Changes in sensitivity to cyclophosphamide of mouse mammary tumors during serial transplantation.

The effect of the cytostatic agent cyclophosphamide on the growth of several transplant generations of a GRS/A mouse mammary tumor line was studied. Cyclophosphamide caused more growth inhibition in transplant generation 6 than in other transplant generations. Transplant generation 6 represented the time of transition in the tumor line from hormone responsiveness to hormone independence. These results suggest that mammary tumors are especially sensitive to chemotherapy at the stage when the first hormone-independent cells appear in the tumor.