Assessment of chronic toxicity and carcinogenicity in rats of Wingstay®100, a rubber antioxidant/antiozonant

Summary The chronic toxicity and carcinogenicity of Wingstay® 100 (W 100), a rubber antioxidant/antiozonant, were studied in Fischer 344 (F 344) rats in two chronic studies. Earlier genetic studies indicated that the product had weak activity in a bacterial mutation assay, but lacked activity in chromosomal aberration assays. In an one year study, both genders of F 344 rats were exposed to 53, 310 or 1900 ppm in NIH-07 diet for 52 weeks, and sacrifices were made at 38, 52 and 64 weeks. No test substance related deaths occurred, although the high dose of 1900 ppm caused a decrease in body weight gain and food consumption in both genders. Red blood cell mean corpuscular volume was significantly increased at 38 weeks, accompanied by a significant decrease in mean corpuscular hemoglobin concentration. At 52 weeks, the red blood cell count and hemoglobin values were also significantly decreased in high dose animals of both genders. Total bilirubin and cholesterol were increased in high dose animals of 38 and 52-week sacrifices. During the 3 month recovery, hematology parameters, bilirubin and cholesterol returned to control values. Total protein was reduced in high dose animals of both genders, throughout the entire exposure and recovery periods. W100 also produced increases in relative liver, spleen, heart and kidney weights in high dose animals. Both genders of all W 100 groups exhibited significant increases in urothelial cell proliferation (measured by PCNA) and adaptive hyperplasia. No regenerative hyperplasia, preneoplasia, or neoplasia were present. There was microscopic evidence of extramedullary erythropoiesis in the spleen and liver of high dose animals in both genders, otherwise no other pertinent microscopic finding was evident. In parallel, an accelerated bioassay (ABA) was conducted, which is a mechanistic initiation/promotion carcinogenicity study designed to assess tumor induction and promotion potential of a test substance in major organs of carcinogenesis. The present study was conducted in male F 344 rats for 38 weeks. The target sites chosen for the ABA were liver and urinary bladder and the dose for W 100 was 1900 ppm previously established to be a toxic dose. The liver tumor initiator was diethylnitrosamine (DEN), and the urinary bladder initiator was N-butyl N-(4-hydroxybutyl) nitrosamine (BBN). The initiators were administered during the first 14 weeks followed by the promoters. The promoters, phenobarbital (PB) for the liver and nitrilotriacetate (NTA) for the urinary bladder, were administered during the last 24 weeks of the study after the test substance. The study had 11 test groups including a negative control. The critical comparisons for initiating activity were conducted between groups 3 (PB) and 6 (W 100 + PB) for the liver and groups 8 (NTA) and 11 (W 100 + NTA) for the urinary bladder. The critical comparisons for promoting activity were conducted between groups 2 (DEN) and 5 (DEN + W 100) for the liver and groups 7 (BBN) and 10 (BBN + W 100) for the urinary bladder. There were 26 and 38-week sacrifices. In this study, most body weight reductions were due to DEN. At 26 weeks, significant increases in liver weights were present in all PB-exposed groups. Significant increases in renal weights occurred in all NTA, BBN and DEN groups. A similar organ weight pattern was present at 38 weeks. At 26 weeks, there were hepatocellular (33 %) and urothelial (67 %) tumors present in positive control groups (DEN/DEN + PB/BBN/BBN + NTA). In contrast, in the DEN + W 100 (5) and the BBN + W 100 (10) groups no tumors were present indicating absence of promotion. In addition, no tumors were present in groups 6 (W 100 + PB) or 11 (W100 + NTA) indicating absence of initiation. At 38 weeks, the incidences of hepatocellular adenomas and carcinomas in positive control group (DEN) was 44 %. The incidence of urothelial adenomas and carcinomas was 67 % in group 7 (BBN). In contrast, groups 5 (DEN + W 100) or group 10 (BBN + W 100) had a significantly lower incidences of 25 % and 33 %, respectively. This also indicates absence of promotion. At 38 weeks, no tumors were present in groups 6 or 11 indicating absence of initiation. In addition, the mean number of neoplasms per animals was significantly reduced from 2.7 per animal in group 2 to 2 in group 5, and from 3.7 in group 7 to 1.5 in group 10. Also, the biggest diameter of carcinomas was reduced in groups 5 and 10 compared to groups 2 and 7. Finally, the incidence of animals with carcinoma of the urinary bladder was 25 % in group 7 and was only 4.2 % in group 10. In conclusion, under the conditions of these 2 experiments, W 100 caused a chronic macrocytic anemia, accompanied by body weight reduction, compensatory extra-medullary erythropoiesis (in liver and spleen), cardio-renal overload and disruption of bilirubin metabolism. These effects are postulated to lead to changes in the renal filtrate and luminal conditions in the urinary bladder, leading to development of adaptive hyperplasia throughout 64 weeks of observation. This hyperplasia did not give rise to either regenerative hyperplasia, preneoplasia or in situ neoplasia. Rather, W 100 administration was associated with the reduction in incidence, size and multiplicity of neoplasms, initiated by DEN in the liver and BBN in the urinary bladder. Furthermore, at the end of 15 months, there were significant reductions in the number of rats with neoplasms in the high dose W 100 group, at 64 weeks, compared to controls (from 100 % in controls to 67 % in W 100), and absence of multiple neoplasms in high dose W 100 rats, compared to 40 % incidence in controls. Accordingly, W 100 did not exhibit either initiating or promoting carcinogenic activites in the liver or urinary bladder of male rats. The combination of a lack of tumor formation in rats chronically exposed to W 100, with the absence of tumor initiating or promoting activities indicate that W 100 is noncarcinogenic under the test conditions.