OECD Accepting Comment on Draft Study Report on Applicability of the key event based TG 442D for in vitro skin sensitisation testing of nano-materials

Yes, OECD TG 442D (KeratinoSens^TM test method) is from a technical point of view applicable for the testing of manufactured nanomaterials. The draft report notes that it was not possible to make any assumption about the relevance for the in vivo correlation due to the scarce availability of data for manufactured nanomaterials that have been tested in vivo. Nevertheless, the work conducted within this project can be seen as a starting point for further work with regard to manufactured nanomaterials safety testing for skin sensitization.

Different manufactured nanomaterials were selected for testing based on an extensive literature review and dependent on information about their skin sensitizing potential. In total, 12 inorganic and organic manufactured nanomaterials were selected (nine test materials, three controls). Testing with KeratinoSens^TM was from a technical point of view possible with all of the selected manufactured nanomaterials.

During the practical part of this study, there were no manufactured nanomaterials identified that could not be tested. The draft study notes that the sample number of 12 manufactured nanomaterials is relatively small in comparison to the variety of manufactured nanomaterials, however, and the authors cannot conclude whether there is one manufactured nanomaterials group that cannot be tested using KeratinoSens^TM. According to the draft study, the critical step might be the sample preparation. If a manufactured nanomaterial cannot be dispersed to be tested in the respective media for KeratinoSens^TM, it cannot be tested in this assay.

According to the experience gained during the testing of selected manufactured nanomaterials with the KeratinoSens^TM and based on the discussion within the two expert workshops in December 2019 and 2021, some recommendations can be made with regard to the dispersion protocols, needs for endotoxin assays, potential to include leaching experiments, and the role of dimethyl sulfoxide (DMSO) as a mediator to assist nanomaterial penetration into cells. Further, the potential for nanomaterials to interact with detection methodologies also brought in the possibility to use two different colorimetric cytotoxicity assays.

Some of the recommendations made under the previous point can be directly addressed by adaptation of the standard operating procedures (SOP) of the KeratinoSens^TM test method, e.g., viability assessment.