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Defined Approaches to Identify Potential Skin Sensitizers

Skin sensitizers are substances with the potential to cause allergic contact dermatitis. Allergic contact dermatitis is an allergic reaction that is often initiated through covalent binding of substances to skin proteins.

Potential skin sensitizers can be identified without animal testing using defined approaches to testing and assessment. A defined approach consists of:

  • Input data generated from identified methods.
  • A data interpretation procedure that is used to translate data produced from a defined set of information sources into a prediction.

An adverse outcome pathway can provide a framework for the development of a defined approach. An adverse outcome pathway is a model that identifies the sequence of biological events required to produce a toxic effect when an organism is exposed to a substance. In vitro methods can be used to observe or measure the biological events and predict whether a substance will produce a toxic effect.

The events that result in allergic contact dermatitis have been described in an adverse outcome pathway for skin sensitization. NICEATM, working with NTP, ICCVAM, and industry scientists, has created and evaluated defined approaches to identify potential skin sensitizers based on this adverse outcome pathway.

Regulatory Applications of Defined Approaches

In June 2021, OECD issued Guideline 497, Defined Approaches on Skin Sensitisation. Drafted and sponsored by NICEATM and international partners, Guideline 497 is the first internationally harmonized guideline to describe a non-animal approach that can be used to replace an animal test to identify skin sensitizers.

Draft guidance released by FDA in February 2020 announced that they will consider skin sensitization data generated using a battery of in silico, in chemico, and in vitro studies that have been shown to adequately predict human skin sensitization with an accuracy similar to existing in vivo methods.

In April 2018, EPA released a draft Science Policy to reduce animal use by using defined approaches to identify potential skin sensitizers. The draft policy is the result of national and international collaboration among ICCVAM, NICEATM, Cosmetics Europe, the European Union Reference Laboratory for Alternatives to Animal Testing, and Health Canada's Pest Management Regulatory Agency.


Defined Approaches Developed by ICCVAM

Defined approaches developed by NICEATM and ICCVAM use non-animal data to predict skin sensitization hazard and potency. These defined approaches combine data from the following sources:

  • Direct peptide reactivity assay (DPRA).
  • KeratinoSens assay.
  • Human cell line activation test (h-CLAT).
  • Read-across predictions generated by the QSAR Toolbox software package.
  • Physical properties (e.g., partition coefficients).

The defined approaches were described in papers published in 2016 and 2017:

  • Strickland et al. 2016 described prediction of murine local lymph node assay outcomes.
  • Strickland et al. 2017 used data from human exposures to predict human skin sensitization hazard.
  • Zang et al. 2017 described prediction of human or animal skin sensitization potency, enabling the classification of skin sensitizers as "weak" or "strong" without animal tests.
To expand the potential applicability of these defined approaches to a broader range of chemical types, ICCVAM agencies nominated over 200 chemicals for additional testing in the DPRA, KeratinoSens, and h-CLAT tests. Chemicals being tested include pesticide ingredients and formulations, industrial chemicals, and personal care product ingredients. NTP conducted this testing, which began in 2017 and was completed in 2020; reports are being prepared for publication in 2022.  A poster describing performance of these test methods individually and in the defined approach for testing of 27 agrochemical formulations (Strickland et al.) was presented at the 11th World Congress on Alternatives and Animal Use in the Life Sciences.

Collaboration With Cosmetics Europe to Evaluate Defined Approaches

NICEATM and Cosmetics Europe collaborated to evaluate multiple defined approaches for skin sensitization safety assessment that had been submitted to OECD as case studies. The collaboration produced two publications:

  • Hoffman et al. 2018 describes a database including data from human, animal, and five non-animal tests for 128 substances. The substances in the database have a variety of chemical structures and use categories. The database is a proposed point of reference for the evaluation and development of new non-animal approaches to skin sensitization safety assessment.
  • Kleinstreuer et al. 2018 describes an analysis of multiple defined approaches for skin sensitization safety assessment of cosmetic ingredients using the database described above. Many of these approaches were found to perform as well or better than animal methods to predict human skin sensitization hazards.

Collaboration With P&G to Develop an Open-Source Integrated Testing Strategy

An integrated testing strategy is a type of defined approach that relies on:

  • Input data generated from identified methods.
  • A data interpretation procedure such as a machine-learning model, flowchart, or decision tree, through which the data are evaluated.
  • A simultaneous assessment of the input data to arrive at either a hazard prediction or a decision that more testing is needed.

NICEATM and other NTP scientists collaborated with scientists at Procter & Gamble (P&G) to develop an integrated testing strategy to identify potential skin sensitizers. Using data from non-animal tests and other information (e.g., substance solubility), the strategy produces a numerical probability that a chemical should be placed in one of the following skin sensitization hazard classes:

  • Strong
  • Moderate
  • Weak
  • Nonsensitizer

This probability could potentially be used to determine if a substance requires hazard labeling. P&G and NTP scientists collaborated to develop this integrated testing strategy using free, publicly available software (Pirone et al. 2014).

P&G updated the integrated testing strategy in 2015. The updated strategy (Jaworska et al. 2015) uses only validated non-animal tests, simplifies the bioavailability inputs, and nearly doubles the size of the database used to derive the previous network. The updated P&G integrated strategy is included in a web tool produced by Edelweiss Connect that can be used to predict a substance's skin sensitization potency.

Files for Running the Open-source Integrated Testing Strategy Analysis

Files to run the analysis described in Pirone et al., including a script that uses the R programming language, are available below.

Additional Resources
  • A bioavailability calculator is available from the National Institute for Occupational Safety and Health (NIOSH) website. Please note that Java must be enabled in your web browser in order to run the calculator.
  • The software for running R can be obtained from the R Project website. Refer to the FAQs on the R Project website for system requirements and installation instructions.
  • The OECD QSAR Toolbox can be obtained from the OECD website. Installation instructions and user documentation are available on this page.