Cutaneous tolerance to nitroxide free radicals in human skin

No data are available on the irritant effect of nitroxide free radicals in human skin. Nitroxides are important biomedical skin probes used in Electron Paramagnetic Resonance spectroscopy and imaging. Our purpose was to study the skin irritation potential of different nitroxide free radical structures in skin of healthy human subjects. We investigated the following nitroxides: Tempo (2,2,6,6-tetramethyl-1-piperidinoxy), Doxo (2,2,5,5-tetramethyl-3-oxazolidinoxy), Proxo (2,2,5,5-tetramethyl- -dihydro-pyrrolinoxy), and Imidazo (2,2,3,4,5,5-hexamethyl-imidazoline-1-yloxyl). Cutaneous irritation was determined in human skin following a single application and after repetitive applications in comparison to the standardized irritant sodium lauryl sulfate (SLS). The response was evaluated clinically as well as by a bioengineering method analyzing transepidermal water loss (TEWL) and skin hydration (capacitance). The nitroxides were classified clinically from nonirritant (Imidazo, Proxo), to slightly irritant (Doxo, 100 mM), or moderately irritant (Tempo 100 mM) after a single application. The TEWL values were significantly increased by Doxo and Tempo, but capacitance values were not changed significantly. In the cumulative irritation test Tempo was scored as a slight irritant (10 mM). TOLH (2,2,6,6-tetramethyl-1-hydroxypiperidin), the hydroxylamine of Tempo, which is the major skin metabolite, did not cause skin irritation after a single or repetitive applications. This may indicate that a loss of cellular reducing equivalents may be involved in the inflammation process caused by Tempo. The order of nitroxide irritation potency (Tempo > Doxo >> Imidazo = Proxo) is inverse to the order of nitroxide biostability in human skin (Imidazo = Proxo >> Doxo > Tempo). In conclusion, nitroxide free radicals are classified as nonirritant to moderately irritant in human skin. Particularly, the pyrrolidine and imidazoline type nitroxides have a low potential to cause acute or subacute skin toxicity.