Protection of normal proliferating cells against chemotherapy by staurosporine-mediated, selective, and reversible G(1) arrest

Background: A major limiting factor in human cancer chemotherapy is toxicit y in normal tissues. Our goal was to determine whether normal proliferating cells could be protected from chemotherapeutic agents by taking advantage of the differential drug sensitivity of cell cycle G(1) checkpoint in norma l and cancer cells. Methods: Normal mammary epithelial cells and mammary ca ncer cells were initially treated with staurosporine at a cytostatic (i.e., nonlethal) concentration, which preferentially arrests normal cells in the G(0)/G(1) phase of the cell cycle without affecting the proliferation of t umor cells. After the selective arrest of normal cells in G(0)/G(1), both n ormal and tumor cells were treated with doxorubicin or camptothecin, two cy totoxic (i.e., lethal) chemotherapeutic agents. Cells were then allowed to recover in drug-free medium for 12 days. Results: After pretreatment of bot h normal and tumor cells with staurosporine followed by treatment with doxo rubicin or camptothecin, tumor cells were selectively killed by chemotherap eutic agents, whereas normal cells resumed proliferation after the drugs we re removed. Pretreatment with staurosporine also protected normal circulati ng lymphocytes that had been induced to proliferate in vitro with phytohema gglutinin from chemotherapeutic agents. Staurosporine-induced arrest of nor mal cells in G(0)/G(1), phase was reversible, and arrested cells tolerated doses of camptothecin that were more than 100-fold higher than necessary to eradicate all tumor cells in culture. Staurosporine-mediated G(0)/G(1) arr est targets the retinoblastoma protein (pRb) pathway and was accompanied by a rapid decrease in cyclin-dependent kinase (CDK) 4 protein levels, increa sed binding of CDK inhibitors p21 and p27 to CDK2, and inhibition of CDK2 a ctivity in normal cells. Conclusions: Breast cancer cells with defective ch eckpoints regulated by the pRb pathway can be targeted specifically with ch emotherapeutic agents, following staurosporine-mediated, selective and reve rsible G(0)/G(1) arrest in normal cells.