Although side effects of
cancer chemotherapy are well known, "opposite effects" of
chemotherapy that enhance the
malignancy of the treated
cancer are not well understood. In this report, we describe the induction of intravascular proliferation, extravasation, and colony formation by
cancer cells, critical steps of
metastasis, by pretreatment of host mice with the commonly used
chemotherapy drug
cyclophosphamide. In contrast, in the unpretreated mice, most
cancer cells remained quiescent in vessels without extravasation. HT1080 human
fibrosarcoma cells, labeled in the nucleus with
green fluorescent protein and
red fluorescent protein in the cytoplasm for imaging, were injected into the epigastric cranialis vein of nude mice. Twenty-four hours before
cancer cell injection,
cyclophosphamide was given i.p. Double-labeled
cancer cells were imaged at the cellular level in live mice with the Olympus OV100 Small Animal Imaging System with variable magnification.
Cyclophosphamide seems to interfere with a host process that inhibits intravascular proliferation, extravasation, and extravascular colony formation.
Cyclophosphamide does not directly affect the
cancer cells because
cyclophosphamide has been cleared by the time the
cancer cells were injected. This report shows an important unexpected "opposite effect" of
chemotherapy that enhances critical steps in
malignancy rather than inhibiting them, suggesting that certain current approaches to
cancer chemotherapy should be modified.