Reaction Chemistry to Characterize the Molecular Initiating Event in Skin Sensitization: A Journey to Be Continued.

The publication by the OECD of the adverse outcome pathway (AOP) for skin sensitization has accelerated the development and validation of mechanistic tests and testing strategies to assess the potential of new molecules to trigger skin allergies. The molecular initiating event (MIE) in the AOP is reaction with skin peptides/proteins. It is followed by a number of cellular events. Currently, only one in chemico test to characterize the MIE was proposed to and underwent adoption by the OECD, while two cell-based assays have completed the process. A multitude of further cellular assays is currently in the validation pipeline, but no further reactivity test has gone through full standardization. Here, we review data on in chemico methods, identify gaps, and discuss how these methods can be improved to better characterize the MIE and to become even more informative. We focus on the importance of kinetics, the information gained from studying adduct formation, and the difficulties posed by side reactions such as peptide oxidation. We then highlight mechanistic insights from reaction chemistry: the relative contribution of different target nucleophiles, the migration of amino acid modifications, and the potential of peptide-cross-linking. We illustrate in a case study how kinetic in chemico methods might have been used to better predict the risk of three preservatives, which have led to serious epidemics of contact dermatitis. In a case study on Michael acceptors, we show the impact of additional substituents around the electrophilic olefin moiety on reactivity, and we highlight the shortcomings which current in silico methods to predict reaction chemistry still have, illustrating the need for experimental in chemico data to improve such models. Finally, based on the information reviewed and the presented case studies, a strong argument is made to continue the journey of developing nonredundant, informative in chemico methods, and not to solely focus on new cell-based methods to further populate the AOP for skin sensitization.