1,5-naphthylene diisocyanate

Endpoint summary

Key value for chemical safety assessment

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEC

0.25 mg/m³

Study duration:

subchronic

Species:

rat

Additional information

In a subchronic toxicity study according to OECD TG 413 (Pauluhn, 2010) 10 male and 10 female Wistar rats per dose group were nose-only exposed for 13 weeks (6 hours /day, 5 days/week) to a solid aerosol of 1,5-Naphthylene diisocyanate (NDI). The mean actual breathing zone concentrations (gravimetric) were 0.065, 0.25, 1.02 and 3.96 mg/m3. Rats exposed under otherwise identical test conditions served as concurrent control group. Additional 10 male and 10 female rats were exposed in the air control and high dose exposure groups and were allowed to recover during a 4-week post-exposure period.

The rats exposed up to and including 1.02 mg/m3 did not display any clinical effects attributable to the test substance. At 3.96 mg/m3 animals displayed responses, such as bradypnea, irregular and labored breathing patterns, dyspnea, breathing sounds, stridor, nasal discharge (serous), nostrils: red encrustations, eye lids: red encrustations, motility reduced, limp, high-legged gait, cyanosis, piloerection, haircoat ungroomed, and bloated abdomen. In regard to visible signs of respiratory tract irritation, female rats appeared to be slightly more susceptible than male rats. Conclusive changes in body temperature, reflexes, body weights or food/water consumption did not occur. Ophthalmology was unobtrusive. There was no evidence of adverse hematological effects and clinical-pathology did not provide pathodiagnostically relevant evidence of NDI-aerosol induced effects. However, an isolated increase in total bilirubin was observed at 3.96 mg/m3 (trend in males, and significant elevations in females). Urinalysis was unremarkable. There were no statistically significant or conclusive changes in absolute or relative organ weights of adrenals, brain, epididymides, kidneys, liver, ovaries, spleen, testes, and thymus. However, at 1.02 and 3.96 mg/m3 , lung weights were significantly increased. In addition, significantly increased heart weights occurred in female rats exposed at 3.96 mg/m3. The elevations in organ weights were not reversible with the 4-week post-exposure period. After 13-weeks, minimal to slight histopathological findings observed in the nasal cavities, pharynx, larynx, trachea and lungs of rats exposed at 1.02 and 3.96 mg/m3 . In the nasal cavities findings were characterized by epithelial degeneration and/or atrophy, goblet cell hyperplasia and increased inflammatory infiltrates. As can be appreciated from the degree of responses at the level I and IV of the nasal cavities, there is an apparent equal susceptibility of regions with respiratory epithelium and olfactory epithelium. This lack of specificity supports a non-specific irritant mechanism. Likewise, also laryngeal epithelium shows irritant-related changes ranging from epithelial alterations, focal inflammatory infiltrations to epithelial squamous metaplasia especially at the ventral aspect of the larynx. In the lungs, bronchiolo-alveolar hypercellularity, minimal septal thickening, inflammatory infiltrates and increased alveolar macrophages with foamy appearance occurred. In addition, females of the dose group 0.25 mg/m3 showed minimal focal inflammatory infiltrations and epithelial alterations in the larynx. After 4 weeks of recovery, in the nasal cavities minimal or slight degeneration and/or atrophy of the olfactory epithelium was still apparent in all rats from the high dose group 3.96 mg/m3. Additionally, unusual nerve-like structures were obvious in the epithelium and neuronal degeneration of nerve bundles was detected in the lamina propria. In the lungs, minimal hypercellularity of the bronchiolo-alveolar region, an increased amount of macrophages and an increased incidence of minimal inflammatory infiltrates was still seen. Histopathological findings of the pharynx, larynx, and trachea recovered entirely. Overall, there was a tendency that female rats were mildly more susceptible than male ones. No respiratory tract lesions were still apparent in male and female rats at 0.25 and 1.02 mg/m3 after the 4-week recovery period.

In summary, this study demonstrates that the test substance causes respiratory tract irritation. The localization of irritation depends on the site of initial deposition and associated local response. Based on portal-of-entry-related endpoints NDI was tolerated without any local or systemic adverse effects up to 0.25 mg/m3. Conclusive evidence of respiratory irritation occurred at 1.02 and 3.96 mg/m3. With regard to histopathological changes, all changes observed were related to portal-of-entry; local irritant effects (nasal passages, larynx, airways and alveoli), i.e, changes that occurred at anatomical structures, to some extent rat-specific, favoring focal deposition of irritant particulates. The minimal laryngeal findings observed in female rats at 0.25 mg/m3 are assessed to be non-adverse. According to the publication of Kaufmann et al. (2009)¹ minimal focal epithelial changes of the laryngeal epithelium are assessed as non-adverse since they may also occur in non-treated animals and were not considered to have a potential for a laryngeal dysfunction. Also, cases of minimal to slight laryngeal squamous metaplasia that are not observed diffusely could occur spontaneously or as treatment-induced lesions and should be assessed as non-adverse. Based on the rationale given, taking all findings into account, 0.25 mg/m3 constitutes a no-observed-adverse-effect-level (NOAEL) based on effects related to respiratory tract irritation.

Based on the results of the 2-week range finding study in rats (Pauluhn, 2008), that showed also minimal to slight histopathological effects of the respiratory tract at concentrations of 1.1 mg/m3 and above (NOAEL = 0.19 mg/m3) it can be concluded that the duration of exposure does not heavily influence the effect threshold. This observation is in line with irritation as the predominant mode of action.

¹ Kaufmann et al. (2009): 1st International ESTP Expert Workshop: Larynx squamous metaplasia - A re-consideration of morphology and diagnostic approaches in rodent studies and its relevance for human risk assessment. Experimental and Toxicologic Pathology 61: 591 -603

Repeated dose toxicity: inhalation - systemic effects (target organ) respiratory: larynx; respiratory: lung; respiratory: nose; respiratory: pharynx; respiratory: trachea

Justification for classification or non-classification

Repeated dose toxicity

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 repeated dose toxicity as (acute) irritation of the respiratory tract is the predominant effect even after repeated inhalation..