In vivo detection of anthralin-derived free radicals in the skin of hairless mice by low-frequency electron paramagnetic resonance spectroscopy.

Free radicals were directly detected in vivo in the skin of hairless mice by low-frequency electron paramagnetic resonance spectroscopy after topical application of anthralin under pertinent therapeutic conditions. The electron paramagnetic resonance signal intensity increased steadily, reaching a maximum after about 1 d and decreased slowly in the following days, probably because of desquamation of the skin. We conclude from the spectroscopic features (single line with a line width of 6 gauss; g = 2.0036) and from the pharmacokinetic pattern that the observed signal arises from the final products of anthralin metabolism (ether-insoluble polymeric structures--"anthralin brown"). Two potential antioxidants, vitamin E and the spin trap tert-butylphenylnitrone, decreased the amount of the anthralin-derived radical that was formed. Neither vitamin E radicals nor tert-butylphenylnitrone spin adducts were observed. We suggest that electron paramagnetic resonance is a valuable tool for the noninvasive and direct in vivo monitoring of drug-induced radical formation in the skin under therapeutic conditions.