This study compared in man the in vivo barrier function of stratum corneum in three racial groups: black, Caucasian and Asian, by two noninvasive technics. They were transepidermal water loss (TEWL) determination measured with an evaporimeter and laser Doppler velocimetry (LDV) to measure the lag time before the vasodilatation induced by application of methyl nicotinate (10 μl of 0.5% solution in ethanol/propylene glycol 95/5 v/v). Both methods were performed simultaneously on each forearm of 7 black, 8 Caucasian and 6 Asian subjects before and after removal of the stratum corneum by stripping. TEWL measurements were higher (p < 0.01) in Asians and Blacks compared to Caucasians. Stripping (8 or 12 strips) increased TEWL in all groups; TEWL increase percentage was higher (p < 0.05) in Asians compared to Caucasians. Vasodilatation lag times assessed by LDV showed that skin permeability was more important in Asians (p < 0.01) and in Caucasians (p < 0.05) than in blacks. Lag times decreased with stripping. After 8 or 12 strips, the order of sensitivity was: Asian > Caucasian > black. Our study showed that, with both noninvasive methods, removal of the stratum corneum increased permeability, with racial differences.

The stratum corneum barrier function of Blacks, Caucasians and Asians were compared in vivo. A noninvasive technic, laser doppler velocimetry (LDV), was used to evaluate the cutaneous penetration of nicotinates by the determination of the lag time before vasodilatation induced by the application of those local vasodilator drugs. The study was performed on untreated skin and after removal of the stratum corneum by 12 strips. The influence of molecular weight and solubility of different nicotinates (methyl, ethyl, hexyl and vitamin E) were also studied on Japanese skin. Vasodilatation lag times assessed by LDV, with methyl nicotinate (MN), showed that skin permeability was more important in Asians (P < 0.01) and in Caucasians (P < 0.05) than in Blacks. Moreover Asian skin was significantly more sensitive to stripping (P < 0.05) than Black skin. A significant shorter lag time was obtained with small and hydrophilic nicotinates (methyl and ethyl) (P < 0.01) compared to a lipophilic one (hexyl). The alteration of the stratum corneum barrier function by stripping showed a more important modification with MN (P < 0.05) than with hexyl nicotinate. Consequently, this noninvasive method can evaluate the modifications of the stratum corneum barrier function and racial origin has to be taken into account in the determination of skin absorption.

Synopsis This study investigated the effect of different treatments on skin permeability, in vivo in man, by two noninvasive methods: transepidermal water loss (TEWL) determination measured with an evaporimeter, and Laser Doppler velocimetry (LDV) to measure the lag time before the vasodilatation induced by application of methyl and hexyl nicotinates. Different treatments were performed on forearms of volunteers: 1. removal of the stratum corneum by stripping, 2. occlusion and hydration of the stratum corneum, 3. application of three surfactants in aqueous solution (Tween 60, sodium dodecyl sulphate and cetyl trimethylammonium bromide). Increase in TEWL and decrease in lag time before the vasodilating effects of nicotinates measured with LDV after all treatments confirmed the modifications in skin permeability. The variations observed were more or less significant depending on the treatment, the investigative method, and the molecule used for the penetration study. With methyl (hydrophilic) nicotinate, the more TEWL increased, the more lag time decreased. A significant decrease in lag time after hexyl (lipophilic) nicotinate application was observed when there was a very high increase in TEWL. The lipophilic nature of the stratum corneum barrier was confirmed in vivo. Use of these two complementary methods, TEWL and LDV, allows the evaluation in vivo of the effect of different treatments on skin permeability in man.