Effect of Magnesium and Calcium Complexation on the Photochemical Properties of Norfloxacin

The fluoroquinolone antibiotics can induce skin photosensitivity in some patients and this has been ascribed to the generation of reactive oxygen species, such as singlet oxygen (O 2 [ 1 Δ g ]). We have studied the photochemical properties of the different ionized forms of the fluoroquinolone norfloxacin upon complexation with Mg 2 + and Ca 2 + ions, as it is proposed that the antibiotic exists mainly as a complex in the blood plasma. We found that the norfloxacin cation (pH < 6) shows no photodegradation after UVA irradiation and has a low quantum yield of O 2 ( 1 Δ g ) generation. The norfloxacin cation does not complex. Ca 2 + or Mg 2 + ions; when these ions are added to the solution, we observed no changes in the fluorescence quantum yields (φ flu ) and singlet oxygen yields (φ Δ ). In contrast, the neutral (6 ± pH > 8.5) and anionic (pH > 9) forms of norfloxacin are able to complex calcium and magnesium, and their generation of O 2 ( 1 Δ g ) is decreased by complexation. The neutral zwitterionic form and the anionic form also quench singlet oxygen by both chemical and physical pathways regardless of complex formation, while physical quenching is observed for the cation. At pH > 7.4, norfloxacin photobleaches and complexation to Ca 2 + and Mg 2 + increases the rate at which photobleaching occurs. Thus, both the pH of the medium and complexation with metal cations may affect the phototoxic potential of this antibiotic.