Enhancement of Tumor Cell Radiosensitivity by a Novel Functional Class of Thalidomide Analogs

Abstract

Materials/Methods: Clonogenic survival assays were used to evaluate cell survival after exposure to CPS45 followed by radiation. DNA damage and repair were evaluated using phosphorylated histone H2AX (gH2AX) and the neutral comet assay. Mechanism of cell death was determined by immunostaining for mitotic catastrophe and apoptosis by flow cytometry. Cell cycle changes were evaluated by staining of phospho-H3 and flow cytometry. In vivo activity was measured using a tumor growth delay assay. Results: Exposure of each cell line to CPS45 for 16 h before irradiation (IR) resulted in an increase in radiosensitivity with dose enhancement factors of 1.8 and 1.5 for U251 and MiaPaCa cells, respectively (PF = 0.43, 0.31 respectively in CPS45 treated cells). CPS45 had no effect on IR-induced apoptosis or on the activation of the G2 cell cycle checkpoint. However, CPS45 did modify the time course ofgH2AX expression in IR cells. There was significant difference in IR-inducedgH2AX foci count at both 6 h and at 24 h after IR in the CPS45 + IR group than either individual treatment. In the neutral comet assay, there was a significant increase in the percentage of DNA damage remaining in cells exposed to CPS45 before IR compared to IR alone. Mitotic catastrophe, measured at 48 and 72 hour, was also significantly increased in cells receiving the CPS45 + IR combination compared with the individual treatments. Tumor growth delay demonstrated a greater than additive effect of the combination of CPS45 and IR than either treatment alone. Conclusions: These results indicate that CPS45 can enhance tumor cell radiosensitivity and suggest that this effect involves an inhibition of double strand breaks repair leading to an increase in mitotic catastrophe.