Dithranol, although a time-honoured treatment and from the beginning of the previous century still going strong, remains an empirical treatment. There is growing evidence that the biochemical basis for the mechanism of action of dithranol at the molecular level is related to the redox activity leading to the production of active oxygen species, which include singlet oxygen, superoxide anion radical and hydroxyl radical. Some authors suggest that epidermal proliferation and/or keratinisation may be the target for dithranol, while others refer to aspects of cutaneous inflammation as crucial in the antipsoriatic effect of dithranol. The present study aims to analyse the effect of single and repeated applications of dithranol on aspects of epidermal proliferation, keratinisation and inflammation in the psoriatic plaque. The most marked effect of dithranol proved to be that on epidermal proliferation (the number of Ki-67-positive nuclei) with an early reduction already 1 day following the single application. This reduction lasted for 16 days. However, such an application induced only a modest clinical improvement. Repeated challenges, resulting in a decrease in the number of Ki-67-positive nuclei of 66%, led to a substantial clinical improvement after 12 days. Repeated challenges resulted in a significant reduction of the number of polymorphonuclear leucocytes. However, this reduction was less pronounced as compared to the effect on epidermal proliferation. It is concluded that epidermal proliferation is a sensitive marker to demonstrate an early effect of dithranol. The dynamics of the cell-biological responses suggest that intermittent applications might be a promising new approach. As dithranol does not reduce the number of T lymphocytes, it is attractive to speculate that the combination of dithranol with immunosuppressive treatments might be a very effective combination.

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