Peptide reactivity assays for skin sensitisation - scope and limitations

The direct peptide reactivity assay (DPRA) is an OECD test guideline method that aims to determine if a chemical is reactive enough to be a skin sensitiser. It involves incubation of the test chemical at 5 mMolar concentration for 24 h with a cysteine-based peptide at 0.5 mMolar concentration and measurement of the percentage depletion (DP) of the peptide. The kinetic direct peptide reactivity assay (kDPRA) is derived from the DPRA and involves incubating the peptide with the test chemical at a range of concentrations and incubation times to produce a data matrix of DP values, which is analysed to give a reactivity parameter logk_max that assigns chemicals to the 1A potency class (high potency) if logk_max reaches the threshold value of -2. Here the DPRA, with a threshold of 47% DP, is compared against the kDPRA for their abilities to distinguish between the 1A and non-1A potency classes. It is found that they perform very similarly against a dataset of 157 chemicals with known potency, with only marginal differences in predictive performance. The thresholds of -2.0 (kDPRA) and 47% DP (DPRA) to distinguish 1A sensitisers are not scientific absolutes but the best compromises for a heterogenous set of data containing classes of chemicals for which different thresholds would be applicable. It is concluded that although the kDPRA represents a major advance towards predicting skin sensitisation potency on a continuous basis without animal testing, it offers no significant advantage over the DPRA for the purpose of 1A classification.