Optimal synchronization of breast cancer cell proliferation by hormonal means may be limited by cellular heterogeneity in sensitivity to the multistep activation of growth following initial hormone binding to the receptor. We hypothesized that induced synchronous growth may be improved by combined manipulation of the polyamine (PA) pathway since we have previously shown that PAs are distal effectors of hormonal action on proliferation in breast cancer. To test our hypothesis, we induced an initial phase of hormone and PA depletion (castration plus administration of the PA synthesis inhibitor α-difluoromethylornithine) in rats bearing N-nitrosomethylurea induced mammary tumors. This was followed by transition phase of hormone repletion in the presence of α-difluoromethylornithine (to push the cells into the proliferative cascade up to the distal step controlled by PA) and finally a phase of hormone and PA repletion. Simultaneously, groups of rats were subjected to hormone/PA depletion/repletion individually. The effects of these manipulations on the labeling indices (LIs) of glandular, myoepithelial, and nonepithelial cells were estimated by autoradiography. The combined hormone/PA manipulation yielded the highest degree of synchronization with LIs of the glandular and myoepithelial cells being ∼-2-fold over intact control after only 2 or 3 days of combined repletion. In contrast, hormone treatment alone restored the LIs of glandular cells only to control levels and minimally influenced those of myoepithelial cells. PA manipulation alone failed to affect the LIs of any cell type. Although the rate of tumor regrowth was highest with the combination treatment, the absolute tumor volumes did not differ significantly at the end of the repletion phase between the three regimens. These results indicate that combined hormone/PA manipulation provides the best “therapeutic window” (LI/tumor volume) for implementation of kinetically based cytotoxic chemotherapy.
This work is supported by Grant PO1 CA40011 from the National Cancer Institute.