Monitoring the (photo)genotoxicity of photosensitizer drugs: Direct quantitation of single-strand breaks in deoxyribonucleic acid using an oligonucleotide chip

Oligonucleotide chip-based assays can be a sample-thrifty, time-saving, routine tool for evaluation of chemical-induced DNA strand breaks. This article describes a novel approach using an oligonucleotide chip to determine photosensitizer-induced DNA single-strand breaks. Surface coverage of fluorophore-labeled oligonucleotides on silicon dioxide chip surfaces was determined on alkaline phosphatase digestion. Fluorescence maxima (at 520 nm) of the solutions were converted to molar concentrations of the fluorescein-modified oligonucleotide by interpolation from a predetermined standard linear calibration curve. The photosensitizing activity of chlorpromazine and triflupromazine toward DNA single-strand breaks was then studied at different drug doses and also as a function of photoirradiation time. Photoinduced single-strand breaks calculated using the method described here agreed with values predicted by theoretical extrapolation of the single-strand breaks obtained for plasmid DNAs from agarose gel electrophoresis, and thereby indirectly validated the chip-based assays. Under UV irradiation (⩾93.6 kJ/m2) chlorpromazine (⩾0.08 mM) was found to have significant photogenotoxicity. However, triflupromazine did not exhibit any (photo)genotoxicity over the concentration range studied (0.04-0.20 mM). The method developed will be useful for quantitative screening of drug genotoxicity in terms of induction of breaks in DNA.