Dexrazoxane induces DSB in tumour cells. (A) HTETOP cells expressing (+TOP2A) or depleted of TOP2A (−TOP2A) by tetracycline (TET) pre-treatment (1 μg·mL–1 for 24 h) were treated with dexrazoxane (DRZ) at the indicated concentrations. Cell viability was determined 24 h following dexrazoxane exposure. *P < 0.05, **P < 0.01, significantly different from +TOP2A. (B) TOP2A-expressing HTETOP cells were treated with 100 μM dexrazoxane or 1 μM doxorubicin (DOX) for specified time periods. γ-H2AX foci were detected by immunofluorescent staining. (C) γ-H2AX foci following 24 h treatment with 100 μM dexrazoxane in HT1080, NYH and DLD-1 cells were determined by immunofluorescent staining. *P < 0.05, **P < 0.01, significantly different as indicated. (D) Neutral comet assay of TOP2A-expressing HTETOP cells performed after 24 h of treatment with 100 μM dexrazoxane. Con: untreated controls, dexrazoxane : dexrazoxane -treated cells. n = 3. *P < 0.05, significantly different as indicated. (E) Immunofluorescent staining of 53BP1 and γ-H2AX in TOP2A-expressing HTETOP cells treated with 100 μM dexrazoxane for 24 h. Quantitative data are mean values from three experiments. *P < 0.05, **P < 0.01, significantly different as indicated.