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.