these two treatments as the RT-GL331 protocol further increased the G2/M fraction of the glioma cells. Conclusion: The induction of GL331 radiosensitization was sequence-dependent, with stronger cytotoxic effects on the gliomas noted where radiation was delivered concomitantly with the beginning of GL331 treatment. Such radiosensitization effects might be, at least partly, related to the increased cell accumulation in the G2/M-phase.