AsA is quickly oxidized and decomposed in aqueous solution and thus is not generally useful as a depigmenting agent. To resolve that problem, Magnesium-L-ascorbyl-2- phosphate (VC-PMG) was synthesized. VC-PMG is stable in water, especially in neutral or alkaline solution containing boric acid or its salt. VC-PMG is hydrolyzed by phosphatases of liver or skin to AsA and thus exhibits vitamin C-reducing activity (41). VC-PMG significantly suppressed melanin formation on purified tyrosinase or cultured cells and inhibited melanin formation without cell growth suppression on cultured human melanoma cells. Inhibition of melanogenesis was stronger when the activity of melanogenic enzymes was relatively high.
VC-PMG is absorbed percutaneously, stays in the skin, and inhibits tyrosinase activity of melanocytes. The addition of 1% to 3% 1,1-methyleneglycol-bis increases the absorption of VC-PMG. In situ experiments demonstrated that 10% VC-PMG cream was absorbed into the epidermis and that 1.6% remained 48 hours after application. When the 10% VC-PMGcream was topically applied to the patients, the lightening effect was significant in 19 of 34 patients with chloasma or senile freckles and in three of 25 patients with normal skin (42).