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Many natural compounds mop up radicals and limit radical reactions and may prove useful in reducing or preventing oxidative stress-related diseases in vivo. Several assays have been developed to measure antioxidant or anti-radical activity. Here, we measured the anti-radical activities of representative antioxidants using different assays. The oxygen radical absorption capacity (ORAC) assay has two mechanistic stages. We classified antioxidant behavior using two characteristic values thought to be related to the two stages — peroxyl radical formation time (lag time) and fluorescein annihilation rate (kobs) — by applying Voronoi polyhedral division. We focused on four class-representative antioxidants, Trolox ®, vitamin C, L-cysteine, and 2,6-di-tert-butyl-p-cresol, and compared their characteristic activities with those of edaravone. Our analysis indicates that edaravone is in the same group as cysteine and may function via a similar mechanism. Our results suggest that analyzing lag time and kobs is a useful method to characterize antioxidants.

Ketoprofen is one of the non-steroidal anti-inflammatory drugs used for fomentation. It is associated with undesirable photosensitivity caused by UV irradiation. Moreover, it induces photodegradation, including decarboxylation, when exposed to UV irradiation. In this study, we attempted to identify compounds that can inhibit the photodegradation of ketoprofen. UV irradiation of ketoprofen follows first-order reaction kinetics and yields three photoproducts. Aqueous solutions containing ketoprofen generate free radicals when it was irradiated with UV. We considered that these free radicals react with ketoprofen. Antioxidants inhibit the photodegradation of ketoprofen. On the other hand, a local anesthetic, dibucaine hydrochloride, was also effective in inhibiting the photodegradation of ketoprofen. We evaluated the difference in the mechanism of inhibition between the antioxidants and the local anesthetic. In conclusion, both antioxidants and dibucaine hydrochloride are effective inhibitors of the photodegradation of ketoprofen.