Dermal absorption and metabolism of the antipsoriatic drug dithranol triacetate

Percutaneous absorption, excretion kinetics, and metabolism of dithranol triacetate (2) have been investigated in Wistar rats. By the use of two differently labelled molecules--3H in the anthracene nucleus and 14C in the acetoxy groups of 2, resp.--the fate of the different parts of the dithranol triacetate molecule could be followed. After injection, large amounts of 2 are cleaved under the influence of enzymes into acetate and dithranol. These deacetylated metabolites lose half their 3H label with formation of 3H2O. In urine, 1,8-diacetoxy-9-anthrone, 1-acetoxy-8-hydroxy-9-anthrone, 1,8-dihydroxy-9,10-anthraquinone and its diacetate were found as metabolites. After dermal application, unchanged 2 is practically not absorbed at all. Arylesterases which, according to in vitro studies, are present in or on the skin, hydrolyse dithranol triacetate to give free dithranol. Up to 33% of the latter are absorbed from under an occlusive dressing. Dithranol triacetate, therefore, shows pro-drug characteristics for the treatment of psoriasis.