Structure-activity relationships in the murine local lymph node assay for skin sensitization: α,β-diketones

The biological activity of skin-sensitizing chemicals is related to their ability to react, either directly or after metabolic activation, with appropriate skin proteins. For direct acting electrophilic compounds, this ability can be modelled, using the RAI (relative alkylation index) approach, by a combination of electrophilicity and hydrophobicity parameters. Several structure-activity relationships based on this approach have been reported, but most of them cover guinea pig sensitization test data on what chemists would classify as relatively soft electrophilic chemicals. In the present work, an electrophilicity parameter based on Taft substituent constants is derived for hard electrophiles having a reactive carbonyl group, and is used to calculate RAI values for the analysis of sensitization test data obtained in the murine local lymph node assay (LLNA) for a series of alpha, beta-diketones. The sensitization potential of these reactive hard electrophilic carbonyl compounds in the LLNA shows a good correlation with the RAI. Overall, the findings reaffirm our view that physical organic chemistry is the key to understanding why some chemicals sensitize more strongly than others, while some do not sensitize at all, and provide further evidence of the value of the LLNA for SAR studies.