A 28-day whole-body inhalation study to evaluate octamethylcyclotetrasiloxane (D4) absorption/distribution in two rat strains.

To investigate the potential toxicity of Octamethylcyclotetrasiloxane (D4), studies in laboratory rats have used primarily one of two strains, Sprague-Dawley (SD) and Fischer-344 (F-344). Reproductive studies used SD rats whereas F-344 rats were used in D4 pharmacokinetics, metabolism, acute/subacute/chronic toxicity and oncogenicity studies. Here, we assessed specific endpoints related to D4 pharmacokinetics and biochemistry in SD and F-344 rats within a single study, which allows for direct comparisons between strain and sex. This assessment included determination of microsomal total P450, NADPH-cytochrome c reductase, epoxide hydrolase, CYP2B1/2, CYP1A1/2, CYP3A1/2, CYP2C11, and CYP2A1. Aside from slight brown pigment in the liver, the treated animals experienced no toxicologically significant weight loss, decrease in food consumption, or clinical signs. Concentrations of D4 in plasma and fat were generally greater in females relative to males in both strains. SD females appeared to have statistically significantly greater plasma and fat concentrations following 28 days of repeated exposure to D4 relative to F-344 rats, suggesting the existence of potential sex and strain differences in D4 pharmacokinetics. The effect of D4 exposure on liver enzyme expression was similar among and between sexes and strain and was consistent with that for phenobarbital-like inducers. Notable differences included a finding of elevated CYP2B1/2 protein levels without a similar magnitude of increase in CYP2B/1 activity and a greater degree of CYP3A1/2 induction (protein and activity) for female SD rats. The importance of these findings is unclear, however reduced CYP2B1/2 activity may give rise to lower rates of D4 metabolism and clearance, consistent with the higher tissue levels of D4 in SD relative to F-344 female rats.

Effects of octamethylcyclotetrasiloxane (D4) on the luteinizing hormone (LH) surge and levels of various reproductive hormones in female Sprague-Dawley rats.

The objectives of this study were to assess the potential for D(4) to suppress the pre-ovulatory lutenizing hormone (LH) surge, to block or delay ovulation, and to evaluate potential effects on reproductive hormones in rats. Female Sprague-Dawley Crl:CD (SD) IGS BR rats received whole-body vapor inhalation exposure to D(4) (0, 700, or 900ppm) 6h per day for 3 days. Trunk blood obtained on proestrus at 10a.m. was evaluated for levels of follicle stimulating hormone (FSH), estradiol (E2), estrone (E1), and progesterone (P4). Other rats had serial blood samples collected via cannula at 2, 4, 6, 8, and 10p.m. on the day of proestrus and plasma evaluated for LH and prolactin (PRL). Trunk blood was collected at 8a.m. of estrus and plasma evaluated for FSH, E2, E1, and P4. At 10a.m. on proestrus, significant increases in E1 levels in the 700 and 900ppm groups and significant increases in P4 levels in the 900ppm group were noted. At 8a.m. on estrus, significant increases in E1, E2, in the E1/E2 ratio and decreases in FSH were noted in the 700 and 900ppm groups. The major effect on the LH profile was observed most clearly when the rats were grouped by ovulatory status, animals that did or did not ovulate. Regardless of treatment, suppression of the LH surge correlated with blocked ovulation. The percentage of rats that ovulated was (700ppm, 42%; 900ppm, 31%) compared to controls (79%). Overall, the data indicate that high exposures to D(4) attenuated the pre-ovulatory LH surge and significantly decreased the portion of female rats that ovulated.

A two-generation reproductive toxicity study of octamethylcyclotetrasiloxane (D4) in rats exposed by whole-body vapor inhalation.

This study evaluated the potential toxicity of whole-body vapor inhalation of octamethylcyclotetrasiloxane (D(4)) on reproductive capabilities in exposed F(0) and F(1) parental animals and the potential effects on neonatal survival, growth, and development of the F(1) and F(2) offspring. F(0) male and female Sprague-Dawley rats (30/sex/group) were exposed to D(4) vapor at concentrations of 0, 70, 300, 500 or 700 ppm 6h per day for at least 70 consecutive days prior to mating and lasted through weaning of the pups on postnatal day (PND) 21. Female exposures were suspended from gestation day (GD) 21 through PND 4 to allow for parturition and permit continuous maternal care for the early neonates. Starting on PND 22, F(1) weanlings were exposed to D(4) as described for the F(0) generation. The F(2) pups were not directly exposed to D(4). F(0) animals were mated once to produce the F(1) generation; F(1) parental animals were mated twice to produce two F(2) litters. In addition, the F(1) males were mated with unexposed females. Prolonged estrous cycles, decreased mating and fertility indices were observed in the F(1) generation exposed to D(4) for the first and second matings. Significant reductions in the mean number of pups born and mean live litter size were observed in the 500 and 700 ppm groups for both the F(0) and F(1) generations. Implantation sites were also reduced at 700 ppm for both F(0) and F(1) generations. No adverse effects were observed at any exposure level on anogenital distance, vaginal patency and preputial separation. No adverse effects were seen on male functional reproductive parameters, spermatogenic endpoints, microscopic evaluation of male reproductive tissue, or when the D(4)-exposed F(1) males were mated with the unexposed females, demonstrating that the reproductive toxicity observed was due to D(4) exposure to the females. Based on the lack of effect on reproduction when the D(4)-exposed males were mated to näive females, the NOAEL for male reproductive toxicity was considered to be 700 ppm. Based on the statistically significant effects on fertility and litter size, NOAEL for female reproductive toxicity was considered to be 300 ppm. The findings observed in this study are consistent with suppression or delaying of LH surge as well as acceleration of the onset of female reproductive senescence in the rat. While analogous pathways control ovulation in both rats and humans, there are significant differences in the mechanism for timing and release of LH and resulting changes in the control of ovulation and mating behavior between the two species. If D(4) delays rather than causes a prolonged suppression or ablation of the LH surge, the reproductive mode of action of D(4) would not likely be relevant for humans.

An inhalation reproductive toxicity study of octamethylcyclotetrasiloxane (D4) in female rats using multiple and single day exposure regimens.

Octamethylcyclotetrasiloxane (D(4)) has been shown to have effects on the female rat reproductive cycle. This study evaluated the phase of the female rat reproductive cycle affected by D(4) using a study design that allowed the complete female reproductive cycle, as well as phases of the cycle, from pre-mating through gestation, to be evaluated. Rats were exposed via whole body vapor inhalation up to 700 ppm D(4) during the overall phase (28 days prior to mating through gestation day (GD) 19), the ovarian phase (31-3 days prior to mating), the fertilization phase (3 days prior to the start of mating through gestation day 3), and the implantation phase (GD 2-GD 5) of the reproductive cycle. D(4) was associated with decreases in implantation sites and litter size in the overall and fertilization phases, but not in the ovarian or implantation phases. In order to further define the sensitive period for D(4) exposure, additional groups of rats were exposed on single days. A single 6h exposure to D(4) on the day prior to mating resulted in a significant reduction in fertility. These data indicate that there is a very narrow window, around the time of ovulation and fertilization, for D(4) to exert effects on the reproductive cycle of the female rat. Subsequent research, reported elsewhere, has elucidated the mode of action and assessed its potential relevance to humans.

A two-generation reproductive toxicity study of decamethylcyclopentasiloxane (D5) in rats exposed by whole-body vapor inhalation.

This two-generation reproduction study assessed the reproductive hazard potential of decamethylcyclopentasiloxane (D(5)). Sprague-Dawley rats (30/sex/group) were exposed by whole-body vapor inhalation to a target concentration of 30, 70, or 160 ppm D(5) or filtered air for 6h/day. Exposures for the F(0) and F(1) generations started at least 70 days prior to mating and lasted through weaning of the respective pups on postnatal day (PND) 21. Female exposures were interrupted from gestation day (GD) 21 through PND 4 to allow for parturition and to permit continuous maternal care for the early neonates. F(2) pups were not directly exposed to D(5). There were no exposure-related mortalities, clinical signs of toxicity, or effects on body weight or food consumption. There were no treatment-related gross findings or organ weight effects at the F(0) and F(1) necropsies. Other than minimal alveolar histiocytosis in all exposed groups, there were no noteworthy microscopic findings. Reproductive parameters (number of days between pairing and mating, mating and fertility indices, gestation length, and parturition), spermatogenic parameters and ovarian primordial follicle counts and numbers of corpora lutea in the F(0) and F(1) parental animals were not significantly changed between treated and control groups. Mean live litter sizes, number of pups born, sex ratios, pup body weights, postnatal pup survival and general physical condition of offspring in each generation were not affected. The slight, but statistically significant, increase in the mean F(1) male pup AGD in the 160 ppm group was not considered to be related to treatment. Vaginal patency and balanopreputial separation were unchanged compared to controls. Thus, the No-Observed-Adverse-Effect-Level (NOAEL) for parental and reproductive toxicity was determined to be 160 ppm D(5).

Chlorosilane acute inhalation toxicity and development of an LC50 prediction model.

The acute inhalation toxicity of 10 chlorosilanes was investigated in Fischer 344 rats using a 1-h whole-body vapor inhalation exposure and a 14-day recovery period. The median lethal concentration (LC50(1)) for each material was calculated from the nominal exposure concentrations and mortality. Experimentally derived LC50(1) values for monochlorosilanes (4257-4478 ppm) were greater than those for dichlorosilanes (1785-2092 ppm), which were greater than those for trichlorosilanes (1257-1611 ppm). Apparent was a strong structure-activity relationship (r2 = .97) between chlorine content and LC50(1) value. Estimated LC50(1) values for mono-, di-, and trichlorosilanes were determined to be 3262, 1639, and 1066 ppm, respectively, utilizing this relationship and the lower limit of the 95% prediction interval. The LC50(1) values determined in this series of studies were greater than that reported for hydrogen chloride (3124 ppm), when expressed on a chlorine equivalence basis (3570-5248 ppm), demonstrating that the acute toxicity of these chlorosilanes is similar to or less than that for hydrogen chloride. The good correlation between chlorine content and LC50(1) provides a sound basis for estimation of LC50(1) for chlorosilanes not already evaluated. The use of structure-activity relationships is consistent with the chemical industry and federal agency initiatives to reduce, refine, and/or replace the use of animals in testing without compromising the quality of health and safety assessments.

Inhalation toxicology of octamethylcyclotetrasiloxane (D4) following a 3-month nose-only exposure in Fischer 344 rats.

Octamethylcyclotetrasiloxane (D4) is a low-molecular-weight cyclic siloxane used primarily in the synthesis of silicone polymers. The objective of the present study was to evaluate the subchronic toxicity of D4 following a 3-month nose-only inhalation exposure. Male and female Fischer 344 rats (20/sex/group) were exposed 6 h/day, 5 days/week for 3 months to vapor concentrations of 0, 35, 122, 488, and 898 ppm D4. Also, an additional 10 per sex in the control and high-exposure groups were allowed a 4-week recovery period to observe reversibility, persistence, or delayed occurrence of any potential adverse effects. Body weights and food consumption were monitored at least twice weekly over the course of exposures. Approximately 18 hours preceding euthanasia, animals were transferred into metabolism cages for urine collection, and were fasted. At necropsy, 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. A concentration-dependent increase in absolute and relative liver weight (488 to 898 ppm) and a significant decrease in ovarian weight (898 ppm) were observed in female rats. Exposure to D4 via nose-only inhalation (35 to 898 ppm) produced minor alterations in hematological and serum chemistry parameters that were considered either incidental and of little toxicological significance (hematology) or suggestive of metabolic adaptation/alteration (serum chemistry) in response to exposure-related hepatomegaly. There were no histopathological findings noted in the liver. Histopathological evidence indicated the primary target organs following D4 inhalation exposure to be components of the female reproductive tract. Reversible histopathological changes were observed in the ovary (hypoactivity) and vagina (mucification) of female rats in the high-dose group only (898 ppm). Although an increase in the incidence and severity of both macrophage accumulation, interstitial inflammation, and eosinophil infiltration was observed in the lungs of male and female rats exposed to D4, the toxicological significance is uncertain as other inhalation studies at similar concentrations failed to show these effects. In summary, nose-only inhalation of a high concentration of D4 resulted in reversible histopathological changes in the female rat reproductive tract. Lower concentrations did not elicit these same effects.