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Fenofibric acid, the major metabolite of fenofibrate, was found to be photolabile. Its irradiation in aqueous solution gave rise to two photoproducts, whose formation involves photodecarboxylation of the dissociated acid to an aryloxy-substituted carbanion, which is directly protonated or, alternatively, undergoes a Wittig rearrangement. A comparative in vitro phototoxicity study has been carried out on the anti-hyperlipoproteinemic drug fenofibrate, its metabolites and the photoproducts of fenofibric acid. Fenofibrate, fenofibric acid and its two photoproducts were found to be active when examined by the photohemolysis test and were able to photosensitize peroxidation of linoleic acid, as evidenced by the UV monitoring of dienic hydroperoxides. In summary, the major metabolite of fenofibrate (fenofibric acid), as well as its photoproducts, are phototoxic in vitro. This behavior can be attributed to the fact that the four compounds retain the benzophenone chromophore present in fenofibrate and is indicative of free radical-mediated photosensitization. In agreement with this rationalization, the metabolites with a reduced ketone functionality exhibit no detectable in vitro phototoxicity.

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Vicinal diols and disulfides are the products of the title reaction, which presumably is initiated by a one‐electron transfer according to (a) (XH). The oxygen transfer in the reaction of dioxetanes and sulfides (XR″) as well as the cleavage of a dioxetane molecule into two ketone molecules, observed in both cases as a side reaction, can also be explained in terms of starting from the radical ion pair in (a). The first experimental proof has been found for the reduction of dioxetanes by glutathione in biological systems actually being decisive for their conversion into non‐toxic compounds.

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