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In an Ames test with the S. typhimurium strains TA 98, TA 100, TA 1535, and TA 1537 1,5-naphthylene diisocyanate (NDI) revealed no mutagenic activity in the absence and in the presence of a metabolic activation system (OECD TG 471; Herbold, 1989).

A second Ames test according to Japanese guidelines showed also no mutagenic activity of NDI in the absence and in the presence of a metabolic activation system with the S. typhimurium strains TA 98, TA 100, TA 1535, TA 1537, and TA 1538 as well as E. coli strain WP2uvrA (JETOC, 1996).

In another test on gene mutation NDI was investigated at the hypoxanthine-guanine phosphoribosyl transferase locus (HPRT test) in Chinese hamster V79 cells according to OECD TG 476 (Entian, 2010). Without and with S9 mix NDI induced decreases in survival to treatment and decreases in relative population growth. These results revealed a significant concentration-related cytotoxicity of NDI. Precipitation of NDI in the culture medium was observed at 48 μg/ml and above. Without and with S9 mix there was a biologically relevant increase in mutant frequency above that of the negative controls. Based on these results, NDI is considered to be mutagenic in the V79/HPRT Forward Mutation Assay, both with and without metabolic activation. This observation is not in line with the results of the two valid and reliabletests with negative result. The reasons for the different results are not known. Overall, the result regarding gene mutation in vitro is equivocal due to the different responses in the Ames test and HPRT test.

In order to clarify the relevance of the positive findings in the in vitro HPRT test, an in vivo unscheduled DNA synthesis (UDS) test according to OECD TG 486 was performed. This approach is in line with ECHA Endpoint Specific Guidance (R. 7A) and the generally agreed principle that in vivo follow up testing should be done in case of positive results from in vitro genotoxicity assays. Ad hoc clarification was needed for a science based solid classification globally. Inhalation was selected as the relevant route of exposure. In this test male rats were once nose-only exposed for 3 hours to NDI (Desmodur 15) aerosol concentrations of 50 and 150 mg/m³ (Nebelung, 2011). All NDI treated animals showed symptoms of toxicity after exposure (bradypnea, irregular breathing patterns, labored breathing patterns, nasal discharge (serous), piloerection, reduced motility, atony (limp) and reduced body temperature). These symptoms demonstrate relevant exposure to the test substance. However, all animals survived until the end of the test. After treatment with NDI no biologically relevant increases were found in the prepared liver cells after evaluation of both sacrifice times (4 and 16 hours), neither concerning nuclear net grains nor concerning cells in repair. Based on these results, NDI was evaluated as negative in the in vivo liver UDS Assay with male rats after single inhalation exposure for 3 hours.

The clastogenic potential of NDI was evaluated in a chromosome aberration test on Chinese hamster V79 cells in the presence and absence of S9 mix according to OECD TG 473 (Nern, 2006). Cultures treated with the test substance in the absence and in the presence of S9 mix showed biologically relevant and statistically significant increased numbers of aberrant metaphases, starting in the absence of S9 mix at 18 µg/ml and in the presence of S9 mix at 12 µg/ml. Based on this test, the test substance is considered to be clastogenic for mammalian cells in vitro.

In order to clarify the relevance of the positive findings in the in vitro chromosome aberration test, an in vivo micronucleus test (MNT) according to OECD TG 474 was performed. This approach is in line with ECHA Endpoint Specific Guidance (R.7A) and the generally agreed principle that in vivo follow up testing should be done in case of positive results from in vitro genotoxicity assays. Ad hoc clarification was needed for a science based solid classification globally. Inhalation was selected as the relevant route of exposure. In this test male mice were once nose-only exposed for 6 hours to NDI aerosol concentrations of 5, 25, 50 and 70 mg/m³ (Herbold, 2009). There was no altered ratio between polychromatic and normochromatic erythrocytes, but at the maximum tolerated concentration of 70 mg/m3 animals showed symptoms of toxicity (decreased body weights, bradypnea, labored breathing patterns, breathing sounds, reduced motility, tremor, gait high-legged, gait staggering, piloerection, stridor and hypothermia). These symptoms demonstrate relevant exposure to the test substance. After inhalative treatment for 6 hours no biologically relevant indications of a clastogenic effect of NDI were found in concentrations up to and including the maximum tolerated concentration of 70 mg/m³. Thus, NDI was concluded to be negative in the mouse micronucleus assay.

In summary, NDI showed no mutagenic properties in two independently performed tests with multiple strains of S. typhimurium and one E. coli strain but a positive results from the in vitro HPRT test. No mutagenic potential was observed in the rat liver UDS test after single 3h-exposure to toxic NDI aerosol concentrations of up to 150 mg/m3. Therefore, the equivocal results from the in vitro gene mutation assay, especially the positive HPRT test could not be confirmed in vivo. No clastogenic potential was observed in the mouse micronucleus test after single 6h-exposure to toxic NDI aerosol concentrations of up to 70 mg/m3. Therefore, the positive results from the in vitro chromosome aberration assay could not be confirmed in vivo in up to clearly toxic concentrations. Using the weight of evidence, it is assumed that in vivo genotoxicity is not of concern for NDI.


Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

Genetic toxicity (in vitro and in vivo)

Not classified under Annex I of Directive 67/548/EEC. According to Annex I of Regulation (EC) No 1272/2008 no classification is required for genetic toxicity.