The cytotoxic action of 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide in Chinese hamster ovary cells is enhanced considerably in the presence of light, when 5-diazoimidazole-4-carboxamide is being continuously generated. A broad shoulder threshold-type survival curve is seen when the cells are exposed to 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide (both in light and dark).

The drug had only slight effects on the survival of cells treated in the dark for short times throughout the cell cycle. However, cells in the G1 and early S phases were slightly more sensitive when treated with the drug in the light. Both light and dark treatments with the drug caused progression delay in S phase. Treatment in the light delayed cells at the G2-M boundary, whereas dark treatments delayed cells at the S-G2 boundary. Both light and dark treatments with the drug caused some G1 cells to progress into S phase 1 hr early, while another fraction of the same population was delayed in G1 phase.

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Work supported by Contract PH 43-66-1156 with Chemotherapy, National Cancer Institute, USPHS, and Damon Runyon Grant 1059.

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