Functional changes in human stratum corneum induced by topical glycolic acid: comparison with all-trans retinoic acid

The effects of topical glycolic acid and all-trans retinoic acid on stratum corneum barrier function and hydration of human skin were investigated in 6 healthy volunteers utilizing non-invasive techniques. In addition, changes in stratum corneum turnover time induced by the substances were examined using the dansyl chloride fluorescence test. Twelve percent glycolic acid in water and 0.1% retinoic acid in ethanol, respectively, were applied for 60 min once daily, over a period of 2 weeks (5 consecutive days weekly) on dansyl chloride-labelled skin and on untreated skin. During a 10-day application period, both glycolic acid and retinoic acid similarly induced a significant increase in TEWL. However, after discontinuing treatment, TEWL in retinoic acid-exposed skin remained increased. Glycolic acid significantly reduced stratum corneum hydration from day 11 to day 18 (p < 0.05), while retinoic acid induced skin dryness after 9 days of treatment, which persisted until day 18 (p < 0.005). Whereas glycolic acid rapidly induced an intense erythema implying a direct non-specific inflammatory response, the retinoic acid-exposed skin gradually developed erythema. Retinoic acid caused scaling to a greater extent than did glycolic acid, even after treatment cessation. Both glycolic acid and retinoic acid significantly decreased stratum corneum turnover time and stratum corneum turnover time50 (the time in days from labelling until approximately 50% of fluorescence disappeared), compared with the vehicle controls. However, glycolic acid shortened stratum corneum turnover time (12.8 +/- 0.9 days) as well as stratum corneum turnover time50 (7.3 +/- 0.7 d) significantly more than did retinoic acid (15.8 +/- 0.7 d and 9 +/- 0.8 d, respectively). While ethanol (vehicle of retinoic acid) slightly but significantly decreased stratum corneum turnover time (p < 0.05), water (vehicle of glycolic acid) did not. This study showed that both glycolic acid and retinoic acid induced certain functional changes in stratum corneum, mirroring their irritation potential. However, changes at retinoic acid-exposed sites appeared longer-lasting, implying a distinct mode of action. An increase in stratum corneum turnover induced by the substances may be, in part, linked with their irritation properties.