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Predicting the Percutaneous Penetration of Cosmetic Ingredients
By: Sara Farahmand, PhD, University of Cincinnati College of Pharmacy; and Howard I. Maibach, MD, PhD, University of California School of Medicine
Posted: March 30, 2010, from the April 2010 issue of Cosmetics & Toiletries.
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Predictivity of the permeation models based on physicochemical properties of molecules such as MW and log Koct has been questioned by the fact that they give little information as to the actual structural features of solutes that influence skin permeability. 13
Eq. 7 was suggested for the prediction of log koct from molecular properties.
Log Koct = 0.088 + 0.562 R2 − 1.054 π 2H + 0.034 Σ α2H − 3.46 Σ β2H + 3.814Vx Eq. 7
In Eq. 7, R2 is an excess molar refraction, π 2H is the solute dipolarity/polarizability, Σ α2H and Σ β2H are the overall hydrogen bond acidity and basicity, respectively, and Vx is the McGown characteristic volume. Therefore, a criticism of Eq. 5 could be that log Koct, as a predictive variable in this equation, may be collinear with the other equation variables.
Although Eq. 5 could show the effect of molecular size on the Cmax, the inclusion of MW in Eq. 4 failed to result in a significant correlation. Again, the complexity of Cmax as an indicator of absorption and elimination, and/or narrow range of MW of studied compounds (162–357) could explain this finding.