Free radicals in skin exposed to dithranol and its derivatives.

The generation of persistent free radical species derived from antipsoriatic drugs in their target tissue under use conditions is reported. Dithranol (anthralin, 1,8-dihydroxy-9-anthrone) and chemical analogues were applied topically to the ear of the pig and the time course of the production of the resulting free radicals was followed by electron spin resonance spectroscopy in biopsy samples. The spectral characteristics observed correspond to data previously reported in vitro for the anthralin-10-yl radical. The role of these radical species in the mode of action of the drug is discussed.