The licorice extract includes liquiritin, isoliquertin (a chalcone) that occurs as a glycoside and during drying is partly converted into liquiritin, liquiritigenin, isoliquiritigenin, and other compounds. Liquiritin causes depigmentation by two mechanism: (i) via melanin dispersibility by means of the pyran ring of the color dispersing flavonoidal nucleus of
liquiritin, and (ii) via amelanodermic and epidermal stain removing property. Acute and chronic toxicity studies have been carried out with no adverse effects. Glabrene and isoliquiritigenin (20, 4′, 4-trihydroxychalcone) in the licorice extract can inhibit both mono – and diphenolase tyrosinase activities. The IC50 values for glabrene and isoliquiritigenin were 3.5 and 8.1 pM, respectively, when tyrosine was used as substrate. The effects of glabrene and isoliquiritigenin on tyrosinase activity were dose-dependent and correlated to their ability to inhibit melanin formation in melanocytes (31).
Liquiritin cream is a new bleaching agent. Amer et al. (32) described that topical liquiritin cream applied at 1 g/day for four weeks is therapeutically effective in melasma. Good to excellent results with complete disappearance of melasma were observed in 18 (90%) out of 20 patients. Yasuaki (33) described the formulation of a liquiritin cream containing 20% liquorice. The cream was applied at 1 g/day to patients with melasma for one to four months and showed good efficacy. Side effects were minimal with mild irritation, which disappeared with continuation of treatment.