36 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 6 iJJ z cI 5 4 3 2 Regression Treatment phase 1 ' , I • I Phase 0 5 10 15 20 DAYS Treatment 3 No Treatment Figure 4. Moisturization efficacy test comparing the effect of a lotion containing 1% phospholipid, 2% cholesterol, and 1% stearic acid, plus 1% glycerol (treatment 3) to a no-treatment control. cessing (14,15). Thus, perturbation of stratum corneum lipid architecture and compo- sition appears to lead to a reduction in the activity of proteases responsible for desmo- some degradation (17,18), and the resultant faulty desquamation leads to the appearance of skin xerosis. Barrier lipids, occlusive agents, and humectants are, therefore, being investigated for their interactions with, and effects on, stratum corneum components and processes (19,20,25,26,27,31). Ceramides and ceramide-like lipids (pseudoceramides) in combi- nation with cholesterol and fatty acids have been shown to be very effective (28-31) for the treatment of experimentally induced skin xerosis. Petrolatum has been shown to mix with the stratum corneum lipids, which may explain its beneficial effects on skin (27). Also, glycerol has been shown to fluidize stratum corneum lipids and prevent their crystallization in low-humidity conditions (25,26). This property probably also influ- ences the effect of glycerol on the proteolysis stratum corneum desmosomes (19,20). In the present investigation we have demonstrated that a lipid mixture consisting of phospholipid, cholesterol, and stearic acid, when combined with low levels of glycerol, functions synergistically to deliver efficacious skin benefits. Indeed, the synergism is apparent in that neither the lipids alone, nor glycerol alone, at the concentrations used in this study, alleviated skin xerosis significantly. Also, the use of the bilayer-forming phospholipid provided a more rapid reduction in xerosis compared to the non-bilayer-

EFFECT OF LIPIDS ON SKIN XEROSIS 37 7 "'6 o34 z3 P0.05* Treatment phase 0 5 10 15 DAYS Regression phase i 20 Treatment 4 Treatment 5 Figure 5. Moisturization efficacy test comparing the effect of a lotion containing 1% phospholipid, 2% cholesterol, and 1% stearic acid, plus 5% glycerol (treatment 4) to a lotion containing 1% petrolatum, 2% cholesterol, and 1% stearic acid, plus 5% glycerol (treatment 5). forming petrolatum. The improved skin xerosis benefit of the phospholipid-containing lotions appears to be related to the bilayer-forming potential of phospholipids, as supported by the in vitro electron microscopy results. It is noted, however, that both the phospholipid and petrolatum-containing lotions were still highly effective in alleviating skin xerosis compared to the no-treatment control. The reasons for the phospholipid-glycerol synergy are not yet fully understood, but glycerol may modulate the lipid-phase behavior of the topically applied lipid mixture, especially allowing the formation of a lameliar phase that may increase the performance of glycerol, or glycerol may allow a better delivery of the lipids into the stratum corneum. Overall, it is hypothesized that improved stratum corneum barrier function, as well as increased stratum corneum water content, is due to the formation of additional exogenous lipid bilayers that in the presence of glycerol improve the hydration of the stratum corneum and allow normal desmosome degradation to occur. At the low levels of lipids and glycerol used in this study, this effect is apparently synergistic. It is noted, however, that glycerol alone, at higher concentrations than those used in this study, has been found to be effective in treating skin xerosis (23). High concentrations of glycerol are, however, aesthetically unpleasant unless formulated carefully. Higher concentra- tions of the lipids and glycerol were not examined in this study. To fully under-