Posted 11 June 2005 - 08:18 AM
Allright, here's some evidence topical Vitamin A (retinol) does in fact convert to the metabolites in the skin. But note that the concentration of the metabolites achieved in the skin is very small compared to applying them directly. Still, not bad for a non-prescription approach. Which is more than likely why you haven't heard about it, there's no real money in it.
Logan
Exp Dermatol. 1998 Feb;7(1):27-34.
In vitro metabolism by human skin and fibroblasts of retinol, retinal and retinoic acid.
Bailly J, Crettaz M, Schifflers MH, Marty JP.
Zyma S.A. Development Department, Nyon, Switzerland. jacques.bailly@roche.com
The metabolism of radio-labelled retinol, retinal and retinoic acid by fresh human skin as well as by human dermal fibroblasts have been investigated in vitro. Surgically removed human skin biopsies were placed at the air liquid interface, and treated topically for 24 h with retinoids. At the end of the treatment period, epidermis and dermis were separated by heat. Epidermis, dermis and medium were subsequently extracted and resulting fractions were analysed by HPLC. Dermal fibroblast cultures were treated and analysed in a comparable manner. Topical application of retinoids resulted in gradient concentrations within the skin. For each fraction, metabolites and unchanged product proportions were determined by HPLC. After treatment with retinol and retinal, low but significant amounts of retinoic acid were detected in the epidermis, as well as in the dermis (30 pmol to 90 pmol). In comparison, treatments with retinoic acid itself, led to higher level of retinoic acid in the epidermis and in the dermis (respectively 2050 and 420 pmol). Cultured human dermal fibroblasts, treated with retinol and retinal, formed retinoic acid as well as several other metabolites (retinol esters, reduction of retinal to retinol...). Taken together, our results are consistent with an action of retinol or retinal on the skin via a retinoic acid formation and a metabolic function of the dermis.
J Invest Dermatol. 1997 Sep;109(3):301-5.
Unoccluded retinol penetrates human skin in vivo more effectively than unoccluded retinyl palmitate or retinoic acid.
Duell EA, Kang S, Voorhees JJ.
Department of Dermatology, University of Michigan Medical School, Ann Arbor 48109-0609, U.S.A.
The formation of all-trans retinoic acid is an oxidative process whereby retinol is converted to retinaldehyde and then to retinoic acid. Because retinol causes qualitative molecular changes similar to those produced by retinoic acid, we compared potency of retinol, retinaldehyde, and retinyl palmitate to retinoic acid and assessed the effects of occlusion. Retinoids were prepared in an experimental vehicle of 95% ethanol:propylene glycol (7:3) with anti-oxidant. Induction of retinoic acid 4-hydroxylase activity was the end point for comparison. Retinoic acid concentrations from 0.001% to 0.05% under occlusion produced a linear dose-response induction of 4-hydroxylase activity. The concentrations of the other retinoids under occlusion required to achieve significant induction of enzyme activity were 0.6% retinyl palmitate, 0.025% retinol, and 0.01% retinaldehyde. The linear dose-response was lost with retinoid concentrations in excess of 0.25% retinol or 0.5% retinaldehyde. Statistical analyses showed no difference in 4-hydroxylase activity between unoccluded and occluded retinol treated sites. By contrast, however, unoccluded sites treated with retinoic acid or retinyl palmitate had less induction of 4-hydroxylase activity than occluded sites. Retinol, retinaldehyde, and retinyl palmitate did not produce erythema but did increase epidermal thickness. Although retinol is a weaker retinoid than retinoic acid, the increased penetration of unoccluded retinol in comparison to unoccluded retinoic acid with this prototypic vehicle confers on retinol a more effective delivery of a retinoidal effect than unoccluded retinoic acid. Retinol at 0.25% may be a useful retinoid for application without occlusion because it does not irritate but does induce cellular and molecular changes similar to those observed with application of 0.025% retinoic acid.