Girish Mahajan (Editor)

Skin whitening

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Skin whitening is the practice of using substances, mixtures, or physical treatments to lighten skin color. Skin whitening treatments work by reducing the content of melanin of the skin. Many agents have been shown to be effective in skin whitening; some have beneficial side effects (e.g.: are antioxidants, nutrients, or decrease the risk of some types of cancer); some are a significant risk to health (for example, those containing mercury).

Contents

Uses

Specific zones of abnormally high pigmentation such as lentigo spots, moles and birthmarks may be depigmented to match to the surrounding skin. In cases of vitiligo, unaffected skin may be lightened to achieve a more uniform appearance.

Discovery and design

Melanogenesis inhibitors have been discovered and developed through several methods, including: screening of synthetic chemical libraries (high throughput screening is occasionally used), screening of plant extracts, computational (in silico) search, found as a side effect of previously known drugs and exploration of structural analogues of previously known tyrosinase inhibitors based on knowledge (in varying degrees) of their structure-activity relationship. Thus, the development and discovery of melanogenesis inhibitors illustrates many of the methods used in drug design. Some of the most potent competitive reversible tyrosinase inhibitors are synthetic compounds with a potency hundreds of times that of kojic acid.

Mechanisms of action

For a review of mechanism of action of skin whitening agents, see Chang (2012) or Ebanks, Wickett, Boissy (2009).

Melanin is the main substance responsible for the color of the skin. Melanin is class of dark polymers generated by the body through the process of melanogenesis. Among the melanin pigmenting the skin and hair, 2 types can be distinguished based on its chemical composition and biological route of synthesis: the black/brown eumelanin and the red/yellow pheomelanin. The variation of skin color among individuals is mostly because of variation of the content of melanin in the skin. Skin with little or no melanin is almost white. Other factors influence skin color in a lesser degree, including the amount of blood in blood vessels (because of the color of blood), skin thickness and content of carotenoids in skin.

Melanin in synthesized in melanosomes which are organelles produced in melanocytes. Melanocytes are cells dedicated to this function that are present in the skin, hair follicles, and other structures of the body. The synthesis of melanin (also called "melanogenesis" and "melanization") involves a chain of enzyme-catalyzed chemical reactions and non-enzyme-catalyzed reactions. The main precursor to melanin is L-tyrosine. The first step of melanogenesis is the conversion of L-tyrosine to L-DOPA; this is the first and rate-limiting step and is catalized by the enzyme tyrosinase (TYR).:1163 Other enzymes involved in the synthesis include tyrosinase-related protein 1 (TRP1) and tyrosinase-related protein 2 (TRP2); TRP2 is also known as "dopachorome tautomerase" (DCT). L-tyrosine is taken by the melanocytes from the intercellular medium, then transported to the melanosomes. L-tyrosine is also synthesized within the melanocytes from L-phenylalanine by the enzyme phenylalanine hydroxylase (PAH).:1164

Melanosomes are transferred to keratinocytes (the most abundant cell type in the skin). Most of the melanin of skin is found in keratinocytes. Additionally, melanocytes interact with keratinocytes through chemical signaling. See § Preventing the transfer of melanosomes to keratinocytes.

Skin whitening agents work by reducing the presence of melanin in the skin. To accomplish this, there are several possible mechanism of actions:

  • Inhibition of the activity of tyrosinase: The catalytic action of tyrosinase is inhibited (slowed or nearly stopped) by the skin whitening agent.
  • Inhibition of the expression or activation of tyrosinase: The antimelanogenic agent causes that less tyrosinase is generated or that tyrosinase is not activated to its functional form.
  • Scavenging of the intermediate products of melanin synthesis.
  • Preventing the transfer of melanosomes to keratinocytes.
  • Directly destroying existing melanin.
  • Destroying melanocytes.
  • Inhibition of the activity of tyrosinase

    Many tyrosinase inhibitors have been discovered or developed. Very many inhibitors of tyrosinase are known; most are of the reversible type. For a review of tyrosinase inhibitors see Chang (2009). Reviews of patents on tyrosinase inhibitors have been published.

    Evaluation of effectivity: The potency (how little of a substance is needed to achieve an effect) of reversible inhibitors is usually given in terms of its IC50. The IC50 is highly dependent on the assay conditions, making it incomparable among different assays (unless designed to be comparable). It is customary practice in studies of tyrosinase inhibitors to assay one or several well known inhibitors as a positive control and point of comparison. The relative activity (RA) of a compound under investigation is its activity divided by the activity of the positive control; in turn the activity of a compound is usually defined as 1/IC50. The RA is less dependent on assay conditions that the IC50 and is suitable to compare the results of different assays provided the same positive control was used. The positive control is commonly kojic acid.

    Upregulation of tyrosinase caused by tyrosinase inhibitors: Several skin whitening agents including some which are tyrosinase inhibitors have been found to cause an increase in the expression of tyrosinase (which by itself would increase melanin synthesis).

    Irreversible inhibitors of tyrosinase include: N-nonyl trans-caffeate, α-Na8SiW11CoO40 (a polyoxometalate), a structural analog of aloe emodin, structural analogues of barbituric acid, structural analogues of chalcone, sodium hydrogen sulfite, structural analogues of coumarin, structural analogues of the following 2 compounds: benzene-1,2-diamine and 2-aminophenol, and 2,3-dihydroxybenzoic acid itself, tetrahydrofolic acid, analogues of pyrimidine and rhodanine, tetrahydropterines, cardol triene (a triene analogue of cardol extracted from cashew), N-(3,5-dihydroxybenzoyl)-6-hydroxytryptamine, aminoethylisothiourea, 8-hydroxynaringenin, NADH, 8-hydroxydaidzein and captopril.

    Inhibition of the expression or activation of tyrosinase

    Microphthalmia-associated transcription factor (MITF) is the master transcription factor that controls the expression of TYR, TRP1 and TRP2, MART1, PMEL17 and many other important proteins involved in the function of melanocytes. Downregulation of MITF decreases melanogenesis and is a mechanism of action of some skin whitening agents. As an heuristic rule, agents acting through downregulation of MITF are more likely to have side effects that selective tyrosinase inhibitors. Various signaling pathways and genetic mutations mutations influence the expression of MITF.

    Inhibitors of melanogenesis whose mechanism of action includes reducing the genetic expression of melanogenic enzymes include caffeoylserotonin, AP736, pomegranate extract, and betulinic acid (extracted from Vitis amurensis root).

    The MC1R receptor and cAMP

    The melanocortin 1 receptor (MC1R) is a transmembrane and G-protein coupled receptor expressed in melanocytes. MC1R is an important target for the regulation of melanogenesis. Agonism (i.e.: activation) of MC1R increases the ratio of eumelanin to pheomelanin and increases the generation of melanin overall.

    MC1R/cAMP signaling pathway: Activation of MC1R causes activation of adenylyl cyclase (AC), which produces cyclic adenosine monophosphate (cAMP), which activates protein kinase A (PKA), which activates (by protein phosphorylation) cAMP response element-binding protein (CREB), which upregulates MITF (CREB is a transcription factor of MITF). Whitening agents that interfere with the MC1R/cAMP signaling pathway have been reviewed by Chang (2012).

    cAMP is degraded by phosphodiesterases (PDE). The PDE5 inhibitors sildenafil and vardenafil, the cAMP-promoter IBMX and 8-CPT-cGMP (a cyclic guanosine monophosphate (cGMP) analogue) increase melanin synthesis.

    MC1R ligands: alpha-melanocyte stimulating hormone (α-MSH), beta-melanocyte stimulating hormone (β-MSH) and adrenocorticotropic hormone are endogenous agonists of MC1R.:1175 Agouti signaling protein (ASIP) appears to be the only endogenous antagonist of MC1R. Synthetic MC1R agonists have been designed; examples include the peptides afamelanotide and melanotan II.

    Mutations of the MC1R gene correlate, and in some individuals are at least partially responsible for red hair, white skin and an increased risk for skin cancer.:1175

    Serotonin signaling

    Melanocytes express serotonin receptors and are capable of producing serotonin. Pharmacological interference with the serotonin system of melanocytes can result in either increased or decreased melanin synthesis. Serotonin itself is a weak inhibitor of tyrosinase with 0.11 times the potency of kojic acid. Nonetheless, serotonin increases synthesis of melanin when its overall effect on melanocytes (as opposed to isolated tyrosinase) is evaluated. Activation of 5-HT2B receptors with BW-723C86 inhibits melanogenesis while activation of 5-HT2A receptors with the amphetamine structural analog DOI promotes melanogenesis. The serotonin reuptake inhibitor (SRI) 6-nitroquipazine inhibits melanogenesis in-vitro.

    Preventing the transfer of melanosomes to keratinocytes

    For an overview of the interaction between keratinocytes and melanocytes see Yamaguchi, Hearing (2009).

    Keratinocytes in the skin: Within the skin, melanocytes are present in the basal layer of the epidermis; from these melanocytes originate dendrites that reach keratinocytes. Keratinocytes are the most abundant cell type in the epidermis. In the skin, there are approximately 36 keratinocytes per melanocyte. Keratinocytes are continuously generated in the basal layer of the epidermis and displace older keratinocytes of the skin towards the surface.

    Melanosome transfer: Melanosomes along with the melanin they contain is transferred from melanocytes to keratinocytes when keratinocytes are low in the epidermis. Keratinocytes carry the melanosomes with them as they move towards the surface. Keratinocytes contribute to skin pigmentation holding the melanin originated in melanocytes and induce melanogenesis through chemical signals directed at melanocytes. The transfer of melanosomes to keratinocytes is a necessary condition for the visible pigmentation of the skin. Blocking this transfer is a mechanism of action of some skin whitening agents. Skin whitening agents that block melanocyte transfer include niacinamide, heparin, madecassoside, soybean and Saccharomyces cerevisiae (a species of yeast).

    The protease-activated receptor 2 (PAR2) is a transmembrane and G-protein coupled receptor expressed in keratinocytes and involved in melanocyte transfer. Antagonists of PAR2 inhibit the transfer of melanosomes and have a skin whitening affects while agonists of PAR2 have the opposite effect, as expected. The common endogenous agonists of PAR2 are serine proteases which irreversibly activate PAR2 by cleaving a part of the extracellular terminal of this receptor thereby exposing a part of it that subsequently works as a ligand tethered to the reset of the receptor at the molecular scale. Some synthetic agonists of PAR2 are short peptides that imitate the aforesaid tethered ligand but do not cleave the extracellular terminal.

    Directly destroying existing melanin

    Several species of fungi produce enzymes that reduce pigmentation by degrading melanin. These enzymes often require the presence of hydrogen peroxide and sometimes the presence of Mg+2 ions to work. They have been proposed as a safer alternative to hydrogen peroxide for cosmetic hair depigmentation.

    The enzyme lignin peroxidase produced by the fungus phanerochaete chrysosporium has been studied as an ingredient suitable for skin-whitening: A double-blind placebo-controlled split-face randomized study found this enzyme to be effective and superior to hydroquinone in skin whitening. In a non-controlled study, this enzyme was applied to volunteers with facial melasma during 8 weeks; the treatment was found effective in reducing pigmentation in both skin affected by melasma and skin unaffected by melasma.

    Destroying melanocytes

    Some compounds are known to destroy melanocytes; this mechanism of action is often used to remove the remaining pigmentation in cases of vitiligo.

    Non-pharmacological treatments

    Most skin-lightening treatments, which can reduce or block some amount of melanin production, are aimed at inhibiting tyrosinase. Many treatments use a combination of topical lotions or gels containing melanin-inhibiting ingredients along with a sunscreen, and a prescription retinoid. Depending on how the skin responds to these treatments, exfoliants ‒either in the form of topical cosmetic or chemical peels‒ and lasers may be used. New development using LED systems are also showing good results.

    Tretinoin

    Research has shown that the use of tretinoin (also known as all-trans retinoic acid) can only be somewhat effective in treating skin discolorations. Users of tretinoin have to avoid sunlight, as the skin can tan. Using tretinoin always makes the skin more sensitive to UVA and UVB rays.

    Hydroquinone

    In medical literature, hydroquinone is considered the primary topical ingredient for inhibiting melanin production. Its components have potent antioxidant abilities. Topical hydroquinone comes in 2% (available in cosmetics) to 4% (or more) concentrations (available from a physician or by prescription), alone or in combination with tretinoin 0.05% to 0.1%. Research has shown hydroquinone and tretinoin to prevent sun- or hormone-induced melasma.

    Hydroquinone is a strong inhibitor of melanin production, meaning that it prevents dark skin from making the substance responsible for skin color. Hydroquinone does not bleach the skin but lightens it, and can only disrupt the synthesis and production of melanin hyperpigmentation. It has been banned in all European countries (e.g. France) because of fears of a cancer risk.

    Some concerns about hydroquinone's safety on skin have been expressed, but the research when it comes to topical application indicates negative reactions are minor or a result of using extremely high concentrations or from other skin-lightening agents such as glucocorticoids or mercury iodine. Any perceived risk is most likely applicable for African women. Hydroquinone has been shown to cause leukemia in mice and other animals. The European Union banned it from cosmetics in 2001, but it shows up in bootleg creams in the developing world. It is sold in the United States as an over-the-counter drug, but with a concentration of hydroquinone not exceeding 2 percent.

    Because of hydroquinone's action on the skin, it can be an irritant, particularly in higher concentrations of 4% or greater and predictably when combined with tretinoin. Some medications have been created that combine 4% hydroquinone with tretinoin and a form of cortisone. The cortisone is included as an anti-inflammatory. The negative side effect of repeated application of cortisone is countered by the positive effect of the tretinoin so that it does not cause thinning of skin and damage to collagen.

    Arbutin

    Some of alternative lighteners are derived from natural sources of hydroquinone. These include Mitracarpus scaber extract, Uva ursi (bearberry) extract, Morus bombycis (mulberry), Morus alba (white mulberry), and Broussonetia papyrifera (paper mulberry). All of these contain arbutin (technically known as hydroquinone-β-D-glucoside), which can inhibit melanin production. Pure forms of arbutin are considered more potent for affecting skin lightening.

    Arbutin is derived from the leaves of bearberry, cranberry, mulberry or blueberry shrubs, and also is present in most types of pears. It can have melanin-inhibiting properties. Arbutin and other plant extracts are considered safe alternatives to commonly used depigmenting agents to make the skin fairer. Medical studies have shown the efficiency of arbutin for skin lightening. There are patents controlling its use for skin lightening. Arbutin actually exists in two isomers, alpha and beta. The alpha isomer offers higher stability over the beta isomer and is the preferred form for skin lightening indications.

    Kojic acid

    Kojic acid is a by-product in the fermentation process of malting rice for use in the manufacturing of sake, the Japanese rice wine. Some research shows kojic acid to be effective for inhibiting melanin production. However, kojic acid is an unstable ingredient in cosmetic formulations. Upon exposure to air or sunlight it can turn brown and lose its efficacy. Many cosmetic companies use kojic dipalmitate as an alternative because it is more stable in formulations. However, there is no research showing kojic dipalmitate to be as effective as kojic acid, although it is a good antioxidant. Further, some controversial research has suggested that kojic acid may have carcinogenic properties in large doses. Other further studies show that kojic acid is not carcinogenic, but can cause allergic contact dermatitis and skin irritation.

    Azelaic acid

    Azelaic acid is a component of grains, such as wheat, rye, and barley. It is applied topically in a cream formulation at a 10-20% concentration. Azelaic acid is used to treat acne, but there also is research showing it to be effective for skin discolorations. Other research also indicates azelaic acid may be an option for inhibiting melanin production.

    Vitamin C

    Vitamin C and its various forms (ascorbic acid, magnesium ascorbyl phosphate, etc.) are considered an effective antioxidant for the skin and help to lighten skin. One study found it raises glutathione levels in the body. Another study found that brownish guinea pigs given vitamin C, vitamin E and L-cysteine, simultaneously, led to lighter skin.

    Glutathione

    Glutathione is a tripeptide molecule found in mammalian bodies. It is an antioxidant that plays an important role in preventing oxidative damage to the skin. In addition to its many recognized biological functions, glutathione has also been associated with skin lightening ability. While skin whitening reduces melanin which serves as the natural protection from UV exposure, glutathione's antioxidant property also protects the skin from UV radiation .

    A double-blind placebo-controlled study found glutathione to be effective as a skin whitening agent and in reducing dark spots; the dose regime was 500 mg per day (split in 2 equal doses per day) for 2 – 4 weeks. In contrast, a study that examined the effect of glutathione and related compounds in-vitro found that glutathione monoethyl ester but not glutathione had a depigmenting effect. A review of the use of glutathione for skin whitening was published in 2016.

    Glutathione is an ingredient in some cosmetics preparations. Glutathione for skin whitening is available in cream, soap, lotion, nasal spray and injectable form. Glutathione that is applied on the skin in the form of lotion is not efficiently absorbed by the skin cells as the thiol group undergoes rapid formation of disulfide. When taken orally, glutathione is hydrolyzed by enzymes in the gastrointestinal tract resulting in reduced bioavailability. The level of glutathione increased in smalls amounts temporarily when large oral doses were administered. As a result, the effectiveness of externally administered glutathione is slowed down by its inability to cross cell membranes efficiently and its rapid degradation by enzymes in the gastrointestinal tract. On the contrary, intravenous glutathione delivers very high doses directly into the systemic circulation and is the preferred mode of administering glutathione. However, this method of administrating the antioxidant might flood the cells with glutathione that may cause reductive stress. This might expose people to potential health risks associated with long-term use of high dose of glutathione. Of all the glutathione products, glutathione tablet remains the most effective type.

    Glutathione can be combined with many other agents like vitamin C to increase its absorption, N-acetyl cysteine to boost its level, and other antioxidants like vitamin E. Some oral intake of glutathione could have dangerous effect when combined with other skin whitening agents such as hydroquinone which is a carcinogenic element and monobenzone which causes irreversible depigmentation.

    Alpha hydroxy acids

    Alpha hydroxy acids (AHAs) – primarily in the form of lactic acid and glycolic acid – are the most researched forms of AHAs because they have a molecular size that allows effective penetration into the top layers of skin. It is generally assumed that in and of themselves AHAs in concentrations of 4% to 15% are not effective for inhibiting melanin production and will not lighten skin discolorations in that manner. It is believed that their benefit is in helping cell turnover rates and removing unhealthy or abnormal layers of superficial skin cells (exfoliation) where hyperpigmented cells can accumulate. However, other research has shown that lactic and glycolic acids can indeed inhibit melanin production separate from their actions as an exfoliant on skin.

    Alpha hydroxy acid peels (using 50% concentrations or greater) may remove skin discolorations. Only a qualified physician should perform these types of facial peels.

    Niacinamide

    Niacinamide is claimed to be a much safer alternative when applied topically for skin or genitalia whitening. According to research by Procter & Gamble, a cosmetics company, niacinamide has no adverse side-effects. It also promotes acne reduction, increases skin moisture, and reduces fine wrinkles.

    Depigmenting agents

    Most commonly, depigmentation of the skin is linked to people born with vitiligo, which produces differing areas of light and dark skin. These individuals, if they so decided to use a lightening process to even out their skin tone, could apply a topical cream containing the organic compound monobenzone to lessen the remaining pigment. Monobenzone may cause destruction of melanocytes and permanent depigmentation. An alternate method of lightening is to use the chemical mequinol over an extended period of time. Increasingly, people who are not afflicted with the vitiligo experiment with lower concentrations of monobenzone creams in the hope of lightening their skin tone evenly. However, monobenzone is not recommended for skin conditions other than vitiligo.

    Mercury

    Many skin whiteners contain toxic mercury, such as mercury(II) chloride or ammoniated mercury as the active ingredient. However, mercury has been banned in most countries for use in skin whitening (1976 in Europe, 1990 in the USA) because it accumulates on skin and it can have the opposite results in the long term. As late as January 2016, the FDA published a warning not to use a particular brand of whitener – Viansilk's "Crema Piel De Seda" ("Silky Skin Cream"), sold in the United States due to its mercury content.

    Tranexamic acid

    Tranexamic acid is sometimes used in skin whitening as a topical agent, injected into a lesion, and taken by mouth, both alone and as an adjunct to laser therapy; as of 2017 its safety seemed reasonable but its efficacy for this purpose was uncertain because there had been no large scale randomized controlled studies nor long term follow-up studies.

    Other

    Other options with some amount of research regarding their potential skin lightening abilities are licorice extract (specifically glabridin).

    There is also a small amount of research showing oral supplements of pomegranate extract, ellagic acid, vitamin E, and ferulic acid can inhibit melanin production.

    Laser treatments

    Both ablative and nonablative lasers can have a profound effect on melasma. However, the results are not always consistent, and problems have been reported (such as hypo- or hyperpigmentation). Laser treatments of this kind are more likely to result in problems for those with darker skin tones.

    Cryosurgery

    Another alternative to laser treatment is cryosurgery using liquid nitrogen. Controlled destruction of skin cells causes the skin to naturally regenerate itself. Excess melanin comes to the surface and peels off in a few days. This is particularly useful in sensitive areas like the genitals where laser treatment could leave a scar. Efficacy of the treatment depends on the depth of the pigment. Freckles in any part of the body can be treated the same way.

    Adverse effects

    There is evidence to suggest that some types of skin-whitening products use active ingredients (such as mercurous chloride) and hydroquinone which can be harmful. Hydroquinone is not available without a prescription in Europe. It is only available when prescribed by a medical doctor (e.g. A general practitioner). This is also the case in many other countries, where hydroquinone can only be prescribed by a doctor for certain skin conditions.

    A test of common skin lightening creams available in Nigeria showed that they caused mutations in bacteria and were possibly carcinogenic. A study that examined skin whitening creams in Mexico found a high concentration of mercury in several of them.

    Society and culture

    In India, the sales of skin lightening creams in 2012 totaled around 258 tons and in 2013 sales were about $300 million. As of 2013 the global market for skin lighteners was projected to reach US$19.8 billion by 2018 based on sales growth primarily in Asia, Africa and the Middle East.

    References

    Skin whitening Wikipedia


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