Assessment of new skin brightening agents

There are numerous internal and external stresses that affect human skin pigmentation. Exposure to certain drugs and chemicals as well as the existence of certain disease states can result in hyperpigmentation. Postinflammatory pigmentation, another skin hiperpigmentation disorder, usually develops after resolution of inflammatory skin eruptions like acne, contact dermatitis, or atopic dermatitis.1,2 The use of skin brighteners is widely extended. In Asia, lighter skin is considered a symbol of beauty and femininity. In Western countries, skin brighteners are applied for the treatment of hyperpigmented spots and the production of a more even skin tone. Current depigmenting agents found in the cosmetic market show numerous adverse effects such as high irritant and sensitising potential, melanocyte cytotoxicity and instability in formulations. A new skin brightening agent is not only required to lighten the skin. In addition, to prove an hypopigmenting effect, other parameters related to toxicity, solubility and stability should be determined to consider a potential candidate for skin brightening applications. The cellular surroundings may also influence the effect of the product. Many inhibitors identified in cell-free enzymatic assays are likely to have some toxicity or delivery problems in cell-based assays.3 The efficacy of the potential skin brightening agent should be tested in melanocyte cultures to validate their efficacy and rule out its possible cytotoxicity. New depigmentation products should include the following features: a) inhibition of the melanogenic pathway; b) lack of toxicologic or mutagenic potential; c) formulation stability; d) clinical efficacy; and e) novelty and patent protection of the agent and/or formulation.4 The efficacy assessment includes different steps. Despite the large number of molecules showing depigmenting properties in vitro, only some of them are able to induce an effective hypopigmenting effect measurable in clinical trials. The in vitro tests are used to prove the mechanism of action of the depigmenting agent and also its bioavailability in cell-based assays. Finally, human testing in vivo is considered of utmost importance to demonstrate the efficacy of the new active. The first step in the development of a potential candidate for skin brightening applications is its screening for brightening efficacy. To do that, it is important to understand the mechanisms underlying skin pigmentation.

Skin pigmentation

The colour of our skin and hair is mainly determined by the amount, distribution and type of melanin. Melanins are pigmented biopolymers that are synthesised by the dendritic melanocytes dispersed at the dermo-epidermal junction. Melanin synthesis takes place in membrane-bound organelles termed melanosomes, which contain specific enzymes controlling the production of the pigments.3 Then, the mature organelles migrate towards the extremities of the melanocyte dendrites where they are transferred in skin to keratinocytes and in hair bulbs to the hair shaft, where the final distribution patterns of the pigment are determined.5 The type and amount of melanin synthesised by the melanocyte and its distribution in the surrounding keratinocytes determine the actual colour of the skin. Mammalian melanocytes can produce two types of melanin: eumelanin, which is black or brown and pheomelanin, which is red or yellow.6 The perceived colour of our skin and hair is determined by the ratio of these two pigments. Tyrosinase inhibition is the most common approach to achieve skin hypopigmentation as this enzyme catalyses the rate-limiting step of pigmentation in melanogenesis. Tyrosinase is a multifunctional, glycosylated, coppercontaining oxidase found exclusively in melanocytes. It catalyses two distinct reactions of melanin synthesis: the hydroxylation of tyrosine into 3,4-dihydroxyphenylalanine (L-DOPA) and the subsequent oxidation of DOPA into DOPAquinone.2 As a result of the key role played by tyrosinase in melanin biosynthesis, most brightening agents act specifically to reduce the function of this enzyme by means of several mechanisms. As well as tyrosinase modulation, there are other mechanisms related to skin pigmentation. We will focus on four approaches for the regulation of skin pigmentation (Fig. 2):