Non-Animal Skin Sensitization Testing Strategies
How to Choose the Right Approach
Introduction: strategy first, then assays
Selecting the right skin sensitization testing strategy is essential for regulatory submissions and confident product development decisions. Because substances vary in physio-chemical properties and complexity, there is no one‑size‑fits‑all plan. Instead, high‑quality strategies combine OECD‑validated non‑animal methods and are tailored to the testing objectives and material properties.
Key takeaways
- There is no one‑size‑fits‑all strategy. Tailor your approach to both regulatory objectives and material properties.
- GARD®skin is central to modern, non‑animal strategies, broadening applicability, filling data gaps, and supporting product development.
- For “difficult‑to‑test” substances, GARD®skin and its adaptations can be pivotal in achieving a decision‑ready outcome.
Author: Andy Forreryd, PhD
In vitro toxicology expert, member of the OECD expert group on respiratory sensitization, and ISO/TC 194/WG 8 on irritation and sensitization.
OECD TG 497: defined approaches in practice
OECD Test Guideline 497 introduces Defined Approaches for Skin Sensitization (DASS), structured combinations of non‑animal methods for predictive performance aligned with human biology.
- “2 out of 3” Defined Approach (2o3 DA): Supports binary hazard identification by requiring concordant outcomes from any two methods that address Key Events (KE1–KE3) of the skin sensitization Adverse Outcome Pathway (AOP).
- Integrated Testing Strategy (ITS): Supports UN GHS potency sub‑categorization using a predefined scoring system that integrates KE1 and KE3 assay data with in silico information.
What drives method selection within DASS?
- Intended use: hazard identification vs potency sub‑categorization.
- Material attributes: solubility and hydrophobicity (e.g. Log P > 3.5); the presence of unknown or poorly characterized constituents (e.g. UVCBs); chemical classes with limited data from conventional cell-based assays (e.g. agrochemicals, metals); and the sample matrix (e.g. neat chemical, mixture, extract, or formulation).
Why GARD® is central
GARD®skin (OECD TG 442E, 497) plays a central role across these scenarios and extends the overall applicability of DASS, helping to fill data gaps in regulatory skin sensitization testing.
Beyond regulatory use, GARD® stand‑alone testing provides human-relevant, mechanistic insights that support confident product development decisions, especially helpful when you need to prioritize candidates, refine formulations, or understand material behavior early.
Recommended non‑animal skin sensitization testing strategies (at‑a‑glance)
Use this structure to map objectives to recommended assay combinations:
Hazard identification — 2o3 DA
- Neat chemicals without challenging properties: DPRA (OECD TG 442C) + GARDskin (OECD TG 442E)
- Difficult‑to‑test substances (e.g. hydrophobic substances, UVCBs, complex formulations): EpiSensA (OECD TG 442D) + GARDskin (OECD TG 442E)
Potency sub‑categorization (GHS 1A/1B) — ITS
- Neat chemicals without challenging properties: DPRA (OECD TG 442C) + GARDskin (OECD TG 442E) + in silico
- Difficult‑to‑test substances: ITS is often not applicable due to the applicability limitation of DPRA; use EpiSensA as a weight‑of‑evidence + GARDskin + in silico
GARD stand‑alone testing for product development
- Hazard identification of neat chemicals & difficult‑to‑test substances: GARDskin provides a comprehensive, human‑relevant solution for hazard identification, especially for substances where other in vitro methods have restricted applicability.
- Hazard identification of medical devices & solid materials: GARDskin adapted protocols for extractables/leachables.
- Substances with unknown sensitization hazard and potency: Tiered approach (GARDskin → GARDskin Dose‑Response for quantitative potency insight).
- Expected sensitizers with unknown potency: GARDskin Dose‑Response only.
Why GARD® helps
GARD®skin measures 196 biomarkers linked to multiple key events in the skin sensitization AOP, delivering human‑relevant, mechanistic insights beyond what early‑stage tests typically provide. The assay offers high performance and broad applicability, including for “difficult‑to‑test” substances outside the domains of conventional OECD in vitro methods.
Adaptations of the assay also enable testing of medical device extracts, solid materials, and quantitative potency assessment to support product development and risk‑management strategies.
What GARD provides:
- Mechanistic data beyond what early-stage testing typically offers.
- Broad applicability to hydrophobic substances, natural extracts, formulations, UVCBs, and more.
- Dose-Response option for quantitative potency insights.
Key benefits:
- Enable earlier decisions during candidate selection, formulation development, and risk evaluation.
- De‑risk later stages and streamline development.
- Save time and resources in the short, medium and long run.
Choosing the right skin sensitization testing strategy: a short checklist
- Define your testing objectives.
- Characterize the test substance.
- Select assays aligned to your objectives and the AOP KEs.
- Consider applicability domains.
- Use GARD® to extend applicability and create a coherent, weight of evidence package where needed.
Need help selecting the right strategy?
Our experts will design a customized, non‑animal skin sensitization testing strategy based on your chemical’s properties and decision needs. Connect with our scientific experts.
About the Author
Andy Forreryd, PhD
In vitro toxicology expert, member of the OECD expert group on respiratory sensitization, member of ISO/TC 194/WG 8
Dr Andy Forreryd is an in vitro toxicology expert with extensive experience in assay development, genomics, and machine learning. He collaborates closely with industry leaders to advance novel applications of New Approach Methodologies (NAMs) for sensitization testing, with the aim to replace animal studies.
Dr Forreryd is a co-developer of the GARDskin assay (OECD TG 442E) for the assessment of chemical sensitizers. He is an active member of the OECD expert group on respiratory sensitization, and ISO/TC 194/WG 8 for skin sensitization testing of medical devices.
References
- NICEATM Report (2025): Evaluation of the GARDskin Sensitization Test Method Using Substances of Regulatory Interest. DOI: 10.22427/NICEATM‑06
- OECD Guideline No. 497 (2025): Defined Approaches on Skin Sensitisation
- OECD Test Guidline 442E (2021): In Vitro Skin Sensitisation
- Joint publication with Corteva Agriscience. Regulatory Toxicology and Pharmacology (2024): GARD™skin and GARD™potency: a proof-of-concept study to investigate the applicability domain for agrochemical formulations.
- Joint publications with Johnson Matthey. ALTEX (2023): The GARD™skin assay: Investigation of the applicability domain for metals.
- Joint publication with Lubrizol, ALTEX (2022): Exploration of the GARDskin applicability domain: Indirectly acting haptens, hydrophobic substances and UVCBs.
