The beneficial use of vitamins in general, and specifically vitamin E, continues to be an active area in dermatology and cosmetic science. It is clear that vitamin E, especially in the form of α-tocopherol, is a potent antioxidant and is widely used by the body to protect lipids in cell membranes from oxidative damage. However, the role of vitamin E in skin is much less clear, both in the understanding of its intrinsic role as well as clearly demonstrating its clinically relevant, in vivo benefits.
α-Tocopherol is stored in the liver and adipose tissue. In the liver it is bound and transferred by a specific cytosolic protein, α-tocopherol transfer protein (α−TTP). When circulated through plasma, tocopherol is transported by several forms of lipoproteins including very low density lipoproteins (VLDL) and high density lipoproteins (HDL).3 It is believed that most α-tocopherol is delivered through HDLs to cells for use in the membrane or within the cell.4 It is not unreasonable to assume that when delivered topically, α- tocopherol needs to associate with a transport protein to gain access to the dermis, especially the fibroblasts, unless some other delivery vehicle is provided in the formulation.
Another significant formulation challenge is to keep α- tocopherol stable until use. The most common approach is to use the ester α-tocopherol acetate. Although the ester is more stable than α-tocopherol, it has a different efficacy profile. For instance, it has been shown that α-TTP binds α- tocopherol to an extent more than 50 times greater than α- tocopherol acetate.
A second approach to vitamin and drug stabilization is phosphorylation. Phosphorylation is the transformation of an alcohol to a phosphate ester through transfer of a phosphoryl group (-PO3H2). Adenosine triphosphate (ATP) is the most common endogenous phosphorylating agent. It has been shown that phosphorylated vitamin C is accumulated into cells as vitamin C. Work in our laboratories has begun to develop evidence for the presence of phosphorylated α-tocopherol in the human body. For this reason, we believe that phosphorylation is a better route to formulation stabilization and that it allows a better opportunity for in vivo efficacy through an endogenous form that can be activated by phosphate ester cleavage.