Melasma is caused by excessive melanin (coloured skin pigment) in the upper layer of the skin (epidermal melasma) and the deeper layer of the skin (mixed melasma). The answer to your question therefore requires a brief discussion on how melanin is formed.
Melanin is produced by specialised pigment producing cells in the lower layer of the epidermis called melanocytes. The production of melanin involves a series of enzyme-catalyzed chemical reactions.
The raw product for melanin production is L-tyrosine. The first step of melanin production is the conversion of L-tyrosine to L-DOPA. This first and rate-limiting step is catalyzed by the enzyme tyrosinase.
This is arguably the most important step as it is the rate-limiting step likened as the flood gate that controls the rate of subsequent steps in melanin production. After a series of intermediate steps, melanin is finally formed and gets transferred out of the melanocytes to the surrounding skin cells called keratinocytes.
Skin lightening creams work by reducing the amount of melanin in the skin. A lightening cream may have several mechanisms of action resulting in a decrease in melanin production and/or delivery to the final destination, the keratinocytes. They include:
1) Inhibiting the activity of tyrosinase e.g. hydroquinone (a major ingredient of Tri-Luma), cysteamine, arbutin, koljic acid, azelaic acid.
2) Preventing tyrosinase from being activated to its functional form e.g. tretinoin (the second major ingredient of Tri-Luma), cysteamine.
3) Removing the intermediate products of melanin synthesis.
4) Preventing the transfer of melanin to the surrounding skin cells e.g. niacinamide
5) Removing existing melanin e.g. tretinoin (a major ingredient of Tri-Luma), exfoliating agents
6) Regulation of melanocytes environment e.g. corticosteroid (the third major ingredient of Tri-Luma)
7) Antioxidants e.g. vitamin C
8) Destroying melanocytes e.g. monobenzyl ether of hydroquinone. This however should be avoided at all cost as it causes permanent and disfiguring complete loss of pigmentation.
Hope this explanation helps you understand how cysteamine and other lightening agents work for melasma!
