The action of the topical antipsoriatic agents is far from clear. The evidence available suggests that, in contrast to systemically administered drugs, these may not be primarily mitotic inhibitors. Consequently, several topical modalities were studied for their effect on human epidermal mitotic activity. The corticosteroids showed a mitotic inhibition not seen with other steroids tested. Tretinoin applied under occlusion to normal skin, or without occlusion to stripped skin, increased mitotic activity.
Crude coal tar cream, liquor carbonis detergens, and anthralin in either a cream base or a modified zinc oxide (Ingram) paste showed no effect on the mitotic activity of either normal or stripped skin. The addition of sunlight or blacklight did not influence these results. It is suggested that the therapeutic action of these antipsoriatic agents may be other than via a direct mitotic inhibition.
Fisher LB, Maibach HI. Topical Antipsoriatic Agents and Epidermal Mitosis in Man. Arch Dermatol. 1973;108(3):374-377. doi:10.1001/archderm.1973.01620240028006