Solvent Solubility Testing of Cosmetics-Relevant Chemicals: Methodology and Correlation of Water Solubility to In Silico Predictions
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Aqueous solubility is one of the main physicochemical parameters used to assess skin absorption. As solvents have great impact on skin absorption, knowledge of chemical solubility in appropriate solvents is key to correlate in vitro skin penetration with in vivo outcomes. For example, acetone:olive oil, ethanol and dimethyl sulfoxide are all relevant to in vitro and in vivo assays. Solubility information is also needed to identify the optimal solvent for skin penetration assays. Therefore, we have measured the solubilities of 54 chemicals related to cosmetics and the reference controls for skin sensitization and genotoxicity, in five different solvents: water, DMSO, ethanol, acetone:olive oil (4:1), 5% ethanol in 0.1 mol·L−1 phosphate buffered saline. The solubility protocol resulted in highly reproducible values, with greatest variability for poorly soluble chemicals, especially those in 0.1 mol·L−1 PBS, which may be due to the high salt content. There was good agreement between experimental and literature values for water solubility (mean difference < twofold). A better correlation of experimental values with in silico predictions was obtained using ACD/Labs software (mean difference < fourfold, R2 = 0.64) than WSKOW from EpiSuite (mean difference < eightfold, R2 = 0.48). Chemicals with a log10P > 2 generally exhibited a poor solubility in water but a much higher solubility in acetone:olive oil, ethanol and DMSO. These five solvents include pH effects, acceptor and donor hydrogen bonding and non-polar interactions. Thus, the solubility profile across these different solvents would help to characterize the chemicals related to their cutaneous absorption with different vehicles and their toxicity assessment.
KeywordsSolubility Measured Predicted Cosmetic ingredients Solvents Bioavailability
EPI (estimation programs interface) suite™
Phosphate buffered saline
We would like to thank David Sanders from Unilever Sharnbrook, UK for all his help, including the analysis of the purity of the chemicals Unilever gifted to the project. David was also integral to the purification of some chemicals and shipment to the participating labs. We would also like to thank Unilever Sharnbrook, UK for the generous donation of seven of the radiolabelled chemicals. Finally, we would like to thank Julien Fernandez from Eurofins I ADME Bioanalysis, France, for all his help with the analytical data collation.
This work was sponsored by Cosmetics Europe.
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