In Vitro Photochemical Induction of Kojic Acid-DNA Adducts

Objective: Kojic Acid (KA) is a natural substance which inhibits skin melanin production and, when used alone or in combination with other compounds, is effective in reducing pigmentation in melasma patients. KA was classified by IARC as presenting limited evidence of carcinogenicity based largely on a single study showing thyroid neoplasms. More recently KA has been shown to be carcinogenic in rat liver, but not mouse skin. KA has produced positive and negative results in both in vivo and in vitro genotoxicity tests and was considered to be genotoxic in vitro by IARC. KA is photochemically unstable causing in vitro breakage of DNA. Consistent with this, photochemically KA is weakly mutagenicity in bacteria and induced chromosome aberrations in Chinese hamster lung cells, although it was negative on mouse epidermis in a photo-micronucleus assay. To assess the potential of KA to photochemically induce formation of DNA adducts, we investigated effects of irradiating KA and DNA with 320 nm light and analyzing for DNA adducts by the nucleotide 32P-postlabeling (NPL) method. Methods: The ability of KA irradiated at 320 nm to induce formation of DNA adducts was compared to direct chemical reaction of chloro-KA with DNA. To ascertain the reliability of this approach, we also studied adduct formation by the photoactivated 8-mehtoxy-(8-MOP) and 4.5`, 8-trimethyl-(TMP) psoralens. Results: All compounds in the presence of UV irradiation produced DNA adducts assessed by one or more of the NPL enhancement methods used. The pattern of major adducts formed photochemically with KA and directly with chloro-KA were similar, but could represent residual KA in chloro-KA samples. Psoralens produced different pattern of adducts. Conclusions: Thus, KA may have the potential to be photoactivated to DNA-damaging products in skin.