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Role of the Nitro Functionality in the DNA Binding of 3-Nitro-10-methylbenzothiazolo[3,2-a]quinolinium Chloride

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journal contribution
posted on 15.09.2008, 00:00 by Iris Gisela Colón, Fernando A. González, Marisol Cordero, Beatriz Zayas, Christian Velez, Osvaldo Cox, Ajay Kumar, Antonio E. Alegría
Interest in DNA binding drugs has increased in recent years due to their importance in the treatment of genome-related diseases, like cancer. A new family of water-soluble DNA binding compounds, the benzothiazolo[3,2-a]quinolinium chlorides (BQCls), is studied here as potential candidates for chemical treatment of solid tumor cells that may encounter low-oxygen environments, a condition known as hypoxia. These compounds are good DNA intercalators; however, no studies have been made of these compounds under hypoxic conditions. This work demonstrates the importance of the nitro-functionality in the DNA binding of 3-nitro-10-methylbenzothiazolo[3,2-a]quinolinium chloride (NBQ-91), which possesses nitro-functionality, and 10-methylbenzothiazolo[3,2-a]quinolinium chloride (BQ-106), which does not. Both NBQ-91 and BQ-106 have similar noncovalent binding affinity toward DNA. Dialysis experiments show that NBQ-91 binds DNA under N2-saturated conditions with increasing concentrations of reducing agent, presumably due to reduction of the nitro-functionality. Conversely, because of the lack of nitro-functionality, the presence of a reducing agent had no effect on BQ-106 binding to DNA under both aerobic and N2-saturated conditions. Clonogenic assays were performed to determine the quinolinium chloride cytotoxicities under both aerobic (95% air and 5% CO2) and hypoxic (80% N2 and 20% CO2) conditions. The calculated ratios of cellular toxicity under aerobic to hypoxic conditions caused by the same concentration of test agent (CTR values) show greater levels of cell death under hypoxia than under aerobic conditions for mitomycin C (MC) (CTR = 0.7 at 1 μM) and NBQ-91 (CTR = 0.4 at 200 μM) than for BQ-106 (CTR = 1.0 at 200 μM), which agreed with the previously reported data for MC and confirmed the importance of nitro-functionality for reactivity under hypoxic conditions. There was no correlation between noncovalent binding affinity constants and their cytotoxicity under hypoxic conditions. Adduct formation between the NBQ-91 and 2′-dG was also assessed by reacting 2′-dG or DNA with NBQ-91 and BQ-106 under N2-saturated conditions in the presence of hypoxanthine and xanthine oxidase (HX/XO). DNA covalent adduct formation was analyzed by two techniques: LC-ESI-MS and Sephadex size exclusion chromatography. LC-ESI-MS results clearly indicate the formation of a prominent molecular ion at masses of 266.0 and 530.58 Da, corresponding to the [M + H]+2 and [M]+ molecular ions of the monitored 2′-dG-NBQ-91 adduct. Results from the Sephadex size exclusion chromatography support these findings because the NBQ-91 elution percentage increases in the presence of HX/XO due to the reduction of the nitro-functionality, which results in covalent binding to DNA. This study reports evidence of the DNA binding capacity of this bioreductive drug. The preferential N2-saturated over aerobic conditions for DNA binding makes NBQ-91 a potential bioreductive compound for hypoxic cell killing.