The effect of post-irradiation tumor oxygenation status on recovery from radiation-induced damage in vivo: with reference to that in quiescent cell populations

Purpose  To elucidate the effect of tumor oxygenation status on recovery from damage following γ-ray or accelerated carbon ion irradiation in vivo, including in quiescent (Q) cells. Methods  SCC VII tumor-bearing mice were continuously given 5-bromo-2′-deoxyuridine (BrdU) to label all proliferating (P) cells. They received γ-ray or accelerated carbon ion irradiation with or without tumor clamping for inducing hypoxia. Immediately after irradiation, cells from some tumors were isolated, or acute hypoxia-releasing nicotinamide was loaded to the tumor-bearing mice. For 9 h after irradiation, some tumors were kept aerobic or hypoxic. Then isolated tumor cells were incubated with a cytokinesis blocker. The response of Q cells was assessed in terms of the micronucleus frequency using immunofluorescence staining for BrdU. That of the total (=P + Q) tumor cells was determined from BrdU non-treated tumors. Results  Clearer recovery in Q cells than total cells and after aerobic than hypoxic γ-ray irradiation was efficiently suppressed with carbon ion beams. Inhibition of recovery through keeping irradiated tumors hypoxic after irradiation and promotion of recovery by nicotinamide loading were observed more clearly with γ-rays, after aerobic irradiation and in total cells than with carbon ion beams, after hypoxic irradiation and in Q cells, respectively. Conclusions  Tumor oxygenation status following irradiation can manipulate recovery from radiation-induced damage, especially after aerobic γ-ray irradiation in total cells. Carbon ion beams are promising because of their efficient suppression of the recovery.     

An attempt to enhance chemosensitivity of quiescent cell populations in solid tumors by combined treatment with nicotinamide and carbogen

cis-Diamminedichloroplatinum(II) (cisplatin) was intraperitoneally injected into mice bearing SCC VII or EMT6/KU tumors after ten administrations of 5-bromo-2-deoxyuridine (BrdU) to label all the proliferating tumor cells. The tumors were excised 1 h after the cisplatin injection, minced, and trypsinized. The tumor cell suspensions were then incubated with cytochalasin-B (a cytokinesis blocker). The micronucleus frequency was determined, using immunofluorescence staining for BrdU. Cells that were not labeled with BrdU were regarded as quiescent. The micronucleus frequency in the total number of tumor cells was determined in tumors that had not been pretreated with BrdU. To modify the sensitivity to cisplatin, nicotinamide was intraperitoneally injected before the administration of cisplatin or mice were placed in a circulating carbogen (95% O₂, 5% CO₂) chamber for 30 min after cisplatin administration. In both tumor systems, the micronucleus frequency in quiescent cells was lower than that in the total cells. Nicotinamide pretreatment increased the micronucleus frequency in total and in quiescent cells in both tumor systems, and to a higher extent in total cells. The combination of nicotinamide and carbogen increased the micronucleus frequency more markedly than treatment with either nicotinamide or carbogen alone. In total cells of both tumors, the nicotinamide injection increased the uptake of [^195mPt]cisplatin. The combined treatment raised the uptake more markedly than did treatment with either agent alone. In total cells of the SCC VII tumor, these increases in micronucleus frequency and the [^195mPt]cisplatin uptake following nicotinamide or combined pretreatment were significant. In both tumors, carbogen breathing also elevated the micronucleus frequency to some degree in total and quiescent cells and the [^195mPt]cisplatin uptake in total cells. The combined nicotinamide and carbogen treatment was considered to be useful for sensitizing tumor cells to chemotherapy with cisplatin in vivo.Abbreviations Q cells quiescent cells - P cells proliferating cells