Celecoxib enhances the radiosensitizing effect of 7-hydroxystaurosporine (UCN-01) in human lung cancer cell lines

Purpose: 7-Hydroxystaurosporine (UCN-01), a Chk1-specific inhibitor, showed promising in vitro and in vivo chemo- or radiosensitizing activity. However, there have been concerns about its limited therapeutic efficacy and risk of side effects. A method of enhancing the treatment efficacy of UCN-01 while not increasing its side effects on normal tissue may therefore be required to apply this drug in clinical settings. Celecoxib is a cyclooxygenase-2 (COX-2)-specific inhibitor that downregulates ataxia telangiectasia and rad3-related (ATR) protein, an upstream kinase of Chk1. In this study, we investigated whether the addition of celecoxib can potentiate the radiosensitizing effect of UCN-01. Methods and Materials: The cooperative radiosensitizing effects and the underlying molecular mechanisms of UCN-01 plus celecoxib were determined by clonogenic assay, tumor growth delay assay, flow cytometry, and Western blotting. Synergism of the three agents combined (UCN-01 plus celecoxib plus radiation) were evaluated using median drug effect analysis and drug-independent action model analysis. Results: The combination of UCN-01 and celecoxib could induce synergistic cytotoxicity and radiosensitizing effects in in vitro and in vivo systems. The combination of both drugs also cooperatively inhibited IR-induced G ₂/M arrest, and increased the G ₂ to mitotic transition. Conclusions: Combined treatment with UCN-01 and celecoxib can exert synergistically enhanced radiosensitizing effects via cooperative inhibition of the ionizing radiation-activated G ₂ checkpoint. We propose that this combination strategy may be useful in clinical applications of UCN-01 for radiotherapy of cancer patients.