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Tox21_202367
Also known as: Mequinol, 150-76-5, 4-hydroxyanisole, P-hydroxyanisole, P-methoxyphenol, Phenol, 4-methoxy-
Molecular Formula
C7H8O2
Molecular Weight
124.14  g/mol
InChI Key
NWVVVBRKAWDGAB-UHFFFAOYSA-N
FDA UNII
6HT8U7K3AM

Mequinol is a synthetic derivate of hydroquinone and depigmenting agent. Although the exact mechanism of the depigmenting effects of mequinol remains unclear, it may exert its effect by oxidation of tyrosinase to cytotoxic products in melanocytes. Mequinol may also act by competitively inhibiting the formation of melanin precursors.
Mequinol is a Skin Lightening Agent. The mechanism of action of mequinol is as a Melanin Synthesis Inhibitor. The physiologic effect of mequinol is by means of Depigmenting Activity.
1 2D Structure

Tox21_202367

2 Identification
2.1 Computed Descriptors
2.1.1 IUPAC Name
4-methoxyphenol
2.1.2 InChI
InChI=1S/C7H8O2/c1-9-7-4-2-6(8)3-5-7/h2-5,8H,1H3
2.1.3 InChI Key
NWVVVBRKAWDGAB-UHFFFAOYSA-N
2.1.4 Canonical SMILES
COC1=CC=C(C=C1)O
2.2 Other Identifiers
2.2.1 UNII
6HT8U7K3AM
2.3 Synonyms
2.3.1 MeSH Synonyms

1. 4-hydroxyanisole

2. 4-hydroxyanisole, Potassium Salt

3. 4-hydroxyanisole, Sodium Salt

4. Hydroquinone Methyl Ether

5. Hydroquinone Monomethyl Ether

6. Leucodinine B

7. Mequinol

8. P-hydroxyanisole

9. Para-methoxyphenol

2.3.2 Depositor-Supplied Synonyms

1. Mequinol

2. 150-76-5

3. 4-hydroxyanisole

4. P-hydroxyanisole

5. P-methoxyphenol

6. Phenol, 4-methoxy-

7. Hydroquinone Monomethyl Ether

8. Leucobasal

9. Mehq

10. Leucodine B

11. Mechinolum

12. P-guaiacol

13. Hydroquinone Methyl Ether

14. Novo-dermoquinona

15. Hqmme

16. P-hydroxymethoxybenzene

17. Para-methoxyphenol

18. 1-hydroxy-4-methoxybenzene

19. Monomethyl Ether Hydroquinone

20. 4-methoxy-phenol

21. Pmf (antioxidant)

22. Phenol, P-methoxy-

23. Usaf An-7

24. Mono Methyl Ether Hydroquinone

25. Nsc 4960

26. Mfcd00002332

27. Bms 181158

28. Bms-181158

29. Nsc-4960

30. 6ht8u7k3am

31. Dtxsid4020828

32. Chebi:69441

33. Nsc4960

34. Mequinol (inn)

35. Ncgc00091390-02

36. Mequinol [inn]

37. Dsstox_cid_828

38. Dsstox_rid_75814

39. Dsstox_gsid_20828

40. Mechinolo [dcit]

41. Mequinolum

42. Mechinolo

43. Mequinolum [inn-latin]

44. Cas-150-76-5

45. Ccris 5531

46. Hsdb 4258

47. Einecs 205-769-8

48. Unii-6ht8u7k3am

49. Mequinol [usan:inn:dcf]

50. 4methoxyphenol

51. Paramethoxyphenol

52. Ai3-00841

53. P- Methoxyphenol

54. P-methoxy Phenol

55. P-methoxy-phenol

56. 4-methoxy Phenol

57. Eastman Hqmme

58. 4-(methoxy)phenol

59. 4ha

60. 4ks

61. Para- Hydroxyanisole

62. 4-(methyloxy)phenol

63. Hqme

64. Mequinol [hsdb]

65. Mequinol [usan]

66. Mequinol (usan/inn)

67. Mequinol, Inn, Usan

68. Mequinol [vandf]

69. Phenol,4-methoxy

70. Hydroxyquinone Methyl Ether

71. Mequinol [mart.]

72. Hydroquinone Monomethylether

73. Chembl544

74. Mequinol [who-dd]

75. Ec 205-769-8

76. Ncimech_000709

77. Wln: Qr Do1

78. Schembl21009

79. Hydroquinone Mono Methyl Ether

80. Mls002454409

81. Mequinol [orange Book]

82. Gtpl6827

83. Zinc1684

84. P-hydroxyanisole [inci]

85. Solage Component Mequinol

86. Schembl12015251

87. Bdbm36295

88. Hms2270f04

89. Hms3264p13

90. Hms3652o08

91. Pharmakon1600-00212037

92. Mequinol Component Of Solage

93. 4-methoxyphenol, Analytical Standard

94. Tox21_111125

95. Tox21_202367

96. Tox21_302876

97. Ccg-35855

98. Nsc760357

99. Stl199145

100. Akos000119852

101. Tox21_111125_1

102. Ac-3292

103. Am10685

104. Cs-w019963

105. Db09516

106. Nsc-760357

107. Ps-3375

108. Sb40551

109. 4-methoxyphenol, Reagentplus(r), 99%

110. Ncgc00091390-01

111. Ncgc00091390-03

112. Ncgc00091390-04

113. Ncgc00256552-01

114. Ncgc00259916-01

115. Bp-23487

116. Hqmme; Hydroxyquinone Methyl Ether

117. Hy-30270

118. Nci60_004190

119. Smr001252253

120. Db-003965

121. B1968

122. Ft-0618865

123. M0123

124. S4077

125. Sw219760-1

126. 4-methoxyphenol, Purum, >=98.0% (hplc)

127. 4-methoxyphenol, Saj First Grade, >=97.0%

128. D04926

129. P17835

130. Ab00641905_06

131. Ab00641905_07

132. A809071

133. Sr-01000865565

134. Q-200491

135. Q2862455

136. Sr-01000865565-2

137. Brd-k45216060-001-06-8

138. F9995-1658

139. Z1262246146

140. 4-methoxybenzyl S-(4,6-dimethylpyrimidin-2-yl)thiocarbonate

2.4 Create Date
2005-03-26
3 Chemical and Physical Properties
Molecular Weight 124.14 g/mol
Molecular Formula C7H8O2
XLogP31.3
Hydrogen Bond Donor Count1
Hydrogen Bond Acceptor Count2
Rotatable Bond Count1
Exact Mass124.052429494 g/mol
Monoisotopic Mass124.052429494 g/mol
Topological Polar Surface Area29.5 Ų
Heavy Atom Count9
Formal Charge0
Complexity75
Isotope Atom Count0
Defined Atom Stereocenter Count0
Undefined Atom Stereocenter Count0
Defined Bond Stereocenter Count0
Undefined Bond Stereocenter Count0
Covalently Bonded Unit Count1
4 Drug and Medication Information
4.1 Therapeutic Uses

Antineoplastic Agents; Antioxidants

National Library of Medicine's Medical Subject Headings online file (MeSH, 1999)


Dyschromias, in particular hyperpigmentation, are major issues of concern for people of color. Pigmentary disorders such as melasma and postinflammatory hyperpigmentation (PIH) can cause psychological and emotional distress and can pose a negative impact on a person's health-related quality of life. The precise etiology of these conditions is unknown. Therapies for melasma and PIH target various points during the cycle of melanin production and degradation. Therapies for these conditions include topical agents and resurfacing procedures. Hydroquinone remains the gold standard of topical agents. Other efficacious agents include kojic acid, azelaic acid, mequinol, and retinoids. Cosmeceutical agents include licorice, arbutin, soy, N-acetyl glucosamine, and niacinamide. Resurfacing procedures that have been used to treat melasma and PIH include chemical peels, microdermabrasion, lasers, and intense pulsed light. These procedures are best used in combination with topical bleaching agents. Given the propensity of darker skin to hyperpigment, resurfacing procedures should be used with care and caution. Maximal results are best achieved with repetitive, superficial, resurfacing modalities. In addition, ultraviolet protective measures such as broad-spectrum sunscreens are fundamental to the successful management of these conditions.

PMID:19608057 Grimes PE; Semin Cutan Med Surg 28(2):77-85 (2009).


Postinflammatory hyperpigmentation (PIH) is a reactive hypermelanosis and sequela of a variety of inflammatory skin conditions. PIH can have a negative impact on a patient's quality of life, particularly for darker-skinned patients. Studies show that dyschromias, including PIH, are one of the most common presenting complaints of darker-skinned racial ethnic groups when visiting a dermatologist. This is likely due to an increased production or deposition of melanin into the epidermis or dermis by labile melanocytes. A variety of endogenous or exogenous inflammatory conditions can culminate in PIH and typically most epidermal lesions will appear tan, brown, or dark brown while dermal hypermelanosis has a blue-gray discoloration. Depigmenting agents target different steps in the production of melanin, most commonly inhibiting tyrosinase. These agents include hydroquinone, azelaic acid, kojic acid, arbutin, and certain licorice (glycyrrhiza) extracts. Other agents include retinoids, mequinol, ascorbic acid (vitamin C), niacinamide, N-acetyl glucosamine, and soy, and these products depigment by different mechanisms. Certain procedures can also be effective in the treatment of PIH including chemical peeling and laser therapy. It is important to note that these same therapeutic modalities may also play a role in causing PIH. Lastly, those lesions that are not amenable to medical or surgical therapy may experience some improvement with cosmetic camouflage.

PMID:21348540 Callender VD et al; Am J Clin Dermatol 12(2):87-99 (2011).


Neither the safety nor effectiveness of Solage Solution for the prevention or treatment of melasma or postinflammatory hyperpigmentation has been established. /Included in US product label/ /Solage/

US Natl Inst Health; DailyMed. Current Medication Information. Solage (03/2007). Available from, as of October 15, 2008: https://dailymed.nlm.nih.gov/dailymed/about.cfm


For more Therapeutic Uses (Complete) data for 4-METHOXYPHENOL (9 total), please visit the HSDB record page.


4.2 Drug Warning

Known hypersensitivity to mequinol or tretinoin or any ingredient in the formulation. /Solage/

American Society of Health-System Pharmacists 2015; Drug Information 2015. Bethesda, MD. 2015


Mequinol and tretinoin should be used with extreme caution in patients with eczema, since tretinoin may produce severe irritation of eczematous skin. In addition, mequinol and tretinoin is a dermal irritant and the risk of long-term sequelae from continued skin irritation for longer than 52 weeks is not known. Irritation of the skin induced by the drug may result in increased reactivity to environmental (eg, wind and cold exposure) stimuli. If local irritation becomes severe, temporary or permanent discontinuance of the drug or dosage reduction should be considered. /Solage/

American Society of Health-System Pharmacists 2015; Drug Information 2015. Bethesda, MD. 2015


Safety and efficacy of mequinol and tretinoin have not been established in patients with moderately or heavily pigmented skin. /Solage/

American Society of Health-System Pharmacists 2015; Drug Information 2015. Bethesda, MD. 2015


Patients with a personal or family history of vitiligo may experience enhanced response to mequinol and tretinoin topical solution. During clinical trials, one patient with a family history of vitiligo experienced hypopigmentation in areas not treated with the drug and some areas continued to worsen for at least 1 month after discontinuance of therapy; hypopigmentation became mild after 6 weeks and resolved in some, but not all, lesions by day 106 post-treatment. /Solage/

American Society of Health-System Pharmacists 2015; Drug Information 2015. Bethesda, MD. 2015


For more Drug Warnings (Complete) data for 4-METHOXYPHENOL (13 total), please visit the HSDB record page.


4.3 Drug Indication

Mequinol is currently primarily available only as an active ingredient in combination products combined with tretinoin that are indicated for the treatment of solar lentigines and related hyperpigmented lesions resulting from chronic sun exposure.


FDA Label


5 Pharmacology and Biochemistry
5.1 Pharmacology

Mequinol is in fact considered a melanocytotoxic chemical which when oxidized in melanocytes results in the formation of toxic entities like quinones. Such cytotoxic compounds subsequently have the potential to damage and destroy pigment cells, therefore causing skin depigmentation. In response, skin cells are naturally capable of protecting themselves against such cytotoxic agents with the help of endogenous intracellular glutathione and the detoxification action of glutathione S-transferase on the cytotoxic compounds. Regardless, it is consequently by way of this seemingly negative and damaging pharmacodynamic profile by which the mechanism of action of mequinol is sometimes described.


5.2 MeSH Pharmacological Classification

Antioxidants

Naturally occurring or synthetic substances that inhibit or retard oxidation reactions. They counteract the damaging effects of oxidation in animal tissues. (See all compounds classified as Antioxidants.)


Antineoplastic Agents

Substances that inhibit or prevent the proliferation of NEOPLASMS. (See all compounds classified as Antineoplastic Agents.)


5.3 FDA Pharmacological Classification
5.3.1 Active Moiety
MEQUINOL
5.3.2 FDA UNII
6HT8U7K3AM
5.3.3 Pharmacological Classes
Established Pharmacologic Class [EPC] - Skin Lightening Agent
5.4 ATC Code

D - Dermatologicals

D11 - Other dermatological preparations

D11A - Other dermatological preparations

D11AX - Other dermatologicals

D11AX06 - Mequinol


5.5 Absorption, Distribution and Excretion

Absorption

The systemic exposure to mequinol was assessed in eight healthy subjects following two weeks of twice-daily topical treatment of a tretinoin and mequinol combination product. About dose of the product corresponding to about 37.3 ug/cm^2 of mequinol was applied to the subjects' backs. The mean Cmax for mequinol was 9.92 ng/mL (range between 4.22 and 23.62 ng/mL) and the Tmax was 2 hours (range between 1 to 2 hours). The safety of mequinol in this combination formulation is supported by the low systemic exposures of the agent in the subjects.


Route of Elimination

Mequinol is predominantly renally eliminated as its metabolites.


Volume of Distribution

The volume of distribution is one that suggests mequinol is distributed throughout the total body water, and intracellular concentrations are not expected to vary greatly from gross measurements.


Clearance

Readily accessible data regarding the clearance of mequinol is not available. The use of mequinol containing products is typically indicated for topical use.


Solage is a combination product composed of 2% mequinol (4-hydroxyanisole) and 0.01% tretinoin (all-trans-retinoic acid) in an ethanolic solution ... The purpose of this study was to evaluate the extent of percutaneous absorption of [(3)H]tretinoin and to estimate the systemic exposure to mequinol from this combination product when topically applied to the backs of healthy subjects. Eight subjects received bid /twice per day/ topical applications of nonradiolabelled 2% mequinol/0.01% tretinoin solution on a 400 sq cm area of the back for 14 days. The subjects then received a single topical application of 2% mequinol/0.01% ((3)H)tretinoin solution. After 12 hr, the radiolabelled dose was removed and bid /twice per day/ treatment with nonradiolabelled 2% mequinol/0.01% tretinoin solution was continued for 7 days. Plasma, urine and faecal samples were analysed for total radioactivity and plasma was analysed for both mequinol and tretinoin by GC/MS procedure. Mean percutaneous absorption of [(3)H]tretinoin based on the cumulative recoveries of radioactivity in the urine and faeces was about 4.5% (median 2.18%). Tretinoin concentrations in plasma did not increase above endogenous levels. This was consistent with the concentrations of radioactivity in plasma, which showed an average Cmax of 91 pg-eq/mL (median 26 ng/mL). Average Cmax and AUC(0-12 hr) values for mequinol were 10 ng/mL and 33 ng h/mL, respectively. Based on the results of this study, systemic toxicity from topical application of tretinoin in this formulation is unlikely, because percutaneous absorption of tretinoin is minimal and because endogenous levels of tretinoin are not increased following bid /twice per day/ dosing with this combination formulation. The safety of mequinol in this combination formulation is supported by the low systemic exposures of the subjects in this study compared with the systemic exposures at the highest doses in the dermal toxicity studies in mice (16.6-fold) and rats (34.6-fold).

PMID:10701701 Everett DW, Franz TJ, Chando TJ, Gale PJ, Lehman PA, Schwarzel EH, Parab PV, D'Arienzo CJ, Kripalani KJ Biopharm; Drug Dispos 20(6):301-8 (1999)


In one study in healthy individuals, following topical application of 0.8 mL of the combination preparation containing mequinol 2% and tretinoin 0.01% on a 400-sq cm back skin area twice daily for 14 days, approximately 4.5% of a radiolabeled dose was recovered as tretinoin in urine and feces. In this study, plasma tretinoin concentrations did not increase above endogenous plasma concentrations and average peak plasma mequinol concentrations were about 10 ng/mL. /Solage/

American Society of Health-System Pharmacists 2015; Drug Information 2015. Bethesda, MD. 2015


There is also indication ... that the material was absorbed in toxic amounts when in solution, especially through abraded skin.

Clayton, G. D. and F. E. Clayton (eds.). Patty's Industrial Hygiene and Toxicology: Volume 2A, 2B, 2C: Toxicology. 3rd ed. New York: John Wiley Sons, 1981-1982., p. 2530


5.6 Metabolism/Metabolites

Urine samples from melanoma patients treated with mequinol were analyzed and various mequinol metabolites were identified, including 3,4-dihydroxyanisole, the two o-methyl derivatives 3-hydroxy-4-methoxyanisole and 4-hydroxy-3-methoxyanisole, and even hydroquinone which may have originated at least partly from mequinol. All these identified metabolites were excreted predominantly as sulphates and glucuronides - only a small portion of the substances were present in urine in an unconjugated form. Ultimately, the 3,4-dihydroxyanisole is considered the most important metabolite of mequinol.


A tyrosinase-directed therapeutic approach for treating malignant melanoma uses depigmenting phenolic prodrugs such as 4-hydroxyanisole (4-HA) for oxidation by melanoma tyrosinase to form cytotoxic o-quinones. However, in a recent clinical trial, both renal and hepatic toxicity were reported as side effects of 4-HA therapy. In the following, 4-HA (200 mg/kg i.p.) administered to mice caused a 7-fold increase in plasma transaminase toxicity, an indication of liver toxicity. Furthermore, 4-HA induced-cytotoxicity toward isolated hepatocytes was preceded by glutathione (GSH) depletion, which was prevented by cytochrome p450 inhibitors that also partly prevented cytotoxicity. The 4-HA metabolite formed by NADPH/microsomes and GSH was identified as a hydroquinone mono-glutathione conjugate. GSH-depleted hepatocytes were much more prone to cytotoxicity induced by 4-HA or its reactive metabolite hydroquinone (HQ). Dicumarol (an NAD(P)H/quinone oxidoreductase inhibitor) also potentiated 4-HA- or HQ-induced toxicity whereas sorbitol, an NADH-generating nutrient, prevented the cytotoxicity. Ethylenediamine (an o-quinone trap) did not prevent 4-HA-induced cytotoxicity, which suggests that the cytotoxicity was not caused by o-quinone as a result of 4-HA ring hydroxylation. Deferoxamine and the antioxidant pyrogallol/4-hydroxy-2,2,6,6-tetramethylpiperidene-1-oxyl (TEMPOL) did not prevent 4-HA-induced cytotoxicity, therefore excluding oxidative stress as a cytotoxic mechanism for 4-HA. A negligible amount of formaldehyde was formed when 4-HA was incubated with rat microsomal/NADPH. These results suggest that the 4-HA cytotoxic mechanism involves alkylation of cellular proteins by 4-HA epoxide or p-quinone rather than involving oxidative stress.

PMID:12228181 Moridani MY, Cheon SS, Khan S, O'Brien PJ; Drug Metab Dispos. 30(10):1063-9 (2002)


Many of the well-known depigmenting agents such as hydroquinone and 4-hydroxyanisole are, in fact, melanocytotoxic chemicals which are oxidized in melanocytes to produce highly toxic compounds such as quinones. These cytotoxic compounds are responsible for the destruction of pigment cells, which results in skin depigmentation. However, cells are capable of protecting themselves against cytotoxic agents by intracellular glutathione (GSH). This protection takes place under the enzymatic action of the detoxification enzyme glutathione S-transferase (GST), which is responsible for the conjugation of toxic species to GSH. The depigmenting effect of hydroquinone is shown to be potentiated by buthionine sulfoximine (BSO) and cystamine as the result of the reduction of intracellular levels of GSH by these two agents. Additionally, BSO and cystamine are shown to inhibit the activity of GST. The combination of all-trans-retinoic acid (tretinoin, TRA) with hydroquinone or 4-hydroxyanisole is also known to produce synergetic skin depigmentation. TRA serves as a potent inhibitor of mammalian GSTs and is known to make cells more susceptible to the cytotoxic effect of chemicals by inhibiting the activity of this enzyme. This agent is also shown to reduce the level of intracellular GSH in certain cells. We have proposed that the mechanism of action of TRA to synergistically enhance the melanocytotoxic effect of chemicals involves the inhibition of GST and the impairment of glutathione-dependent cytoprotection against melanocytotoxic agents.

PMID:12771467 Kasraee B, Handjani F, Aslani FS; Dermatology 206(4):289-91 (2003)


Yields 1,4-dimethoxybenzene in guinea pig, rat, rabbit, mouse. Yields 4-methoxycatechol, p-methoxyphenyl-beta-d-glucuronide, & p-methoxyphenyl sulfate in rabbit. /From Table/

Goodwin, B.L. Handbook of Intermediary Metabolism of Aromatic Compounds. New York: Wiley, 1976., p. M-15


4-Methoxyphenol has known human metabolites that include (2S,3S,4S,5R)-3,4,5-Trihydroxy-6-(4-methoxyphenoxy)oxane-2-carboxylic acid.

S73 | METXBIODB | Metabolite Reaction Database from BioTransformer | DOI:10.5281/zenodo.4056560


5.7 Biological Half-Life

Mequinol demonstrated an elimination half-life of 30 to 90 minutes following intravenous infusion of 5 or 10 grams/m^2 over 3 to 5 hours in melanoma patients; similar values were reported after intra-arterial infusion.


5.8 Mechanism of Action

Solar lentigines and related hyperpigmented lesions are localized, pigmented, macular lesions of the skin, usually on the areas of the body which have been chronically exposed to sunlight. These lesions are characterized by increased numbers of active melanocytes and increased melanin production. Although the mechanism of action of mequinol is not fully elucidated, when employed as an active ingredient in combination with other agents like tretinoin in skin depigmentation products, a synergy between a number of potential mechanisms is proposed. Firstly, mequinol is in fact considered a melanocytotoxic chemical which when oxidized in melanocytes results in the formation of toxic compounds like quinones. Such cytotoxic agents are subsequently capable of damaging and destroying pigment cells, which results in skin depigmentation of solar lentigines or other related hyperpigmented lesions. Nevertheless, skin cells are naturally capable of protecting themselves against such cytotoxic entities by endogenous intracellular glutathione (GSH). This protection is elicited through the enzymatic action of glutathione S-transferase (GST), which is responsible for the conjugation of agents toxic to glutathione. Conversely, tretinoin has been observed to serve as a potent inhibitor of mammalian GSTs and to be capable of reducing the level of intracellular GSH in various cells. As a result, the combination of mequinol with tretinoin seemingly allows for a synergistic enhancement of a melanocytotoxic effect that involves the inhibition and impairment of GSH and GST cytoprotection. Secondly, even though mequinol is a substrate for the enzyme tyrosinase and therefore acts as a competitive inhibitor of the formation of melanin precursors by way of tyrosinase facilitated reactions, the clinical significance of this action is unknown.


We examined changes in the levels of chaperone proteins to evaluate the toxic effects of environmental chemicals in human cells in vitro. Some chaperones are up-regulated by estrogenic chemicals, but the effect is not necessarily dependent on the receptor. Thus we also investigated whether a chemical-induced change in chaperone protein expression is human estrogen receptor (hER)-dependent or not, using cultured human cell lines transfected with hERalpha cDNA or an empty vector. In the hERalpha-expressed cells, the protein levels of the heat shock protein 27 (HSP27), the glucose-regulated protein 78 (GRP78/BiP), and GRP94 increased after exposure to beta-estradiol (E(2)) (from 10(-9)M to 10(-6)M) and bisphenol A (BPA) (from 10(-6)M to 10(-5)M). On the other hand, the increase was not observed in the cells without hERalpha expression. These results suggest that the E(2)- and BPA-induced increase in the protein levels were hERalpha dependent. We next examined the effect of four phenolic chemicals similar in structure to BPA, and found that among them, 4-methoxyphenol (from 10(-6)M to 10(-5)M) increased the levels of the chaperone proteins with hERalpha dependency. Thus the human cultured cells would be suitable for evaluating whether an increase in chaperone proteins occurs upon exposure to environmental chemicals and whether the effect is ER-dependent.

PMID:19269315 Kita K et al; Toxicol In Vitro. 23(4):728-35 (2009).


Many of the well-known depigmenting agents such as hydroquinone and 4-hydroxyanisole are, in fact, melanocytotoxic chemicals which are oxidized in melanocytes to produce highly toxic compounds such as quinones. These cytotoxic compounds are responsible for the destruction of pigment cells, which results in skin depigmentation. However, cells are capable of protecting themselves against cytotoxic agents by intracellular glutathione (GSH). This protection takes place under the enzymatic action of the detoxification enzyme glutathione S-transferase (GST), which is responsible for the conjugation of toxic species to GSH. The depigmenting effect of hydroquinone is shown to be potentiated by buthionine sulfoximine (BSO) and cystamine as the result of the reduction of intracellular levels of GSH by these two agents. Additionally, BSO and cystamine are shown to inhibit the activity of GST. The combination of all-trans-retinoic acid (tretinoin, TRA) with hydroquinone or 4-hydroxyanisole is also known to produce synergetic skin depigmentation. TRA serves as a potent inhibitor of mammalian GSTs and is known to make cells more susceptible to the cytotoxic effect of chemicals by inhibiting the activity of this enzyme. This agent is also shown to reduce the level of intracellular GSH in certain cells. We have proposed that the mechanism of action of TRA to synergistically enhance the melanocytotoxic effect of chemicals involves the inhibition of GST and the impairment of glutathione-dependent cytoprotection against melanocytotoxic agents.

PMID:12771467 Kasraee B, Handjani F, Aslani FS; Dermatology 206(4):289-91 (2003)


Mequinol, the monomethyl ether of hydroquinone, is a substrate for the enzyme tyrosinase and acts as a competitive inhibitor of the formation of melanin precursors. The precise mechanism of action of mequinol is not known. /Solage/

American Society of Health-System Pharmacists 2015; Drug Information 2015. Bethesda, MD. 2015


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