Potentially lethal damage repair by quiescent cells in murine solid tumors

We investigated potentially lethal damage repair by quiescent tumor cells in vivo. SCC VII tumor-bearing C3H/He mice were irradiated after being given 10 injections of 5-bromo-2'-deoxyuridine (BUdR) to label all the proliferating cells in their tumors, and the tumors were then excised and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (Cyt-B, a cytokinesis blocker), and the micronucleus frequency in cells without BUdR labelling was determined using immunofluorescence staining to BUdR. The micronucleus frequency was then used to determine the surviving fraction of unlabelled cells on the basis of the regression line obtained for the micronucleus frequency and the surviving fraction of all tumor cells not labeled by BUdR, which can be regarded as the quiescent cells in a tumor for all practical purposes. Assessment performed 0, 3, 6, 9, and 24 hr after irradiation showed that quiescent cells had more potentially lethal damage repair capacity than the tumor cell population as a whole. Assays were also performed immediately after irradiation alone, 24 hr after the injection of cis-diamminedichloroplatinum(II) (CDDP), mitomycin C (MMC), or misonidazole [1-(2-nitro-1-imidazolyl)-3-methoxy-2-propanol] (MISO) following irradiation, and 24 hr after irradiation alone. It was found that CDDP and MISO (especially the latter) inhibited potentially lethal damage repair more strongly in quiescent cells than in the tumor cell population as a whole. This assay method thus appears to be quite useful for detecting the responses of quiescent tumor cells to various chemical agents.