Subchronic inhalation toxicity study of caprolactam (with a 4-week recovery) in the rat via whole-body exposures.

Abstract

This study was designed to assess the potential subchronic inhalation toxicity of caprolactam when administered as a 3-micron aerosol from an aqueous solution to Sprague-Dawley CD rats (10/sex/group) via whole-body exposure. The study was enhanced with the inclusion of motor activity measurements and a functional observational battery to assess the neurotoxic potential of caprolactam. The rats were exposed at least 65 times over a 13-week period for 6 h per day, 5 days per week, to target concentrations (3 microns, mass median aerodynamic diameter) of 0, 25, 75, and 250 milligrams per cubic meter (mg/m3). An additional 10 animals/sex/group were similarly exposed and then held for a 4-week recovery period. Exposure levels were determined gravimetrically six times daily; one daily sample was analyzed by high-pressure liquid chromatography. No deaths were observed in the study during the exposure or recovery periods. Treatment-related responses such as labored breathing and nasal discharge were seen during many of the exposures. Similar responses as well as moist rales were seen during the nonexposure periods during the 13 weeks of exposure. However, these responses abated during the 4-week recovery period. There were no clearly treatment-related responses observed with ophthalmoscopic examinations, body weight measurements, food consumption measurements, neurobehavioral evaluations, clinical pathology evaluations, organ weight measurements, or macroscopic pathology examinations. Microscopic findings that were considered related to exposure to the test material were seen in the nasoturbinal tissues (hypertrophy/hyperplasia of goblet cells in the respiratory mucosa and intracytoplasmic eosinophilic material in epithelial cells of the olfactory mucosa) of the two higher-exposure group animals and in the laryngeal tissues (squamous/squamoid metaplasia/hyperplasia of the pseudostratified columnar epithelium covering the ventral seromucous gland) of all three exposure group animals. These changes were considered to be adaptive responses to an irritant (caprolactam). The keratinization of the metaplastic epithelium in the larynx was considered to be an adverse effect. By the end of the 4-week recovery period, there was complete regression of the keratinization in the larynx, but recovery of the adaptive nasoturbinal effects had not completely resolved. In conclusion, the whole-body exposure of Sprague-Dawley rats to caprolactam as a respirable aerosol for 6 h/day, 5 days/week, for 13 weeks at gravimetrically determined levels of 24, 70, and 243 mg/m3 resulted in respiratory tract effects (laryngeal) at the highest exposure level with complete recovery within 4 weeks postexposure. The results indicate that the no-observed-adverse-effect level for caprolactam is 70 mg/m3, based on upper respiratory effects, with 243 mg/m3 representing a no-observed-effect level for systemic toxicity, neurotoxicity, and lower respiratory tract effects.