Tyrosinase is the rate-limiting enzyme of melanin production and, accordingly, is the most prominent target for inhibiting hyperpigmentation. Numerous tyrosinase inhibitors have been identified, but most of those lack clinical efficacy because they were identified using mushroom tyrosinase as the target. Therefore, we used recombinant human tyrosinase to screen a library of 50,000 compounds and compared the active screening hits with well-known whitening ingredients. Hydroquinone and its derivative arbutin only weakly inhibited human tyrosinase with a half-maximal inhibitory concentration (IC) in the millimolar range, and kojic acid showed a weak efficacy (IC > 500 μmol/L). The most potent inhibitors of human tyrosinase identified in this screen were resorcinyl-thiazole derivatives, especially the newly identified Thiamidol (Beiersdorf AG, Hamburg, Germany) (isobutylamido thiazolyl resorcinol), which had an IC of 1.1 μmol/L. In contrast, Thiamidol only weakly inhibited mushroom tyrosinase (IC = 108 μmol/L). In melanocyte cultures, Thiamidol strongly but reversibly inhibited melanin production (IC = 0.9 μmol/L), whereas hydroquinone irreversibly inhibited melanogenesis (IC = 16.3 μmol/L). Clinically, Thiamidol visibly reduced the appearance of age spots within 4 weeks, and after 12 weeks some age spots were indistinguishable from the normal adjacent skin. The full potential of Thiamidol to reduce hyperpigmentation of human skin needs to be explored in future studies.

Background Hyperpigmentary disorders like melasma, actinic and senile lentigines are a major cosmetic concern.

SummaryInteractions between melanocytes and neighboring cells in the skin are important in regulating skin color in humans. We recently demonstrated that the less pigmented and thicker skin on the palms and soles is regulated by underlying fibroblasts in those areas, specifically via a secreted factor (DKK1) that modulates Wnt signaling. In this study, we tested the hypothesis that dermal fibroblasts regulate the constitutive skin color of individuals ranging from very light to very dark. We used microarray analysis to compare gene expression patterns in fibroblasts derived from lighter skin types compared to darker skin types, with a focus on secreted proteins. We identified a number of genes that differ dramatically in expression and, among the expressed proteins, neuregulin-1, which is secreted by fibroblasts derived from dark skin, effectively increases the pigmentation of melanocytes in tissue culture and in an artificial skin model and regulates their growth, suggesting that it is one of the major factors determining human skin color.

Although photodegradation of the retinal pigment epithelium (RPE) melanin may contribute to the etiology of age-related macular degeneration, the molecular mechanisms of this phenomenon and the structural changes of the modified melanin remain unknown. Recently, we found that the ratio of pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA) to pyrrole-2,3,5-tricarboxylic acid (PTCA) is a marker for the heat-induced cross-linking of eumelanin. In this study, we examined UVA-induced changes in synthetic eumelanins to confirm the usefulness of the PTeCA/PTCA ratio as an indicator of photo-oxidation and compared changes in various melanin markers and their ratios in human melanocytes exposed to UVA, in isolated bovine RPE melanosomes exposed to strong blue light and in human RPE cells from donors of various ages. The results indicate that the PTeCA/PTCA ratio is a sensitive marker for the oxidation of eumelanin exposed to UVA or blue light and that eumelanin and pheomelanin in human RPE cells undergo extensive structural modifications due to the life-long exposure to blue light.

Racemic RS-4-(4-hydroxyphenyl)-2-butanol (rhododendrol, RD) was used as a topical skin-whitening agent until it was recently reported to induce leukoderma. We then showed that oxidation of RD with mushroom tyrosinase rapidly produces RD-quinone, which is quickly converted to RD-cyclic quinone and RD-hydroxy-p-quinone. In this study, we examined whether either or both of the enantiomers of RD can be oxidized by human tyrosinase. Using a chiral HPLC column, racemic RD was resolved optically to R(-)-RD and S(+)-RD enantiomers. In the presence of a catalytic amount of l-dopa, human tyrosinase, which can oxidize l-tyrosine but not d-tyrosine, was found to oxidize both R(-)- and S(+)-RD to give RD-catechol and its oxidation products. S(+)-RD was more effectively oxidized than l-tyrosine, while R(-)-RD was less effective. These results support the notion that the melanocyte toxicity of RD depends on its tyrosinase-catalyzed conversion to toxic quinones and the concomitant production of reactive oxygen species.

Objective: Skin ageing is a multifactorial process involving formation of reactive oxygen species, consecutive inflammation with reduced epidermal and dermal cell viability and resulting damage to the extracellular matrix. Effective dermocosmetic treatment modalities should ideally address these hallmarks in a holistic approach.Here, we determined the corresponding activity profile of bakuchiol, a plant-derived meroterpene, in an array of in vitro, ex vivo and in vivo studies and compared it to retinol, currently considered as gold standard in topical antiageing cosmetics. Methods:The antioxidative capacity and power of bakuchiol and retinol were analysed by measuring 2,2′-diphenyl-1-picrylhydrazyl (DPPH) reduction via its absorption decay and electron spin resonance spectroscopy, respectively. Effects on prostaglandin E2 (PGE2), macrophage migration inhibitory factor (MIF), fibroblast growth factor 7 (FGF7), collagen type I and VII (COL1A1, COL7A1), fibronectin (FN) levels as well as the metabolization of water-soluble tetrazolium 1 (WST-1) were determined in human dermal fibroblasts. Epidermal regeneration was assessed utilizing an in vitro wound healing model. FN protein levels were analysed ex vivo after treatment with a formulation containing bakuchiol, retinol or vehicle using suction blister fluid. Skin condition improvement was determined in vivo in a split-face comparison study after application of bakuchiol or vehicle. Results:In contrast to retinol, bakuchiol demonstrated high antioxidative efficacy. Levels of PGE2 and MIF were significantly decreased by both bakuchiol and retinol. Bakuchiol but not retinol significantly increased FGF7 protein levels. WST-1 metabolization levels were significantly augmented by bakuchiol and retinol. Bakuchiol and retinol application led to a significant augmentation of COL1A1, COL7A1 and FN protein levels. Wounds supplemented with bakuchiol but not retinol displayed a significant increase in epidermis regeneration.