There is considerable concern regarding the
biological plausibility of the response of certain chemicals in the in vitro photoclastogenicity assay, suggesting that this assay is oversensitive and lacks specificity. To explore this further, four coded compounds (
aminotriazole,
propantheline bromide,
cycloheximide and
disulfoton) were evaluated for their potential response in a photoclastogenicity assay in cultured Chinese hamster ovary (CHO) cells. None of the four compounds were shown to absorb ultraviolet radiation (UVR) or visible light in the 290- to 700-nm region of the electromagnetic spectrum. A fifth coded compound,
tetracycline, which absorbs UVR, was also tested as this has previously been shown to be phototoxic in vitro (3T3-NRU assay) and is cytotoxic, but not genotoxic, at high concentrations in standard 'dark' genotoxicity assays in mammalian cells. The results showed that
cycloheximide,
disulfoton and
tetracycline were clastogenic in CHO cells following UVR exposure (solar-simulated light at 700 mJ/cm(2)) but not in the absence of UVR.
Aminotriazole and
propantheline were negative in the presence and absence of UVR exposure. Follow-up testing showed that neither
cycloheximide nor
disulfoton was positive in the 3T3-NRU assay, the standard in vitro regulatory test for
phototoxicity, a result consistent with their inability to absorb UVR. These data suggest that both
cycloheximide and
disulfoton are pseudophotoclastogens, like
zinc oxide. Together, these data question the specificity of the in vitro photoclastogencity assay in CHO cells and raises further concern regarding its use for the assessment of chemical photosafety for regulatory purposes. At the very least, a review of the current guidance documents for the photosafety evaluation of
pharmaceuticals and
cosmetics should be undertaken urgently.