N-acetyl-4-S-cysteaminylphenol and N-propionyl-4-S-cysteaminylphenol are derived from homologues of phenols with melanocytotoxic activity. Like Hydroquinone and Monobenzyl Ether of Hydroquinone, these belongs to the phenol/catechols class. N-acetyl-4-S-cysteaminylphenol is a tyrosine-amine derivative analogue that is less irritating than hydroquinone. As an alternative substrate of tyrosinase, it may therefore inhibit tyrosinase activity. By decreasing intracellular glutathione, it favors the pathway of forming pheomelanin rather than eumelanin. A clinical study using 4% preparation of N -acetyl-4-S-cysteaminyphenol for melasma showed marked-to-moderate improvement after 2-4 weeks of application, with minimal adverse effects. N -propionyl-4-S-cysteaminylphenol is more potent and has more cytotoxic properties as compared to the N-acetyl form.
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