Screening and categorising azo-based substances

In Stage 2 of the Inventory Multi-Tiered Assessment and Prioritisation (IMAP) Framework, we continue to carry out pre-prioritisation work. This includes screening and categorising about 3,000 unassessed azo-based substances listed in the Australian Inventory of Chemical Substances (AICS).

Azo-based substances are a group of aromatic and aliphatic substances with the general chemical structure in the form of ‘R1-N=N-R2’ (aliphatic when R1 and R2 are alkyl; aromatic if one or more of R1 or R2 are aryl). Azo substances are generally large molecules that are poorly absorbed.

The azo linkage is usually the most labile portion of an azo substance and may easily undergo enzymatic reduction to release aromatic amines from aromatic azo substances. The released aromatic amines are much more readily absorbed across biological membranes compared with the azo substance itself. These aromatic amines may potentially cause human health effects related to skin sensitisation, mutagenicity and carcinogenicity (aromatic amines of concern).

Our aim was to identify azo-based substances that, with up to date and currently available knowledge, would not be efficiently reduced to aromatic amines of concern. These substances are poorly bioavailable (absorbed). Trace amines, which may be more readily available, are not expected to cause unreasonable risks. Therefore these azo-based substances are considered low concern for human health. Azo-based substances not considered as low concern for human health under this methodology will be further evaluated.

Screening methodology

We have developed a semi-automated screening methodology based on the following main factors:

  • cosmetic use information
  • known hazards
  • potential to release aromatic amines of concern, or
  • bioavailability of the parent azo compound.

Cosmetic use

Azo-based substances with existing cosmetic uses were considered to present greater exposure and, therefore, potential for human health effects. These substances will be considered for further evaluation.

Known hazard

Azo-based substances with known human health hazards were considered to be of potential concern for human health and will be considered for further evaluation.

Aromatic amines of concern

Azo-based substances that were not benzidine derivatives or did not have existing human health hazard concerns or known cosmetic uses were evaluated for their potential to release aromatic amines of concern. The aromatic amines of concern were derived from the following lists:

  • European Union (EU) 22 amines regulated in clothing
  • Bruschweiler et al. 2014; 2017
  • IMAP assessment outcomes

We used the Simplified Molecular-Input Line-entry System (SMILES) representation derived from the Organisation for Economic Co-operation and Development (OECD) quantitative structure–activity relationship (QSAR) Toolbox or other publicly available databases. Substances with unknown constituents or no verified SMILES representation will be considered for further evaluation.

We then used the OASIS Pipeline Human Health Profiling scheme, a software program customised for NICNAS. The metabolism of the substances was simulated using OASIS Tissue Metabolic simulator (TIMES). Genotoxicity potential of the metabolites was predicted using OASIS TIMES and Vega genotoxicity QSAR models and the Danish EPA QSAR database.

Substances that metabolised, through model simulation, to aromatic amine metabolites of concern will be considered for further evaluation.

Bioavailability

Parent azo compounds that are not easily accessible for the enzymes (e.g. low bioavailability) are not expected to be efficiently reduced to aromatic amines. The main factors considered are:

  • high molecular weight
  • high degree of sulfonation (increased water solubility), and
  • azo bond protected from cleavage as part of a transition metal complex.

The influence of factors reducing bioavailability is expected to be greater for substances with more than one azo bond.

Substances that are not considered to be of low bioavailability were directed for further evaluation.

Categorisation outcomes

Azo-based substances identified as low concern for human health have the following features:

  • do not have known cosmetic use(s)
  • do not have known human health hazard concerns
  • are not metabolised into EU22 metabolites of concern (potent carcinogens),
  • are not metabolised to other amines of identified concern, and
  • are considered to have very low bioavailability.

Substances that meet these features are determined to meet the Tier I criteria for human health.

View the IMAP Stage 2 list of low concern azo-based substances to human health.

More information on the semi-automated screening methodology is available here.

Last update 30 October 2017