[Chemical instability of the antipsoriatic dithranol and chrysarobin in aqueous systems: oxidation behavior]

Decomposition of dithranol (1) and chrysarobin (4) in homogeneous and heterogeneous buffer solutions (pH 7.5) were studied by means of HPLC-chromatography. Half lives of (1) under homogeneous conditions were 1 h 15 min., under heterogeneous conditions 2 h 45 min., whereas in the case of (4) 3 h and 30 min. respectively were measured. (1) was degraded to dihydroxyanthrachinon (2) and dimeric dithranol (3), the amount of (2) and (3) depending of the chosen conditions, whereas degradation of (4) under heterogeneous conditions revealed no oxidation products. On the other hand two metabolites of (4) were detected in homogeneous buffer solutions. The presenting studies clearly show the different ways of decomposition of (1) and (4) in homogeneous and heterogeneous solutions.