ADRA.

OECD TG 442C: Amino Acid Derivative Reactivity Assay

Amino Acid Derivative Reactivity Assay (ADRA) is an in chemico method that assesses the skin sensitization hazard of chemicals.

The assay evaluates a substance’s ability to react with synthetic model amino acid derivatives containing either lysine or cysteine, providing a binary classification that identifies test substances as either skin sensitizers or non‑sensitizers.

Targeting Key Event 1 of the skin sensitization AOP

ADRA is included in OECD Test Guideline 442C and addresses Key Event 1 (KE1) of the Adverse Outcome Pathway (AOP) for skin sensitization: covalent binding to skin proteins.

For poorly soluble and complex substances, replacing DPRA

Compared to peptide-based assays such as DPRA, ADRA uses amino acid derivatives, improving both solubility and analytical robustness.

  • Improved solubility handling extends applicability to poorly soluble substances and complex mixtures and UVCBs, where peptide‑based systems may be limited.
  • Support for fluorescence detection alongside UV improves detection of co‑eluting components in HPLC. This is particularly beneficial for complex mixtures.

Together, these features make ADRA a well-suited KE1 metod for poorly soluble and complex substances.

As with other KE1 assays, certain limitations remain. Substances requiring metabolic activation (pro‑haptens) or containing metals fall outside the assay’s applicability domain.

*Reference: OECD TG 442C, Appendix II

Defined approaches for “difficult-to-test” substances (OECD TG 497)

For regulatory testing of “difficult-to-test” substances under Defined Approaches for Skin Sensitization (DASS, OECD TG 497), use GARD®skin as the central method and combine it with ADRA, EpiSensA, and in silico methods to tailored testing strategies:

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At a glance

Test systemIn chemico.
ComplianceOECD TG 442C.
GLP-compliant laboratory.
What it measures Percent amino acid derivative depletion (cysteine‑ and lysine‑based).
ReadoutBinary prediction: Skin sensitizer or non-sensitizer.
Turnaround time2-4 weeks for standard studies.
Fast track available.

How it works

The Amino Acid Derivative Reactivity Assay (ADRA) is an in chemico test method used to assess a chemical’s potential to cause skin sensitization. It evaluates this potential by measuring the chemical’s ability to form covalent bonds with proteins, the key molecular initiating event in the skin sensitization process.

This approach is based on a well‑established biological principle: most skin sensitizers are electrophilic substances that react with nucleophilic amino acid residues in skin proteins. To model this process, test substances are incubated with two synthetic amino acid derivatives representing key nucleophilic binding sites:

  • The cysteine derivative NAC: N-(2-(1-naphthyl)acetyl)-L-cysteine (CAS. 32668-00-1)
  • The lysine derivative NAL: α-N-(2-(1-naphthyl)acetyl)-L-lysine (CAS. 397841-92-8)

The derivatives are incubated with the test substance for approximately 24 hours under controlled conditions, allowing covalent binding reactions to occur. After incubation, the remaining (non‑reacted) derivatives are quantified using High‑Performance Liquid Chromatography (HPLC) with gradient elution and UV detection at 281 nm. If needed, fluorescence detection (Ex/Em 284/333 nm) can be used to address co‑elution, where compounds elute at the same time and are difficult to distinguish.

The reduction in derivative concentration, expressed as percent depletion, reflects the protein reactivity of the test substance. The depletion values are then evaluated using a defined prediction model. Based on established thresholds, each substance is assigned a binary classification as either a skin sensitizer or non‑sensitizer, supporting hazard identification.

ADRA results are typically interpreted within Defined Approaches or Integrated Testing Strategies under OECD TG 497, rather than used as a stand‑alone assessment.

For a complete description of the protocol and data interpretation, see OECD Test Guideline 442C.

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