Efficiency of Fatty Acids as Chemical Penetration Enhancers: Mechanisms and Structure Enhancement Relationship

Purpose. The present study evaluated the effects of fatty acids commonly present in cosmetic and topical formulations on permeation enhancement across human epidermal membrane (HEM) lipoidal pathway when the fatty acids saturated the SC lipid domain without cosolvents (Emax). Methods. HEM was treated with neat fatty acids or fatty acid suspensions to determine Emax. A volatile solvent system was used to deposit fatty acids on HEM surface to compare fatty acid enhancer efficiency in topical volatile formulations with Emax. To elucidate permeation enhancement mechanism(s), estradiol (E2 beta) uptake into fatty acid-treated SC lipid domain was determined. Results. Emax of fatty acids was shown to increase with their octanol solubilities and decrease with their lipophilicities, similar to our previous findings with other enhancers. Emax of solid fatty acids was shown to depend on their melting points, an important parameter to the effectiveness of the enhancers. The E2 beta uptake results suggest that enhancer-induced permeation enhancement across HEM is related to enhanced permeant partitioning into the SC lipid domain. Conclusions. The results suggest Emax as a model for studying the permeation enhancement effect of the fatty acids and their structure enhancement relationship.