Development of photo-amino acid derivative reactivity assay: a novel in chemico alternative method for predicting photoallergy

[In chemico skin sensitization alternative method: development of ADRA and listing to OECD test guideline]

Amino acid Derivative Reactivity Assay (ADRA) is an alternative method developed based on the principle of covalent bonding between sensitizer and proteins in the early stage of the mechanism of skin sensitization. The Direct Peptide Reactivity Assay (DPRA) with same principle previously listed in the OECD test guidelines (TG) have some problems such as precipitation of the test chemical in the reaction solution and co-elution of the peptide with the test chemical. While, instead of DPRA, the ADRA was developed using two chemically synthesized nucleophilic reagents-namely, NAC and NAL in which naphthalene rings with a high molar absorbance coefficient (MAC) in the ultraviolet range have been introduced to N-termini of the cysteine and lysine that can react with the test chemical. Therefore, in March 2016, we set up a validation team with the aim for adoption in the OECD TG, ADRA's validation tests were conducted. After reporting the results of validation study, holding a third-party evaluation meeting and two commenting rounds, ADRA was able to be adopted in the OECD TG in June 2019. In addition, since the introduction of naphthalene with a high MAC has made it possible to reduce the concentration, enabling the following items. 1) Decrease in the frequency of precipitation of the test chemicals in the reaction solution. 2) Decrease in the frequency of co-eluting of the nucleating reagent and the chemical. 3) Evaluation of chemicals with unknown molecular weight using the gravimetric approach. 4) High-sensitivity detection of nucleophilic reagents by the fluorescence method. 5) Evaluation of the mixture by a combination of the gravimetric approach and fluorescence detection.

The within- and between-laboratories reproducibility and predictive capacity of Amino acid Derivative Reactivity Assay using 4 mM test chemical solution: Results of ring study implemented at five participating laboratories

Amino acid derivative reactivity assay (ADRA) for skin sensitization was adopted as an alternative method in the 2019 OECD Guideline for the Testing of Chemicals (OECD TG 442C). The molar ratio of the nucleophilic reagent to the test chemicals in the reaction solution was set to 1:50. Imamura et al. reported that changing this molar ratio from 1:50 to 1:200 reduced in false negatives and improved prediction accuracy. Hence, a ring study using ADRA with 4 mM of a test chemical solution (ADRA, 4 mM) was conducted at five different laboratories to verify within- and between-laboratory reproducibilities (WLR and BLR, respectively). In this study, we investigated the WLR and BLR using 14 test chemicals grouped into three classes: (1) eight proficiency substances, (2) four test chemicals that showed false negatives in the ADRA with 1 mM test chemical solution (ADRA, 1 mM), but correctly positive in ADRA (4 mM), and (3) current positive control (phenylacetaldehyde) and a new additional positive control (squaric acid diethyl ester). The results showed 100% reproducibility and 100% accuracy for skin sensitization. Hence, it is clear that the ADRA (4 mM) is an excellent test method in contrast to the currently used ADRA (1 mM). We plan to resubmit the ADRA (4 mM) test method to the OECD Test Guideline Group in the near future so that OECD TG 442C could be revised for the convenience and benefit of many ADRA users.

In chemico sequential testing strategy for assessing the photoallegic potential

Several non-animal testing methods to assess photoallergic potential have been developed so far, while none of them have yet to be validated and regulatory accepted. Currently, some photoreactivity assays such as UV-VIS spectral analysis and ROS assay are generally used for initial photosafety assessments because of their high sensitivity. However, they have a low specificity, generating a high percentage of false positive results, and the development of a follow-up assessment method is desired. Therefore, this study aimed to develop an in chemico photoallergy testing method, photo-direct peptide reactivity assay (photo-DPRA). Based on photosafety information, 34 photoallergens and 16 non-photoallergens were selected and subjected to UV-VIS spectral analysis, ROS/micellar ROS assays, photo-DPRA, sequential testing strategy (STS) consisting of all three methods, and 3T3 neutral red uptake phototoxicity testing (3T3 NRU PT). Combination of the methods addressing the key events of photoallergy exhibited high prediction performance. Our results showed the proposed strategy would be useful to predict the photoallergic potential of chemicals as the follow-up assessment for false positive chemicals by UV/VIS spectral analysis and ROS assay.

Reactivity-directed analysis - a novel approach for the identification of toxic organic electrophiles in drinking water

Drinking water consumption results in exposure to complex mixtures of organic chemicals, including natural and anthropogenic chemicals and compounds formed during drinking water treatment such as disinfection by-products. The complexity of drinking water contaminant mixtures has hindered efforts to assess associated health impacts. Existing approaches focus primarily on individual chemicals and/or the evaluation of mixtures, without providing information about the chemicals causing the toxic effect. Thus, there is a need for the development of novel strategies to evaluate chemical mixtures and provide insights into the species responsible for the observed toxic effects. This critical review introduces the application of a novel approach called Reactivity-Directed Analysis (RDA) to assess and identify organic electrophiles, the largest group of known environmental toxicants. In contrast to existing in vivo and in vitro approaches, RDA utilizes in chemico methodologies that investigate the reaction of organic electrophiles with nucleophilic biomolecules, including proteins and DNA. This review summarizes the existing knowledge about the presence of electrophiles in drinking water, with a particular focus on their formation in oxidative treatment systems with ozone, advanced oxidation processes, and UV light, as well as disinfectants such as chlorine, chloramines and chlorine dioxide. This summary is followed by an overview of existing RDA approaches and their application for the assessment of aqueous environmental matrices, with an emphasis on drinking water. RDA can be applied beyond drinking water, however, to evaluate source waters and wastewater for human and environmental health risks. Finally, future research demands for the detection and identification of electrophiles in drinking water via RDA are outlined.