MECHANISMS OF CHEMICALLY INDUCED PHOTOSENSITIVITY

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Summary of Work: Photosensitization can result when light interacts with endogenous or exogenous chemical agents in the skin and eyes. This process can produce undesirable Photosensitization can result when light interacts with endogenous or exogenous chemical agents in the skin and eyes. This process can produce undesirable clinical consequences, such as phototoxicity (exaggerated sunburn), photoallergy, or photocarcinogenicity; or it can have beneficial effects as in tumor photodynamic therapy (PDT) and coal tar, anthralin or psoralen (PUVA) therapy for psoriasis. The objective of this research project is to elucidate the photochemical mechanisms whereby photosensitizers exert their toxic or therapeutic effects. Fluoroquinolones (FQ) are a relatively new class of antibacterials that are useful in the treatment of gram-negative bacterial infections. When used in humans FQ?s often cause phototoxicity. Recent studies have shown that lomefloxacin and fleroxacin cause squamous cell carcinomas in hairless mice injected with these drugs and irradiated with UV-A (315-400) nm. We have studied the photochemical properties of lomefloxacin and related FQ?s to determine why these drugs as a class are phototoxic and why lomefloxacin and fleroxacin are photocarcinogenic. Singlet oxygen (1O2) and superoxide yields for the FQ antimicrobials do not correlate with their phototoxic potentials. However, photocleavage of pBR322 DNA by the FQ antibiotics is at least 10-fold more efficient for difluorinated quinolones (lomefloxcin and fleroxacin) than for monofluorinated analogs. 1O2 does not induce photocleavage. Futhermore, the inhibitory effect of O2 on the induction of frank strand breaks makes it unlikely that superoxide could play a major role in the photocleavage of DNA by these antibiotics.We have found that the F-8 atoms of lomefloxacin and fleroxacin are photoeliminated as fluoride. Similarly the Cl-8 chlorine atoms of clinafloxacin and Bay y3118 are photoeliminated as chloride. In contrast the non- photocarcinogenic, less phototoxic FLQ's norfloxacin and ciprofloxacin did not exhibit UVA-induced halide loss. We propose that the carbene generated by halide loss is responsible for the photogenotoxicty of the FLQ's. Berberine is an alkaloid from Goldenseal, an herb that is widely used for medical applications including eyewash and skin disinfectant. Goldenseal is currently undergoing testing by the National Toxicology Program. We have found that concentrations of berberine as low as 1uM killed HaCaT keratinocytes when irradiated by UVA. This effect appears to be related to the generation of singlet oxygen and free radicals upon irradiation. The DNA damaging effect of UVA/berberine are being studied using the comet assay. able clinical consequences, such as phototoxicity (exaggerated sunburn), photoallergy, or photocarcinogenicity; or it can have beneficial effects as in tumor photodynamic therapy (PDT) and coal tar, anthralin or psoralen (PUVA) therapy for psoriasis. The objective of this research project is to elucidate the photochemical mechanisms whereby photosensitizers exert their toxic or therapeutic effects. Fluoroquinolones (FQ) are a relatively new class of antibacterials that are useful in the treatment of gram-negative bacterial infections. When used in humans FQ's often cause phototoxicity. Recent studies have shown that lomefloxacin and fleroxacin cause squamous cell carcinomas in hairless mice injected with these drugs and irradiated with UV-A (315-400) nm. We have studied the photochemical properties of lomefloxacin and related FQ's to determine why these drugs as a class are phototoxic and why lomefloxacin and fleroxacin are photocarcinogenic. Singlet oxygen (1O2) and superoxide yields for the FQ antimicrobials do not correlate with their phototoxic potentials. However, photocleavage of pBR322 DNA by the FQ antibiotics is at least 10-fold more efficient for difluorinated quinolones (lomefloxcin and fleroxacin) than for monofluorinated analogs. 1O2 does not induce photocleavage. Futhermore, the inhibitory effect of O2 on the induction of frank strand breaks makes it unlikely that superoxide could play a major role in the photocleavage of DNA by these antibiotics. We have now found that upon UVA-irradiation the F-8 fluorine atoms of lomefloxacin and fleroxacin are lost as fluoride with the concomitant generation of a carbene at C-8. In contrast non- photocarcinogenic FLQ's norfloxacin and ciprofloxacin did not exhibit UVA-induced fluoride loss. Oxazepam is a commonly prescribed anti-anxiety drug that has been shown to induce hepatocellular adenomas and carcinomas in mice. Studies conducted by the National Toxicology Program have shown that mice chronically treated with this drug develop cataracts. We have found that while the drug itself is a poor generator of singlet oxygen, one of its metabolites, 6-chloro-4-phenyl-2(1H)-quinazoline, is able to sensitize the formation of 1O2 with high efficiency. Finally, 5,7,9(11),22- ergosta-tetraen-3 -ol and 5,7,9(11)-cholestatrien-3-ol have been identified as potential chromophores responsible the bioeffects of UVA in the skin.