Riddelliine is a naturally occurring pyrrolizidine alkaloid, a class of compounds occurring in rangeland plants of the genera Crotalaria, Amsinckia, and Senecio. Two-week and 13-week rodent toxicity studies of riddelliine were conducted because riddelliine can be a contaminant of foodstuffs, such as meat, grains, seeds, milk, herbal tea, and honey. In addition to histopathology, evaluations included clinical pathology and reproductive toxicity. In vitro genetic toxicity studies included assessments of mutagenicity in Salmonella typhimurium and of the induction of chromosomal aberrations and sister chromatid exchanges in Chinese hamster ovary cells. Riddelliine was also evaluated in vivo for the induction of micronuclei in mouse bone marrow and in peripheral blood and for the induction of S-phase synthesis and unscheduled DNA synthesis in the liver of rats and mice.
In the 2-week studies, groups of five male and five female F344/N rats and B6C3F1 mice were administered riddelliine in 0.1 M phosphate buffer by gavage at dose levels of 0, 0 . 33, 1.0, 3.3, 10, or 25 mg/kg body weight five times per week, for a total of 12 doses. Four of five male rats in the 25 mg/kg group died or were killed moribund before the end of the study. Mean body weight gains of male rats in the 10 and 25 mg/kg groups were depressed. No deaths or body weight effects were observed in female rats. Male rats had dose-related hemorrhagic centrilobular hepatic necrosis, hepatocytic karyomegaly and cytologic alterations, pulmonary hemorrhage and/or edema, splenic extramedullary hematopoiesis, and pancreatic edema. Female rats exhibited fewer and less severe lesions than identically treated male rats. Heart weights of treated male and female rats were lower than those of the controls.
No deaths or effects on body weight were observed in treated mice. Dose-related increases in absolute and relative liver weights and increased incidences of hepatic cytomegaly were the only treatment-related findings in male and female mice administered riddelliine.
In the 13-week studies, groups of 20 male and 20 female F344/N rats and B6C3FI mice were administered riddelliine in 0.1 M phosphate buffer by gavage five times per week for 13 weeks. Rats received 0, 0.1, 0.33, 1.0, 3.3, or 10 mg/kg and mice received 0, 0.33, 1.0, 3.3, 10, or 25 mg/kg. Ten animals from each dose group were killed after 13 weeks of treatment. The remaining 10 animals in each dose group were observed without further treatment for up to 14 weeks; five animals from each dose group were killed after 7 weeks of recovery, and the remaining five animals per dose group were killed at the end of the 14-week recovery period.
During the 13-week treatment period, 19 of 20 male rats in the high-dose group died; all others survived. Body weight gains were decreased with increasing dose at Week 13. During the 14-week recovery period, all male rats survived, but five high-dose females died. Mean body weight gains of dosed and control male rats were similar throughout the 14-week recovery period; the final mean body weights of the treated males approached the final mean body weight of the controls. Similarly, mean body weight gains among the treated female rats were similar to the control value at the end of the 14-week recovery period. However, the final mean body weight of female rats given 1.0 or 3.3 mg/kg remained lower than that of controls at the end of the 14-week recovery period.
In the 13-week study, the most significant treatment-related histopathologic lesions in rats occurred in the liver and included hepatocyte cytomegaly and karyomegaly, cytoplasmic vacuolization, centrilobular necrosis, mixed inflammatory cell infiltration, and bile duct hyperplasia. Vascular lesions in the kidneys and lungs were observed in most high-dose rats after 13 weeks of riddelliine administration. Additional lesions were found in the heart, spleen, kidneys, and pancreas at 13 weeks. At the end of the 14-week recovery period, hepatocyte karyomegaly, cytomegaly, and cytoplasmic vacuolization persisted. In addition, the incidence of bile duct hyperplasia was markedly increased in dosed female rats, and foci of cytologic alteration or hyperplastic hepatocytes were observed in dosed rats that were allowed to recover for up to 14 weeks.
Adenomas of the liver occurred in 2 of 10 females in the 10 mg/kg group at 13 weeks and in one of five females in this group after the 14-week recovery period; no adenomas were found in the livers of control females.
Serum activities of alkaline phosphatase in male rats and sorbitol dehydrogenase in female rats increased with increasing dose. Reticulocyte counts consistently increased and platelet counts consistently decreased with increasing dose in treated male and female rats. The clinical pathology findings were indicative of liver damage and erythrocyte and platelet sequestration.
In mice in the 13-week study, no deaths related to riddelliine treatment occurred. Body weight gains were depressed at the two highest dose levels (10 and 25 mg/kg); the depression in body weight persisted throughout the 14-week recovery period. Dose-related increases in erythrocyte counts in male mice and in reticulocyte counts in female mice were observed. Dose-related decreases in platelet counts were also observed in both males and females. Centrilobular cytomegaly in the liver was noted at 13 weeks in males and females administered 25 mg/kg riddelliine; this lesion persisted through the recovery period in females. At the end of the 14-week recovery period, bile duct hyperplasia was seen in the liver in high-dose female mice. Epithelial hyperplasia of the forestomach was noted in male and female mice in the 10 and 25 mg/kg groups after 13 weeks of treatment, but this lesion became less severe during the recovery period.
In male rats administered up to 3.3 mg/kg and in male mice administered up to 25 mg/kg for 13 weeks, riddelliine did not adversely affect any of the reproductive end points evaluated. In female rats given 10 mg/kg and in female mice given 25 mg/kg, the length of the estrous cycle was increased. However, no unequivocal adverse effects were noted on fertility, pup growth and survival, or weight gain of dams during pregnancy during the mating trial in rats, although mean body weights of dams given 0.1 or 1.0 mg/kg were significantly lower than the mean body weight of the controls throughout gestation and lactation. In contrast, riddelliine administered at a dose of 25 mg/kg was toxic to the dams in the mouse mating trial, resulting in lower body weights at the beginning of gestation and throughout lactation. Administration of 25 mg/kg riddelliine to mouse dams also affected fetal growth and survival; the average live litter size was significantly reduced, the number of pups born dead was increased, and the average pup weight was reduced throughout the 21-day postpartum period.
Riddelliine was mutagenic in Salmonella typhimurium strain TA100 with, but not without, S9 activation; results of mutagenicity testing were negative in strains TA97, TA98, and TA1535. Riddelliine induced sister chromatid exchanges in Chinese hamster ovary (CHO) cells with and without S9. Chromosomal aberrations were induced in CHO cells only in the presence of S9. The frequency of micronucleated erythrocytes in mouse peripheral blood samples was not elevated after 4 or 13 weeks of daily gavage treatments; however, a weakly positive response was noted in the peripheral blood and bone marrow of male mice administered a single, high dose of riddelliine by gavage.
Unscheduled DNA synthesis was detected in cultured hepatocytes from male and female rats and mice following 5 or 30 days of riddelliine treatment by gavage. In addition, an increase in S-phase DNA synthesis was observed in cultured hepatocytes of male and female rats treated for either time period.
In summary, the administration of riddelliine to rodents by gavage for up to 13 weeks resulted in a spectrum of neoplastic and nonneoplastic effects similar to those previously described for other pyrrolizidine alkaloids. Rats were found to be somewhat more sensitive than mice, and males more sensitive than females, to the toxic effects of riddelliine. The no-observed-adverse-effect level (NOAEL) for histopathologic changes in the 13-week studies was 3.3 mg/kg body weight for mice and 0.1 mg/kg body weight for rats. The liver was the primary target of riddelliine-induced injury that resulted in lesions characterized by cytomegaly and cytologic alteration in rats and mice and also by marked necrotic and proliferative changes in rats. Riddelliine is carcinogenic to female F344/N rats, based on the occurrence of hepatocellular adenomas.