Absorption and metabolism of (all-trans-) retinol acetate, retinol, and retinoic acid in the single-pass, albumin-perfused rabbit ear.

Background: Retinoic acid, a metabolite of retinol, is formed in the skin of various species.

Purpose: Formation rates have not been determined in a dynamic skin perfusion model which may show the dehydrogenation of retinol to be rate limited.

Methods: (all-trans-) Retinol acetate, retinol, or retinoic acid was applied dermally with isopropyl myristate. The ears were single-pass perfused with 6% hetastarch or 5% bovine serum albumin in the buffer solution. The effluent was analyzed by HPLC for these substances.

Result: No ester hydrolysis of retinol acetate was observed, nor did this substrate appear in the effluent. Retinol or retinoic acid were detected in the effluent. The absorption rate was linearly related to the dermally applied concentration density. The absorption rate of retinoic acid was 70 times larger than that for retinol. The formation rate of retinoic acid from retinol was rate limited (apparent Vmax: 0.08 pmol/min/cm2 skin).

Conclusion: Since retinoic acid is a recognized pharmacologically active ingredient in the skin, the dehydrogenation of retinol or retinal was also observed in the effluent of this skin perfusion model. The percutaneous retinol adds to the endogenous pool of retinoic acid by cytosolic dehydrogenation or microsomal oxidation elsewhere. The steady-state absorption rate of 0.1% retinoic acid in isopropyl myristate, referred to 500 cm2 of the human skin, would contribute about 20% of the daily requirement of vitamin A, without increase in endogenous retinoic acid plasma concentration.