DEVELOPMENT OF ALTERNATIVE METHODS TO IN VIVO AND IN VITRO ASSAYS FOR THE PREDICTION OF SKIN PERMEABILITY TO CHEMICAL COMPOUNDS
Creators: name: ROCCO, PAOLO
Publisher: name: Universit� degli Studi di Milano
Year: 2015

description: Percutaneous absorption is an interdisciplinary topic which is relevant to a number of divergent fields. Indeed, the knowledge of the diffusion of a given compound after skin contact plays a crucial role for the risk assessment of toxic substances, the safety of cosmetic ingredients and the design and optimization of pharmaceutical dosage forms. The skin?s barrier capacity is related to the molecular organization of the lipids in the extracellular space of the stratum corneum (SC). The well-known equation by Potts and Guy, correlating the SC permeability coefficient Kp to the permeant's partition coefficient and molecular volume or weight, can already explain almost 70% of the variability in the data contained in Flynn's set. Since experimental Kp values in the data set may have up to 30% variability, improving the Potts and Guy equation is difficult. Though room for a quantitative improvement of existing predictive models seems to be limited, there is still a quest for a better understanding of the skin permeation process at a molecular level. In this respect, Molecular Dynamics is a promising tool, since it naturally accounts for the non homogeneity and anisotropy of the SC. In this work, after building a stable and equilibrated model for the stratum corneum (SC) lipid bilayer, SMD simulations were performed to study the penetration of 80 permeants of known permeability coefficient through the SC. MD has been utilized before by other Authors for studying permeation of chemicals through plasma membranes, notably the work by Marrink and Berendsen, whose approach was later applied to the SC by Das, Noro and Olmsted. A screening of a large number of molecules with MD simulations, though has never been attempted to our knowledge. The goal of the screening was to assess the possibility of using MD to assist in the development of predictive equations of skin permeability, more than gaining new insight in the mechanics of SC permeation. The approach developed (for plasma membranes) by Marrink and Berendsen for the prediction of permeability couldn't be applied, though, because of the unfeasibility of Free Energy calculations from SMD simulations in the screening of 80 permeants, due to the large amount of trajectories required. Instead, SMD has been used in an indirect way as a mean to explore the conformational and property space of the permeants in the different microenvironments of the stratum corneum. SMD was also used in a direct way to calculate averaged diffusion coefficients in different regions of the stratum corneum. What proved to be the best option, was a mixed approach where the averaged diffusion coefficients in the plane parallel to the bilayer surface were calculated in different regions of the SC, and then used together with the physicochemical properties in the correlation equation. This approach has allowed us to correlate the permeability coefficient to averaged physicochemical properties, improving on existing semi-empirical methods (employing the same quantities determined experimentally). The obtained equation compares well with the Potts and Guy equation. Further improvement of the correlation coefficient seems difficult, considering the heterogeneity of the experimental data sets and the experimental error involved in the measurement of the permeability coefficient. Further study of SMD data may still allow us to gain a better understanding of the physical process. Performing multiple SMD simulations for a small number of permeants, or designing a new protocol using only equilibrium simulations in specific regions of the lipid matrix model, will allow us to make free energy calculations. So, the work done here is just a first step in the direction of actually employing Molecular Dynamics in developing viable predictive equations of skin permeability. As a final point, our work has shown that, as stated above, the uncertainty of experimental values of the permeability coefficient is too high to expect significant improvements in the predictive power of equations. The Fully Validated set represents a great advance in reducing the bias of Flynn's set and making the variability of experimental conditions explicit. We feel, though, that this is not enough. As an effect of Regulation (EC) No 1223/2009 on cosmetic products, testing of finished cosmetic products and ingredients on animals is prohibited in the European Union, as well as the marketing of cosmetic products which have been tested on animals. If governmental Authorities feel that the development of alternative methods to in vivo and in vitro assays for the determination of skin permeability to chemical compounds is worth consideration and funding, then developing a research project for measuring skin permeability coefficients of a large set of molecules in controlled and reproducible conditions would be an important step in the right direction.