Integrated skin sensitization assessment based on OECD methods (II): Hazard and potency by combining kinetic peptide reactivity and the "2 out of 3" Defined Approach.

Depending on regulatory requirements, the skin sensitization risk for new chemicals with potential consumer skin contact must be assessed by experimental testing by (i) binary hazard assessment to identify sensitizers, (ii) subclassification of sensitizers according to the Global Harmonized System (GHS), and (iii) derivation of a point of departure (PoD) for risk assessment. The Organisation for Economic Co-operation and Development (OECD) recently published a test guideline incorporating the “2 out of 3” defined approach (2o3 DA) for skin sensitization hazard assessment and added the kinetic direct peptide reactivity assay (kDPRA) as a stand-alone test guideline method for GHS subclassification. The 2o3 DA requires that at least two in vitro tests are conducted. The cell-based tests and the kDPRA generate, next to a binary outcome with a fixed threshold, continuous concentration-response data, which can be used in quantitative regression models to derive a PoD. The sequence of testing for the 2o3 DA is flexible. Here we compare different testing sequences and how they can be combined with kDPRA data to provide a PoD in parallel to hazard identification (hazard ID) and GHS subclassification. A set of 188 chemicals with available in vitro data was evaluated for the final PoD using these dif-ferent testing sequences. The results indicate that testing can start with DPRA / kDPRA and either of the cell-based assays, and that testing can stop after two congruent tests without major impact on the final PoD for chemicals within the applica-bility domain of the kDPRA.