Hydroxy acids are being used to facilitate desquamation and improve lipid biosynthesis together with barrier function. The influence of alpha-and beta-hydroxy acids (115) on desquamation is now well established, but new lipophillic variants of salicylic acid appear to influence corneodesmolysis differently. Whereas lactic and salicylic acid act on all corneodesmosomes, LSA only acted in the stratum disjunctum corneodesmosomes. These lipophillic variants appear to act on the whole structure of the corneodesmosomes whereas the “ordinary” acids fractionate the corneodesmosomes. Fartarsch et al. (116) also demonstrated that the action of glycolic acid on corneodesmolysis was restricted to the stratum disjunctum suggesting a targeted action without compromising barrier function. Medium chain fatty acids have also been reported to not only improve SC flexibility but also assist in the relief of dry skin in combination with barrier lipids. Further enhanced dry skin relief was observed in the presence of barrier lipids (117), and the L isomer, in particular, increased SC extensibility and keratinocyte proliferation as reported by Rawlings et al. (118). Rawlings et al. also reported that longer chain hydroxy acids were more effective than short chain fatty acids at facilitating corneocyte cell release in the presence of several calcium chelators. This may be due to a fluidizing effect of these longer chain fatty acids on the lamellar lipids as in SC extensibility studies using extensions where only lipids are believed to being extended longer chain alpha-hydroxy acids plasticize the corneum (119).
SC turnover time measured by dansyl chloride (a measure of epidermal proliferation matched by desquamation) increased by 15% by applying a moisturizing cream at pH 3.8. However, further increases were observed with increasing concentration of the free acid of glycolic acid or by decreasing the pH of the base. At 8% glycolic acid concentration (4% free acid) Johnson (120) reported approximately 30% increase in SC turnover time. The increased turnover time needs to be matched by increased desquamation; otherwise, retention hyperkeratosis would occur, which clearly it does not. In fact, the opposite occurs. So desquamation must also be enhanced by further activating acidic optimum enzymes or by also chelating calcium, which is known to reduce the final processing steps involved in Cdsn degradation.
Nevertheless, not all hydroxy acids perform equally, and, in fact, some appear to enhance the skin’s sensitivity to UV irradiation, especially glycolic acid. However, glucanolactone and tartaric acid have been shown to be not only superior to glycolic acid and lactic acid in improving barrier function but have been shown to not increase in sunburn cell formation (121).