The underlying factors that explain why nucleophilic reagents rarely co-elute with test chemicals in the ADRA

Proof of concept testing of a positive reference material for in vivo and in vitro sensitization testing of medical devices

In vivo skin sensitization tests are required to evaluate the biological safety of medical devices in contact with living organisms to provide safe medical care to patients. Negative and positive reference materials have been developed for biological tests of cytotoxicity, implantation, hemolysis, and in vitro skin irritation. However, skin sensitization tests are lacking. In this study, polyurethane sheets containing 1 wt/wt % 2,4-dinitrochlorobenzene (DNCB-PU) were developed and evaluated as a positive reference material for skin sensitization tests. DNCB-PU sheet extracts prepared with sesame oil elicited positive sensitization responses for in vivo sensitization potential in the guinea pig maximization test and the local lymph node assay. Furthermore, DNCB-PU sheet extracts prepared with water and acetonitrile, 10% fetal bovine serum-containing medium, or sesame oil elicited positive sensitization responses as alternatives to animal testing based on the amino acid derivative reactivity assay, human cell line activation test, and epidermal sensitization assay, respectively. These data suggest that the DNCB-PU sheet is an effective extractable positive reference material for in vivo and in vitro skin sensitization testing in medical devices. The formulation of this reference material will lead to the development of safer medical devices that contribute to patient safety.

Alternative Methods for Skin-Sensitization Assessment

Skin sensitization is a term used to refer to the regulatory hazard known as allergic contact dermatitis (ACD) in humans or contact hypersensitivity in rodents, an important health endpoint considered in chemical hazard and risk assessments. Information on skin sensitization potential is required in various regulatory frameworks, such as the Directive of the European Parliament and the Council on Registration, Evaluation and Authorization of Chemicals (REACH). The identification of skin-sensitizing chemicals previously required the use of animal testing, which is now being replaced by alternative methods. Alternative methods in the field of skin sensitization are based on the measurement or prediction of key events (KE), i.e., (i) the molecular triggering event, i.e., the covalent binding of electrophilic substances to nucleophilic centers in skin proteins; (ii) the activation of keratinocytes; (iii) the activation of dendritic cells; (iv) the proliferation of T cells. This review article focuses on the current state of knowledge regarding the methods corresponding to each of the key events in skin sensitization and considers the latest trends in the development and modification of these methods.

Theoretical Validation of In Chemico Skin Sensitization Assay "ADRA" Using the Products Formed by Nucleophilic Reagents and Chemicals

Amino acid derivative reactivity assay (ADRA) is an in chemico assay for assessing the skin sensitization potential of chemicals by evaluating the reactivity of nucleophilic reagents that mimic skin proteins. N-(2-(1-Naphthyl)acetyl)-l-cysteine (NAC) and α-N-(2-(1-naphthyl)acetyl)-l-lysine (NAL), used as nucleophilic reagents, are small-molecule derivatives of two different amino acids, each with a naphthalene ring attached. The rate of decrease in the amount of NAC or NAL in the reaction solution is evaluated in this assay as an indicator of the test substance's skin sensitization ability. However, the products formed between the nucleophilic reagent and the test substance, which play an important role in vivo, are not directly identified. Therefore, six highly reactive chemicals, including the proficiency substances listed in the OECD Test Guidelines─squaric acid diethyl ester, 2-methyl-2H-isothiazol-3-one (MI), p-benzoquinone, palmitoyl chloride, diphenylcyclopropenone (DPCP), and imidazolidinyl urea (IU)─were used to determine each formed product. Samples were prepared according to the standard ADRA method, and the formed products were predicted on the basis of the reaction mechanism. Excluding DPCP, the estimated structures were validated using mass spectrometry and nuclear magnetic resonance spectrometry on the synthesized samples. In this manner, the products of each nucleophile were confirmed for all examined test substances. The estimated structure products were obtained through a series of reactions initiated by the nucleophilic attack of NAC's thiol group or NAL's amino group on the test substance's electron-deficient carbonyl carbon. However, contrary to expectations, disulfide-linked-type ring-opened products were detected in the case of MI, and products with free formaldehyde in solution were detected in the case of IU. In summary, all skin sensitizers tested herein reacted with NAC and/or NAL to give products. This supports the theoretical validity of ADRA, which provides an indirect evaluation of the formed products based on a decrease in nucleophilic reagents.

Expansion of the Cosmetics Europe skin sensitisation database with new substances and PPRA data

The assessment of skin sensitisation is a key requirement in all regulated sectors, with the European Union's regulation of cosmetic ingredients being most challenging, since it requires quantitative skin sensitisation assessment based on new approach methodologies (NAMs). To address this challenge, an in-depth and harmonised understanding of NAMs is fundamental to inform the assessment. Therefore, we compiled a database of NAMs, and in vivo (human and local lymph node assay) reference data. Here, we expanded this database with 41 substances highly relevant for cosmetic industry. These structurally different substances were tested in six NAMs (Direct Peptide Reactivity Assay, KeratinoSens™, human Cell Line Activation Test, U-SENS™, SENS-IS, Peroxidase Peptide Reactivity Assay). Our analysis revealed that the substances could be tested without technical limitations, but were generally overpredicted when compared to reference results. Reasons for this reduced predictivity were explored through pairwise NAM comparisons and association of overprediction with hydrophobicity. We conclude that more detailed understanding of how NAMs apply to a wider range of substances is needed. This would support a flexible and informed choice of NAMs to be optimally applied in the context of a next generation risk assessment framework, ultimately contributing to the characterisation and reduction of uncertainty.

Applicability of ADRA (4 mM) for the prediction of skin sensitization by combining multiple alternative methods to evaluate key events

The amino acid derivative reactivity assay (ADRA) is an alternative method for evaluating key event 1 (KE-1) in the skin sensitization mechanism included in OECD TG442C. Recently, we found that ADRA with a 4-mM test chemical solution had a higher accuracy than the original ADRA (1 mM). However, ADRA (4 mM) has yet to be evaluated using Integrated Approaches to Testing and Assessment (IATA), a combination of alternative methods for evaluating Key-event. In this study, the sensitization potency of three Defined Approach (DA) using ADRA (4 mM) as KE-1 was predicted and compared with those of two additional ADRAs or DPRA: (i) "2 out of 3" approach, (ii) "3 out of 3" approach,, and (iii) ITS. In the hazard identification of chemical sensitizers, the accuracy of human data and LLNA remained almost unchanged among the three approaches evaluated. Potency classifications for sensitization were predicted with the LLNA and human datasets using ITS. The potency classifications for the sensitization potency prediction accuracy of LLNA data using any alternative method were almost unchanged, at approximately 70%, and those with ITS were not significantly different. When ITS was performed using DPRA, the prediction accuracy was approximately 73% for human data, which was similar to that of the LLNA data; however, the accuracy tended to increase for all ADRA methods. In particular, when ITS was performed using ADRA (4 mM), the prediction accuracy was approximately 78%, which proved to be a practical level.

[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.

Skin sensitization in silico protocol

The assessment of skin sensitization has evolved over the past few years to include in vitro assessments of key events along the adverse outcome pathway and opportunistically capitalize on the strengths of in silico methods to support a weight of evidence assessment without conducting a test in animals. While in silico methods vary greatly in their purpose and format; there is a need to standardize the underlying principles on which such models are developed and to make transparent the implications for the uncertainty in the overall assessment. In this contribution, the relationship between skin sensitization relevant effects, mechanisms, and endpoints are built into a hazard assessment framework. Based on the relevance of the mechanisms and effects as well as the strengths and limitations of the experimental systems used to identify them, rules and principles are defined for deriving skin sensitization in silico assessments. Further, the assignments of reliability and confidence scores that reflect the overall strength of the assessment are discussed. This skin sensitization protocol supports the implementation and acceptance of in silico approaches for the prediction of skin sensitization.

The amino acid derivative reactivity assay with fluorescence detection and its application to multi-constituent substances

The Amino acid Derivative Reactivity Assay (ADRA) is an in chemico alternative to animal testing for the prediction of skin sensitization potential. Although co-elution of test chemicals and nucleophilic reagents during HPLC analysis is sometimes problematic when using the Direct Peptide Reactivity Assay (DPRA), it rarely occurs when using ADRA. Nevertheless, the application of either of these tests to multi-constituent substances requires nucleophilic reagents capable of selective detection. With this issue in mind, the authors developed an ADRA fluorescence detection method (ADRA-FL), which utilizes the natural fluorescence of ADRA nucleophilic reagents. In this study, we demonstrate the efficacy of ADRA-FL by testing 82 test chemicals used in the development of both DPRA and the conventional ADRA (ADRA-UV) as well as establish a threshold value for distinguishing sensitizers and non-sensitizers. Our results show that not only are depletion values obtained using ADRA-FL virtually identical to those obtained using ADRA-UV, the threshold value for either test is 4.9%. Additionally, in order to demonstrate the applicability of ADRA-FL to multi-constituent substances, we prepared test samples that consisted of a set of 10 non-sensitizers combined with one of 10 different sensitizers and tested each using ADRA-FL. The test results were concordant with those obtained using ADRA-UV. Also, because ADRA-FL chromatograms showed a significant decrease in multiple peaks as well as extremely stable baselines, we conclude that ADRA-FL is a highly selective and highly accurate mans of quantifying nucleophilic reagents that is applicable to a wide variety of chemical substances.

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

Photoallergy test of cosmetics and several types of pharmaceutical substances is often necessary for obtaining approval from authorities. However, there are no official test guidelines for photoallergy evaluation. Therefore, we tried to establish a photoallergy test by utilizing an in chemico alternative sensitization method, amino acid derivative reactivity assay (ADRA). To determine the criteria for judging the photoallergy potential, photo-ADRA with or without photoirradiation was performed using 60 photoallergenic chemicals, and cysteine and lysine derivatives were detected using high-performance liquid chromatography either by absorbance or fluorescence measurement. The accuracy of prediction was 81.4% (48 of 59) and 80.0% (48 of 60) using the absorbance and fluorescence methods, respectively. However, as chemicals can breakdown into multiple chemicals during photoirradiation, the absorbance method often cannot perform accurate detection due to co-elution, whereas the fluorescence method can do this due to lack of co-elution. Moreover, all eight chemicals that were found to be negative or false-positive for photoirritation in the 3T3 neutral red uptake phototoxicity test were confirmed as positive for photoallergy using this method. Furthermore, we prepared three types of pseudo-mixtures where we added one photoallergen along with five nonphotoallergens and performed the photo-ADRA by the ultraviolet and fluorescence methods. The result of the fluorescence method was almost the same as that obtained with the use of a single photoallergen and hence the outcome was not affected by the mixture. Thus, this study not only showed a method of evaluating the photoallergy potential of a single chemical but also a mixture, making it useful as an in chemico photoallergy alternative test.

Oxidation of a cysteine-derived nucleophilic reagent by dimethyl sulfoxide in the amino acid derivative reactivity assay

The amino acid derivative reactivity assay (ADRA), which is an in chemico alternative to the use of animals in testing for skin sensitization potential, offers significant advantages over the direct peptide reactivity assay (DPRA) in that it utilizes nucleophilic reagents that are sensitive enough to be used with test chemical solutions prepared to concentrations of 1 mm, which is one-hundredth that of DPRA. ADRA testing of hydrophobic or other poorly soluble compounds requires that they be dissolved in a solvent consisting of dimethyl sulfoxide (DMSO) and acetonitrile. DMSO is known to promote dimerization by oxidizing thiols, which then form disulfide bonds. We investigated the extent to which DMSO oxidizes the cysteine-derived nucleophilic reagents used in both DPRA and ADRA and found that oxidation of both N-(2-(1-naphthyl)acetyl)-l-cysteine (NAC) and cysteine peptide increases as the concentration of DMSO increases, thereby lowering the concentration of the nucleophilic reagent. We also found that use of a solvent consisting of 5% DMSO in acetonitrile consistently lowered NAC concentrations by about 0.4 μm relative to the use of solvents containing no DMSO. We also tested nine sensitizers and four nonsensitizers having different sensitization potencies to compare NAC depletion with and without 5% DMSO and found that reactivity was about the same with either solvent. Based on the above, we conclude that the use of a solvent containing 5% DMSO has no effect on the accuracy of ADRA test results. We plan to review and propose revisions to OECD Test Guideline 442C based on the above investigation.

Applicability of amino acid derivative reactivity assay for prediction of skin sensitization by combining multiple alternative methods to evaluate key events

Amino acid derivative reactivity assay (ADRA) has previously been developed as an alternative method to direct peptide reactivity assay (DPRA) to evaluate key event 1 in skin sensitization mechanisms. However, when using alternative methods for skin sensitization, integrated approaches to testing and assessment (IATA) that combine the results of multiple tests evaluating different key events are generally required. To verify whether ADRA can be used in IATA, we replaced DPRA with ADRA in five IATA methods combining DPRA, KeratinoSens, and h-CLAT: (i) the "2 out of 3" approach, (ii) the "3 out of 3" approach, (iii) sequential testing strategy (STS), (iv) integrated testing strategy by scoring approach (ITS-SA), and (v) the "ITS by two methods approach" (ITS-2MA). The prediction accuracy of the "2 out of 3" approach using ADRA (1 mM) and ADRA (0.5 mg/mL) was 90.0% and 91.1%, respectively, for human data, and was very similar to that obtained using DPRA (91.1%). The "3 out of 3" approach also showed good predictability (83.2%) using either ADRA (1 mM) or ADRA (0.5 mg/mL) compared to DPRA. Regarding the accuracy of the prediction of sensitization intensity for the human data by the third classification, prediction accuracy using ADRA was almost the same as STS, ITS-SA, or ITS-2MA using DPRA. As a result, this study showed that ADRA can be used as a test method for key event 1 in the evaluation of skin sensitization by combining multiple alternative methods.

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

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.