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.