Protein kinase C inhibitor as a potent inhibitor of accelerated repopulation during radiotherapy: I. growth inhibitory, cytotoxic, and indirect radiosensitizing effects of a staurosporine derivative (UCN‐01) in vitro

Abstract

AbstractRecently, various chemicals that attack the regulatory system for cell growth have been developed as potential antitumor agents. We have tested the effect of an inhibitor of protein kinase C (PKC) on cell proliferation and cytotoxicity together with the interaction with radiations. A staurosporine derivative, UCN‐01 (7‐hydroxy staurosporine), was selected because of its relative specificity to PKC. Mouse FSa‐II fibrosarcoma cells were cultured in McCoy's 5a medium in 60 mm Petri dishes. To investigate cell proliferation, the number of cells was counted after trypsinization. Cell survival following drug treatment with or without irradiation was obtained by the in vitro colony formation assay; namely, single cells were plated and treatment was initiated ∼20 hr later. After drug treatment, cells were washed and incubated with fresh medium at 37°C for 7–9 days for colony formation. Survival was determined by normalizing with plating efficiency and cell multiplicity. The growth of FSa‐II cells was inhibited with 100–500 nM UCN‐01 during a 3 day treatment period, whereas no significant growth inhibition was observed following a treatment with 50 nM. In one of these experiments, cells treated with UCN‐01 for 3 days were replated in drug‐free medium. Cells pretreated with 100 nM quickly resumed their proliferation, but cells pretreated with 250 or 500 nM showed a substantial delay of repopulation. Cell survival studies showed the cytotoxic effect of UCN‐01 on FSa‐II cells. Survival curves were best‐fitted by the linear‐quadratic equation. To study whether the growth inhibition is due to the cytotoxic effect or this agent inhibiting cell proliferation, the effect of UCN‐01 on repopulation during fractionated irradiation was investigated. This result confirmed that this agent has a growth inhibitory effect on tumor cells together with a cytotoxic effect, although its synergism with radiation is weak. This result suggests that it can inhibit the cell repopulation during fractionated doses; thus, it could reduce the tumor control dose if used during the course of fractionated radiotherapy. © 1995 Wiley‐Liss, Inc.