Data on dermal irritation by copper and its compounds are scant and the role of copper as an irritant/sensitizer remains controversial3. A recent review on copper hypersensitivity concluded that copper is a weak sensitizer as compared with other metallic compounds14. However, with the prevalence of skin permeation enhancement methods (such as microneedles and laser) used in cosmetic and pharmaceutical industry15, copper compounds may pose a higher risk to cause skin irritation, which necessitates a thorough skin toxicity testing. Animals tests for acute skin irritation assessment usually follow the Draize rabbit test while other accepted assays for skin sensitization include Local Lymph Node Assay (LLNA) and Guinea Pig Maximisation Test (GPMT). However, LLNA is deficient in detecting metals and organometallic compounds16.
Furthermore, due to questionable significance of animal data and ethical opposition to animal testing, effort has been to put into finding alternative testing methods to identify potential skin toxins17,18,19. Most considerations of non-animal alternatives for skin irritation/sensitization testing tend to examine a single aspect of the process (e.g., chemical reactivity, epidermal bioavailability, dendritic cell responses). However, experts in the area generally concur that a combination of data from multiple endpoints is needed to discriminate sensitizing and/or irritant substances in vitro. Besides, Direct Peptide Reactivity Assay (DPRA) is the assay recommended by EURL ECVAM for skin sensitization testing. However, DPRA is not suitable for metallic compounds, because they may form bonds with the nucleophilic residues in histidine20.