The proliferative state of clonogenic cells in the Lewis lung tumour after treatment with cytotoxic agents.

The proportion of clonogenic cells from the Lewis lung carcinoma which are in S-phase of the cell cycle has been measured as the fraction killed by a short exposure to hydroxyurea in vitro. Estimates of the proportions of S-phase cells before and 30 min after doses of gamma-radiation of 1000--2000 rad suggest no alternation in the cell cycle age distribution due to these doses of radiation. As the survivors of these high doses of radiation are predominantly hypoxic, the results imply that hypoxic cells have the same cell cycle age distribution as oxygenated cells in Lewis lung tumours. After treatment with cyclophosphamide or CCNU, the proportion of S-phase cells among the survivors exceeds the faction of S-phase cells in untreated populations. This increase is consistent with a relative resistance of S-phase cells to alkylating agents and nitrosoureas.