EFFECT OF LACTIC-ACID ISOMERS ON KERATINOCYTE CERAMIDE SYNTHESIS, STRATUM-CORNEUM LIPID-LEVELS AND STRATUM-CORNEUM BARRIER FUNCTION

Alpha-hydroxy acids are effective agents for the treatment of skin xer osis and it is known that, following treatment with lotions containing D,L-lactic acid, the stratum corneum prevents xerosis more effectivel y. To date, the relative efficacy of the different isomers of lactic a cid has not been evaluated and the mode of action of lactic acid in im proving stratum corneum resilience is not known. The objective of the present studies was to determine the effects of lactic acid isomers on keratinocyte ceramide biosynthesis, stratum corneum barrier function and the resistance of the stratum corneum to the appearance of skin xe rosis. In vitro, lactic acid enhanced the production of ceramides by k eratinocytes. L-Lactic acid was more effective than the D isomer (300% increase vs 100% increase). Carbon label from lactic acid was incorpo rated into all keratinocyte lipid species and a greater incorporation of label into ceramides was achieved with L-lactate than with D-lactat e. In vivo, lactic acid increased the levels of stratum corneum cerami des. Whereas, lotions containing L-lactic acid resulted in the greates t increase (48% increase) followed by D,L-lactic acid (25% increase), D-lactic acid had no effect on stratum corneum ceramide levels. The in creases in stratum corneum ceramide levels following lactic acid treat ment also led to improvements in stratum corneum barrier function, mea sured by transepidermal water loss following a challenge to the skin w ith SLS and in the regression phase of a moisturization efficacy study . Significant improvements in barrier function and resistance to the a ppearance of skin xerosis were observed following L-lactic acid and D, L-lactic acid, but not following D-lactic acid treatment. From these r esults we believe that lactic acid, particularly the L isomer, stimula tes ceramide biosynthesis leading to increased stratum corneum ceramid e levels which results in superior lipid barrier and a more effective resistance against xerosis.