Inhalation toxicology of decamethylcyclopentasiloxane (D5) following a 3-month nose-only exposure in Fischer 344 rats.

D5 is a low-molecular-weight cyclic siloxane used for industrial and consumer product applications. The objective of the present study was to evaluate the subchronic toxicity of D5 following a 3-month nose-only inhalation exposure. In addition, animals from both sexes of the control and high dose groups were allowed a 4-week recovery period to observe reversibility, persistence, or delayed occurrence of any potential adverse effects. Male and female Fischer 344 rats were exposed for 6 h/day, 5 days/week for 3 months to target concentrations of 0 (30/sex/group), 26 (20/sex/group), 46 (20/sex/group), 86 (20/sex/group), and 224 (30/sex/group) ppm D5. Recovery groups (0 and 224 ppm) comprised 10 rats/sex/group. Body weights and food consumption were monitored at least twice weekly over the course of exposures. Approximately 16 h preceding euthanasia, animals were transferred into metabolism caging for urine collection and were fasted. Rats were anesthetized with pentobarbital and euthanized by exsanguination. Blood was collected for hematological and clinical biochemical analyses. Selected organ weights were measured and a complete set of tissues was taken for histopathological examination. There were several minor changes observed in clinical biochemistry parameters; the most notable was an increase in gamma glutamyl transferase (gamma-GT) in both sexes at the high dose. In females, this effect was dose-related (46-224 ppm) and did not recover upon cessation of exposure. Additionally, there was an decrease in serum lactate dehydrogenase (LDH) observed in females at 86 and 224 ppm which was not resolved during recovery. There was an increase in absolute and/or relative liver weight in rats of both sexes. Taken together, these data suggest that the female rat is more sensitive to the actions of D5 on the liver. Exposure-related increases in absolute and relative lung weights were observed in both sexes at terminal necropsy. This observation was not noted in males in the recovery phase, but was still present in females. Finally, histopathological evidence indicated the primary target organ following D5 inhalation exposure is the lung, with an increase in focal macrophage accumulation and interstitial inflammation in the lungs of male and female rats exposed to 224 ppm D5. This observation did not appear to resolve at the end of a 1-month period of nonexposure. The incidence of these changes was also slightly increased in rats of both sexes exposed to 86 ppm D5. These data suggest that nose-only D5 vapor inhalation provokes minimal changes in the lung which are similar in incidence and severity to spontaneously occurring changes in control animals after nose-only exposures. There were no histopathological findings noted in the livers which support this organ as a target in this study, despite the observed changes in organ weight and in some serum chemistry parameters.