Tracing progress: The evolution of a protocol for relative metal release in surrogate gastric fluid

Grouping and read-across are applied to human health toxicity classification of metal substances under EU REACH to reduce animal testing. Metal ion release is responsible for the systemic (and often local) toxicity of metal-containing materials. Metal release in simulated gastric fluid is relevant to the oral route of exposure. In 2010, the metals industry initiated the development of a method to assess relative metal release in HCl pH 1.5 from different substances of the same metal, including metal-containing materials such as massive and powder forms of alloys, metals, inorganic metal compounds, and other inorganic complex metal-containing materials. These data could also identify matrix effects in complex materials that increase or decrease metal release compared to what could be predicted from the content of the classified ingredients. The method underwent a round robin and multiple reviews over the past 15 years (e.g., by EURL ECVAM, ESAC and OECD), leading to revi­sions of the protocol. This paper provides a history of the protocol’s evolution and details supporting studies and revisions made to address the comments and concerns of its reviewers. The 2025 version of the protocol (publicly available via TSAR) significantly improved the clarity and robustness of the method without changing the way in which data are generated. The method is relevant to the oral route and is suitable for supporting grouping and read-across of metal-containing materials and refining the classification of metal materials showing a matrix effect, which might contribute to reducing animal testing.