The Organization for Economic Cooperation and Development (OECD) has published a March 23, 2018, report entitled Evaluation of in vitro methods for human hazard assessment applied in the OECD Testing Programme for the Safety of Manufactured Nanomaterials. The Testing Program for the Testing of Manufactured Nanomaterials tested 11 manufactured nanomaterials to generate information, including their physico-chemical properties, environmental fate, and environmental and mammalian toxicity. The purpose of the evaluation was to review the Testing Program dossiers and evaluate which of the existing in vitro OECD Test Guidelines (TG) were used, what other non-guideline methods were applied, and what were the potential limitations of each assay used for testing manufactured nanomaterials. The report lists the following general observations:
- A number of in vitro methods were used in the OECD Testing Program to assess manufactured nanomaterials, however, many in vitro data reported were not generated using OECD TGs;
- A number of in vitro endpoints have been assessed, but a full set of in vitro assays for a specific manufactured nanomaterial is not available;
- The existing in vitro TGs have not been used extensively (g., skin sensitization and irritation);
- There are many inconsistencies and omissions within the dossiers related to the use of existing OECD TGs, representation of dose and dose metrics, physico-chemical parameters assessed, consideration of manufactured nanomaterial interference with assay parameters, and protocols used, including inconsistencies in reporting;
- Characterization of manufactured nanomaterials in in vitro conditions (g., in culture media) and cellular uptake have not been reported; and
- No in vitro data based on TGs have been reported for nanoclay and dendrimers and no in vitro cytotoxicity data has been reported for silver and nanoclay within the Testing Program.
The report identifies the following potential next steps:
- Based on TG 428 and the reported studies, there is no apparent limitation in the application of the TG 428 to manufactured nanomaterials. Some critical factors for the evaluation of skin absorption tests with nanomaterials were not addressed in detail in these studies, however, and might need to be further explored. Applicability of TG 431 (In Vitro Skin Corrosion: Reconstructed Human Epidermis Test Method) should be further discussed;
- Applicability of TG 437 (Bovine Corneal Opacity and Permeability Test Method) should be further discussed. Literature findings indicate that nanomaterials can aggregate/agglomerate in the suspension or can absorb the dispersant and the dye, causing possible artefacts. Also, some nanomaterials present in opacity measurements may affect the result;
- TG 471 (Bacterial Reverse Mutation Test) may be amended with the acknowledgement that it is not applicable for most types of manufactured nanomaterials (no uptake into the bacteria);
- Modification of TG 487 (In Vitro Micronucleus Assay) should be considered to include specific recommendations regarding addition of cytochalasin B when testing manufactured nanomaterials and the verification of intracellular uptake of manufactured nanomaterials;
- Since several genotoxicity assays were applied in the Testing Program, it might be most efficient to consult the expert group on genotoxicity to prioritize the assays for further harmonization; and
- In addition to the work that is currently underway at the OECD Working Party on Manufactured Nanomaterials (WPMN) and International Organization for Standardization (ISO) Technical Committee (TC) 229 to develop standards and guidance for some cytotoxicity assays, the Testing Program should consider evaluation and further development of the assays that are routinely used to test manufactured nanomaterials. For instance, the assays used to determine the potential of manufactured nanomaterials to cause oxidative stress and those used to assess endpoints specific to immunological implications of exposure to manufactured nanomaterials.