DETECTION OF SUBPOPULATIONS RESISTANT TO DNA-DAMAGING AGENTS IN SPHEROIDS AND MURINE TUMORS

Chinese hamster V79 monolayers, V79 spheroids, and SCCVII murine tumou rs were examined for DNA damage using the alkaline comet assay and for cell killing by measuring clonogenicity following a l-h exposure to d oxorubicin, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 4-nitroquinol ine-N-oxide (4-NQO), etoposide, or 3-amino-1,2,4-benzotriazine-1,4-dio xide (tirapazamine). Greater heterogeneity in DNA damage was evident i n spheroids compared to monolayers exposed to these drugs, and cell su rvival was correlated with the fraction of cells which lacked sufficie nt DNA damage following treatment with tirapazamine or doxorubicin. Ce ll sorting experiments verified that subpopulations of cells resistant to DNA damage were also more resistant to cell killing. Significant h eterogeneity was observed in cells from SCCVII tumours exposed to tira pazamine and etoposide, and comet DNA content was used to independentl y assess DNA damage to aneuploid tumour cells and diploid host cells. These results suggest that, for some drugs, the comet assay may be an effective method of identifying drug-resistant cells in solid tumours.