Topical 9-cis-retinaldehyde for delivery of 9-cis-retinoic acid in mouse skin

Retinoids in cosmeceuticals

Retinoids are natural and synthetic vitamin A derivatives. They are lipophilic molecules and easily penetrate the epidermis. Their biologically active forms can modulate the expression of genes involved in cellular differentiation and proliferation. Retinoic acid (tretinoin), its 13-cis isomer isotretinoin, as well as various synthetic retinoids are used for therapeutic purposes, whereas retinaldehyde, retinol, and retinyl esters, because of their controlled conversion to retinoic acid or their direct receptor-independent biologic action, can be used as cosmeceuticals. These natural retinoic acid precursors are thus expected to be helpful in (i) renewing epidermal cells, (ii) acting as UV filters, (iii) preventing oxidative stress, (iv) controlling cutaneous bacterial flora, and (v) improving skin aging and photoaging. Retinol and retinyl esters are not irritant, whereas demonstrating only a modest clinical efficiency. On the other hand, retinaldehyde, which is fairly well tolerated, seems to be the most efficient cosmeceutical retinoid; it has significant efficiency toward oxidative stress, cutaneous bacterial flora, epidermis renewing, and photoaging.

Pharmacology of RALGA, a Mixture of Retinaldehyde and Glycolic Acid

BACKGROUND

Retinoids and alpha-hydroxy acids (AHAs) are major compounds in topical therapy. They exert distinct but potentially complementary activities. However, their association is limited by their respective irritating potential. Recently, the first association between a retinoid and an AHA has been achieved; this formulation (RALGA) associates retinaldehyde (RAL)--a precursor of retinoic acid (RA)--and glycolic acid (GA)--an AHA.

OBJECTIVE

To study the pharmacological properties of RALGA.

METHODS

The bioavailability of RAL into the skin after topical RALGA was studied by HPLC, and its bioconversion to RA was analysed by measuring the enzyme activity of retinaldehyde dehydrogenase and the RA content in the epidermis and dermis. The retinoid activity of RALGA was studied on the modulation of Hhb4 keratin mRNA on the tail of C57BL/6 mice, and its comedolytic properties on the size and density of dermal cysts and the morphology of sebaceous glands in hairless mice.

RESULTS

Epidermal and dermal concentrations of RAL and RA were higher after RALGA treatment, as compared to both RAL 0.1% alone and RA 0.05% alone; this indicates that the presence of GA favours the bioavailability and biotransformation of RAL into RA. The retinoid activity of RALGA (suppression of Hhb4 mRNA keratin) was similar to that of RAL alone, indicating that the presence of GA does not interfere with specific retinoid activity; GA alone had no effect in this test, which confirms the specificity of Hhb4 mRNA keratin modulation for retinoid activity. The diameter and the density of dermal cysts as well as the size of sebaceous glands were significantly decreased by RALGA.

CONCLUSION

These observations indicate that the addition of an AHA such as GA to a retinoid such as RAL results in a better bioavailability of the retinoid, thus a higher delivery of RA, which potentiates the biological activities of the retinoid. This combination allows a delivery of high amounts of RA in the skin while preventing the side-effects usually observed with high concentrations of topical RA.

Topical delivery of retinoids counteracts the UVB-induced epidermal vitamin A depletion in hairless mouse

UVB irradiation depletes all-trans-retinol (ROL) and all-trans-retinyl esters (RE) from the hairless mouse epidermis. Prevention of this may be of relevance in counter-acting the long-term side effects of UVB exposure. We studied the effects of a topical treatment with natural retinoids before and after UVB exposure on three parameters involved in vitamin A metabolism: the amount of epidermal ROL and RE, the level of functional cellular retinol-binding protein I (CRBP-I), which is likely to protect ROL from UVB, as well as the cytosolic and microsomal enzyme activities which generate ROL and RE, i.e. all-trans-retinaldehyde (RAL) reductase, acylCoA:retinol acyltransferase (ARAT) and retinyl-ester hydrolase (REH). Topical pretreatment with retinoids promoted a dramatic increase of epidermal ROL, RE and CRBP-I levels, a transient increase of RAL reductase and ARAT activities as well as a decreased activity of REH, indicating a direction of epidermal vitamin A metabolism toward storage. In untreated mice UVB irradiation induced a depletion of epidermal ROL and RE in 10 min and a 50% decrease of CRBP-I after 24 h. In mice treated with topical retinoids, and then exposed to UVB, epidermal RE levels were higher than in vehicle-treated, nonirradiated mice. In contrast, ROL was as much depleted after UVB in pretreated as in untreated animals in spite of an induction of CRBP-I, indicating that CRBP-I does not actually protect ROL from UVB-induced depletion in this model. However, the reconstitution of both epidermal ROL and RE, after their depletion induced by UVB, was accelerated by previous topical treatment with RAL. Our results indicate that topical delivery of retinoids partly counteracts UVB-induced vitamin A depletion and promotes recovery.