Anthralin (1,8‐dihydroxy‐9anthrone, dithranol) is an older anti-psoriatic agent that was first synthesized as a derivative of chrysarobin, obtained from the araroba tree in Brazil over 100 years ago. Adverse effects of anthralin include irritation and discoloration of the skin [A27277]. This specific property of the molecule inspired workers to study details of its pharmacology. It is important to consider that the drug is relatively innocuous, yet effective, and systemic side effects have not been observed with this anthralin, in contrast to a wide variety of systemic and topical therapies for psoriasis [L1935]. Anthralin is also known as dithranol. It is a main active ingredient in topical skin formulations for the treatment of psoriasis. Various formulations of the drug are available, including solutions, foams, and shampoos [L1979]. The chemical structure of anthralin allows for dual solubility, permitting the compound to be absorbed well through the epidermis [A27276]. Anthralin has also been studied in the treatment of warts, showing promising results [A32307]. Salicylic acid is frequently added to anthralin to augment the stability of anthralin and to increase its penetration and efficacy [L1979].

Anthralin inhibits the proliferation of keratinocytes (epidermal skin cells), prevents the action of T-cells, and promotes cell differentiation, likely through mitochondrial dysfunction. In addition, the production of free radicals may contribute to its anti-psoriatic effect [L1979]. In vitro studies demonstrate that anthralin prolongs the prophase component of mitosis for keratinocytes and leukocytes [L1932]. Prophase is the first step of mitosis, the process separating the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells [L1980]. In vivo studies demonstrate that anthralin blocks DNA synthesis and can increase the release of reactive oxygen species [L1932]. Anthralin is believed to normalize the rate of epidermal cell proliferation and keratinization by reducing the mitotic activity of the epidermal hyperplasia in psoriasis [L1979]. Anti-proliferative and anti-inflammatory effects of anthralin have been demonstrated on both psoriatic and healthy skin. The anti-proliferative effects of anthralin are thought to be due to a combination of inhibition of DNA synthesis and its strong reducing properties. The effectiveness of anthralin as an anti-psoriatic agent is partly owed to its ability to promote lipid peroxidation and reduce the concentration of endothelial adhesion molecules, which are found to be elevated in psoriatic patients [L1933], [L1979]. Recent studies suggest that its ability to prevent T-lymphocyte activation and normalize keratinocyte differentiation may occur by a direct effect on mitochondria [A32313].

Anthralin is administered topically. Although the extent of systemic absorption after topical application has not been determined, no traces of anthraquinone metabolites were detected in the urine of treated subjects in a limited clinical study of anthralin cream [L1932], [L1933]. Anthralin does not inhibit hepatic microsomal enzyme activity [L1933].

Some mild adverse effects include alterations in nail coloring, hair coloring, increase in photosensitivity, and skin irritation [L1933]. The most common side effects of anthralin are skin irritation and staining of nearby skin, nails, clothing, and other objects that come in contact with the treated patient. The incidence of irritation of psoriatic/surrounding healthy skin is higher in patients who leave anthralin on the skin without rinsing than in those who use short-contact therapy of 2 hours or less, followed by rinsing [L1939]. If the psoriatic plaques are well circumscribed, the surrounding normal skin may be protected by the use of a coating agent such as zinc oxide ointment. Anthralin should be applied cautiously to the face and intertriginous areas due to the risk of severe skin irritation [L1979]. There is no current evidence of any long-term anthralin toxicity related either to skin exposure or to systemic issues [L1939]. Some long-term studies in mice have demonstrated anthralin to be tumorigenic in mouse skin. This carcinogenic potential has not been thoroughly evaluated. Tumorigenic and carcinogenic effects of anthralin have not been observed in humans at this time [L1932]. Anthralin is classified as FDA pregnancy risk category C drug [L1979]. It is not known if anthralin can cause fetal harm when administered during gestation. Because of the lack of evidential human data, anthralin should be used during pregnancy only when clearly required [L1933].