Mack, P. C., Jones, A. A., Gustafsson, M. H., Gandara, D. R., Gumerlock, P. H. and Goldberg, Z. Enhancement of Radiation Cytotoxicity by UCN-01 in Non-small Cell Lung Carcinoma Cells. Radiat. Res. 162, 623–634 (2004).

Thoracic ionizing radiation is a standard component of combined-modality therapy for locally advanced non-small cell lung cancer. To improve low 5-year survival rates (5– 15%), new strategies for enhancing the effectiveness of ionizing radiation are needed. The kinase inhibitor UCN-01 has multiple cell cycle effects, including abrogation of DNA damage-induced S- and G2-phase arrest, which may limit DNA repair prior to mitosis. To test the hypothesis that therapy-induced cell cycle effects would have an impact on the efficacy of a combination of UCN-01 plus ionizing radiation, the cell cycle responses of the non-small cell lung cancer cell lines Calu1 (TP53-null) and A549 (wild-type TP53) to 2 Gy ionizing radiation were correlated with clonogenic survival after irradiation plus UCN-01. Irradiated cells were exposed to UCN-01 simultaneously and at 3-h increments after irradiation. In Calu1 cells but not A549 cells, sequence-dependent potentiation of radiation by UCN-01 was observed, with maximal interaction occurring when UCN-01 was administered 6 h after irradiation. This coincided with the postirradiation time with the greatest depletion of cells from G1. Abrogation of G2 arrest was observed regardless of TP53 status. The role of TP53 was investigated using siRNA to achieve gene silencing. These studies demonstrated that radiation plus UCN-01 was more effective in cells with diminished TP53 activity, associated with a reduced G1 checkpoint arrest. These studies indicate that simultaneous elimination of multiple DNA damage-induced checkpoints in G1, S and G2 may enhance the effects of radiation and that drug scheduling may have an impact on clinical efficacy.

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