Use of tumor lines with selectable markers in assessing the effect on experimental metastases of combination chemotherapy with alkylating agents.

High-dose chemotherapy with a 3-day regimen of cyclophosphamide, cisplatin, and carmustine was used to treat mice bearing experimental lung metastases of mammary tumor cell lines with selectable markers (lines 66cl4 and 4TO7). Cloning of lung cells at various times after treatment revealed a rapid 3 to 4 log loss of clonogenic tumor cells, down to undetectable levels. However, after several weeks, clonogenic tumor cells reappeared in the lungs; few cures were obtained even when mice had a relatively low tumor burden when treated with chemotherapy. Splenocyte numbers and response to Concanavalin A indicated a transient immunosuppression. In one experiment, mice were treated with a second round of chemotherapy 3 weeks after the first. The number of clonogenic cells per lung again dropped, but regrowth of cells was rapid, and no cures were obtained. Inoculation of tumor-bearing mice s.c. after chemotherapy with lethally irradiated cells of the highly immunogenic tumor cell line 4TO7-IL-2 had little effect on the rate of reappearance of line 4TO7 in lungs, but subsequent growth of tumor cells in lungs was slowed. This model system can be used to test the efficacy of additional immunotherapy and chemotherapy regimens on minimal residual metastatic disease after high-dose chemotherapy, when remaining metastatic cells are apparently dormant.