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## PHOTOCLEAVAGE OF DNA BY THE FLUOROQUINOLONE ANTIBACTERIALS Lydia J. Martínez*,and Colin F. Chignell† Laboratory of Pharmacology and Chemistry, National Institute of Environmental Health Sciences, National Institutes of Health, P.O. Box 12233, Research Triangle Park, NC 27709 running title: DNA photocleavage by fluoroquinolones Corresponding author: Dr. Colin F. Chignell, Laboratory of Pharmacology and Chemistry, National Institute of Environmental Health Sciences, National Institutes of Health, P.O. Box 12233, Research Triangle Park, NC 27709 Phone number: (919) 541-4575 Fax number: (919) 541-5750 E-mail: chignell@niehs.nih.gov Keywords: fluoroquinolones, lomefloxacin, fleroxacin, norfloxacin, nalidixic acid, enoxacin, DNA, photocleavage. Footnotes * Present address: Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada † To whom correspondence should be addressed. Abbreviations: ssb, single strand-breaks; SOD, superoxide dismutase; Abstract We have determined the relative efficiencies for the formation of single strand-breaks (ssb) after the UVA irradiation of pBR322 DNA and various fluoroquinolone (fleroxacin, lomefloxacin, norfloxacin) and naphthyridine (nalidixic acid, enoxacin) antibacterials. After correcting for the differences in absorption the relative order for DNA photocleaving activity under anaerobic conditions is: fleroxacin, lomefloxacin > nalidixic acid >> norfloxacin > enoxacin. In general, fluoroquinolones having fluorine substituents at the C-6 and C-8 positions (lomefloxacin and fleroxacin) are 10-fold more efficient in generating ssb's than those having only a C-6 fluorine atom (norfloxacin). The effect of oxygen on photoinduced DNA damage caused by these antibacterials is complex, but our data imply that active oxygen species are not necessary for DNA scission by these molecules, and indeed, may sometimes inhibit it. Lomefloxacin ethyl ester, which cannot undergo decarboxylation, is as active as lomefloxacin itself. Thus the free radical generated by decarboxylation is unlikely to be the active species involved in photoinduced DNA cleavage. For lomefloxacin and fleroxacin, DNA damage probably results from the generation of a carbene at C-8 as a result of photoinduced loss of their F8 atom as fluoride upon UVA irradiation. Stable photoproducts may also play a role at least for lomefloxacin. Fluoroquinolones lacking a C-8 fluorine atom must operate by a different mechanism. Although it is not apparent at the present time whether DNA damage is involved in phototoxicity, this adverse side effect is also most severe for lomefloxacin and fleroxacin as compared to mono-fluorinated fluoroquinolones such as norfloxacin, and it is possible that the same active intermediates may play a large role in the phototoxic response.