[Stability of cignolin (dithranol) in ointments containing tar with and without the addition of salicylic acid. Oxidation to danthron and dithranol dimer].

The stability of dithranol in white petrolatum was investigated over a period of 10 weeks in dithranol ointments containing liquor carbonis detergens (LCD) and/or coal tar, in the presence and absence of salicylic acid. Qualitative and semiquantitative analysis using high-pressure liquid chromatography (HPLC) showed that dithranol (0.1%, 0.5%, 1.0%) remained rather stable in white petrolatum for 10 weeks, regardless of the presence or absence of salicylic acid. The addition of 5% LCD or 5% crude coal tar resulted, however, in the rather rapid oxidation of dithranol into its products, danthron and dithranol dimer. The addition of 2% salicylic acid to these combined ointments did not stabilize the dithranol: rather, it reduced the amount of danthron but resulted in a considerable increase in the amount of dithranol dimer. The oxidation of dithranol into its products danthron and dithranol-dimer was almost complete in 0.1% ointments and ranged between 16.3% and 53.8% in 0.5% white petrolatum after 10 weeks. These findings may explain why the addition of tar minimizes dithranol-induced erythema, as reported in clinical studies. Such combinations may have an important influence on the antipsoriatic activity of dithranol and the appearance of brownish discoloration.