title: Efficient, Mg(2+)-dependent photochemical DNA cleavage by the antitumor quinobenzoxazine (S)-A-62176.
abstract: The quinobenzoxazines, a group of structural analogues of the antibacterial fluoroquinolones, are topoisomerase II inhibitors that have demonstrated promising anticancer activity in mice. It has been proposed that the quinobenzoxazines form a 2:2 drug-Mg(2+) self-assembly complex on DNA. The quinobenzoxazine (S)-A-62176 is photochemically unstable and undergoes a DNA-accelerated photochemical reaction to afford a highly fluorescent photoproduct. Here we report that the irradiation of both supercoiled DNA and DNA oligonucleotides in the presence of (S)-A-62176 results in photochemical cleavage of the DNA. The (S)-A-62176-mediated DNA photocleavage reaction requires Mg(2+). Photochemical cleavage of supercoiled DNA by (S)-A-62176 is much more efficient that the DNA photocleavage reactions of the fluoroquinolones norfloxacin, ciprofloxacin, and enoxacin. The photocleavage of supercoiled DNA by (S)-A-62176 is unaffected by the presence of SOD, catalase, or other reactive oxygen scavengers, but is inhibited by deoxygenation. The photochemical cleavage of supercoiled DNA is also inhibited by 1 mM KI. Photochemical cleavage of DNA oligonucleotides by (S)-A-62176 occurs most extensively at DNA sites bound by drug, as determined by DNase I footprinting, and especially at certain G and T residues. The nature of the DNA photoproducts, and inhibition studies, indicate that the photocleavage reaction occurs by a free radical mechanism initiated by abstraction of the 4'- and 1'-hydrogens from the DNA minor groove. These results lend further support for the proposed DNA binding model for the quinobenzoxazine 2:2 drug-Mg(2+) complex and serve to define the position of this complex on the minor groove of DNA.
authors.affiliation: Division of Medicinal Chemistry and Institute for Cellular and Molecular Biology, The University of Texas at Austin, 78712, USA.
authors.forename: H
authors.initials: H
authors.lastname: Yu
authors.forename: Y
authors.initials: Y
authors.lastname: Kwok
authors.forename: L H
authors.initials: LH
authors.lastname: Hurley
authors.forename: S M
authors.initials: SM
authors.lastname: Kerwin
mesh_terms: D000332:Aerobiosis; D000693:Anaerobiosis; D000890:Anti-Infective Agents / Q000494:pharmacology; D000970:Antineoplastic Agents / Q000494:pharmacology*; D001483:Base Sequence; D001665:Binding Sites; D001761:Bleomycin / Q000494:pharmacology; D004247:DNA / Q000187:drug effects* / Q000528:radiation effects*; D004249:DNA Damage; D003850:Deoxyribonuclease I / Q000378:metabolism; D024841:Fluoroquinolones; D016166:Free Radical Scavengers / Q000494:pharmacology; D005609:Free Radicals; D017665:Hydroxyl Radical; D008274:Magnesium; D008751:Methylene Blue / Q000494:pharmacology; D008956:Models, Chemical; D008969:Molecular Sequence Data; D010078:Oxazines / Q000494:pharmacology*; D010787:Photosensitivity Disorders; D015363:Quinolones / Q000494:pharmacology*
chemical_list: D000890:Anti-Infective Agents; D000970:Antineoplastic Agents; D024841:Fluoroquinolones; D016166:Free Radical Scavengers; D005609:Free Radicals; D010078:Oxazines; D015363:Quinolones; D001761:Bleomycin; C091925:A 62176; D017665:Hydroxyl Radical; D004247:DNA; D003850:Deoxyribonuclease I; D008274:Magnesium; D008751:Methylene Blue