Tachpyridine induces G₂/M arrest and radiosensitizes HCT 116 TP53^+/+ and HCT 116 TP53^-/- cells but does not radiosensitize normal diploid fibroblasts. (A) HCT 116 TP53^+/+ or (B) HCT 116 TP53^-/- cells were treated with 10 μM tachpyridine for 24 hours, and cell-cycle distribution was determined by flow cytometry. (C) HCT 116 TP53^+/+ or (D) HCT 116 TP53^-/- cells were pretreated with 7.5 μM tachpyridine for 12 hours, followed by irradiation at 1, 2, and 4 Gy, and clonogenic survival was determined as described in “Materials and methods.” (E) HCT 116 TP53^+/+ cells were pretreated with 7.5 μM tachpyridine for 12 hours, followed by irradiation at 1, 2, and 4 Gy, and viability was determined by the MTT assay as described in “Materials and methods.” (F) MRC5 fibroblast cells were pretreated with 25 or 30 μM tachpyridine for 12 hours, followed by irradiation at 6 and 10 Gy, and viability was determined by the MTT assay. Data shown are the mean and SE of 3 independent experiments performed in triplicate.