Precipitation of test chemicals in reaction solutions used in the amino acid derivative reactivity assay and the direct peptide reactivity assay

Abstract

The Amino acid Derivative Reactivity Assay (ADRA) was developed by the authors as an in chemico alternative to animal testing for skin sensitization potential. Although ADRA is based on the same scientific principles as the Direct Peptide Reactivity Assay (DPRA), a comparison of the results from these two test methods shows a far lower incidence of precipitation of test chemicals in reaction solutions for ADRA than for DPRA. Specifically, a comparison of the results for 82 test chemicals that were tested using both DPRA and ADRA showed that while there were 30 chemicals tested using DPRA for which precipitation was found in the reaction solution, there were just three chemicals tested using ADRA for which even slight turbidity was found in the reaction solution. In contrast to the fact that many DPRA test chemicals with a n-Octanol/Water Partition Coefficient (LogKow) of 2.0 or higher exhibited precipitation, there were only three ADRA test chemicals that exhibited turbidity, and these were all highly hydrophobic with a LogKow of greater than 6.0. Moreover, one of the DPRA test chemicals that exhibited precipitation also gave a false negative result, suggesting that anytime a test chemical exhibits precipitation in the reaction solution during DPRA testing the results must be interpreted with the greatest care, although all false positives are not caused by precipitation of test chemicals. Therefore, since relatively few ADRA test chemicals exhibited precipitation relative to DPRA, we consider ADRA to be an extremely useful means of testing a wide variety of chemical substances.