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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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Eq. 1 is an empirical model developed by Potts and Guy that is widely used to predict the permeability coefficient (Kp) based on octanol-water partition coefficient (log Koct) and MW.
Log Kp = −2.72 + 0.71 log Koct − 0.0061 MW Eq. 1
Variability in the proposed models is partly due to the experimental uncertainties and individual skin variations, which limit the prediction of skin permeability coefficients. However, the correlation of in vivo skin permeation descriptors such as plasma concentration and molecular characteristics has not been evaluated in the literature.
According to Eq. 2, where A is the patch area and Ke is the elimination rate constant,11 after transdermal drug delivery, plasma concentration depends on steady-state flux (Css) of the drug per unit area (J) and is inversely related to the drug’s volume of distribution (Vd ).
Css = A*J/Ke* Vd Eq. 2