The effect of fumaric acid esters and dithranol on acanthosis and hyperproliferation in psoriasis vulgaris.

In this study we investigated the histological changes, regression of acanthosis and rate of proliferation, that accompany the healing of psoriatic lesions after fumaric acid esters and dithranol treatment. Biopsies were taken before and during therapy as well as from neighbouring untreated, clinically uninvolved skin and healthy, non-psoriatic volunteers. Specimens were assessed using computer-supported image analysis and immunohistology. The parameters primarily examined were the height of the rete pegs and of the epithelium above the papillary body, the rate of proliferation, the actual number of cells in the two epidermal compartments and the cellular density in the epidermis. Both fumaric acid esters and dithranol reduce the degree of acanthosis; however, the mechanism and the rate of the reduction differ. While under fumaric acid esters the reduction is more rapid at first but subsequently slows down, dithranol leads to a slow but steady decrease of epidermal thickness, so that at the end of our study the degree of acanthosis was less under dithranol. As an underlying mechanism of action, we found that fumaric acid esters reduce the rate of proliferation and thereby decrease the number of cells per rete peg as well as the size of the individual keratinocytes. Dithranol in contrast does not reduce cell renewal. The decrease of the number of cells in the rete pegs might be caused by an increased differentiation time.