Determination and drug modification assessed by micronucleus frequency assay of potentially lethal damage repair in quiescent cell populations within murine solid tumors.

We investigated potentially lethal damage repair (PLDR) by quiescent (Q) tumor cells in vivo. SCC VII tumor-bearing C3H/He mice were irradiated with 60Co gamma-rays after being given 10 injections of 5-bromo-2'-deoxyuridine (BrdU) to label all proliferating (P) cells in their tumors, and the tumors were than excised and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (Cyt-B, a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling was determined using immunofluorescence staining for BrdU. Thus, the MN frequency was determined for cells not labeled by BrdU; for all practical purposes, such cells can be regarded as the Q cells in a tumor. The MN frequency in the total (P + Q) tumor cell population was determined from irradiated tumors that were not pretreated with BrdU. Assays were performed immediately after irradiation alone, 24 hours after the injection of cis-diaminedichloroplatinum(II) (CDDP), mitomycin C (MMC), misonidazole [1-(2-nitro-1-imidazolyl)-3-methoxy-2-propanol] (MISO), 3-aminobenzamide (3-AB), camptothecin (CPT) or caffeine (CAF) following irradiation, and 24 hours after irradiation alone. Q cells were more radioresistant and had a greater capacity for PLDR than the tumor cell population as a whole. CDDP and MISO (especially the latter) inhibited PLDR more strongly in Q cells than in the tumor cell population as a whole. However, CPT and CAF exerted similar inhibition of PLDR in Q cells and in the tumor cell population as a whole. This assay method appears to be useful for detecting the responses of Q tumor cells to various chemical agents.