Senna is used as a stimulant laxative in the management of constipation resulting from opioid use or when treatment with bulking or osmotic agents has failed. Increased use of senna was expected due to the removal of the stimulant laxatives danthron and phenolphthalein from the market. Senna was nominated for study by the Center for Drug Evaluation and Research, United States Food and Drug Administration (FDA) due to the wide use of laxative preparations, positive genotoxicity in vitro for some senna components or metabolites, and unknown carcinogenic potential. Because a 2-year rat study was ongoing by the manufacturer, the FDA requested that the NTP conduct a senna study in the p53+/− mouse. In this study, the potential for carcinogenic effects of senna was studied in the C3B6.129F1/Tac-Trp53tm1Brd N12 haploinsufficient (heterozygous F1 p53+/−) mouse model as an ongoing goal of the NTP to develop and test model systems for toxicology and carcinogenesis studies, especially those that can provide mechanistic information relative to understanding an agent’s mode of action. C57BL/6NTac mice were exposed to senna in feed for 5 weeks; heterozygous F1 p53+/− mice were exposed to senna in feed for 40 weeks. Genetic toxicology studies were conducted in Salmonella typhimurium, Escherichia coli, and mouse peripheral blood erythrocytes.
5-Week Study in C57BL/6NTAC Mice
Groups of five male and five female mice were exposed to 0, 625, 1,250, 2,500, 5,000, or 10,000 ppm senna (equivalent to average daily doses of approximately 115, 245, 490, 975, or 2,075 mg senna/kg body weight to males and 160, 310, 625, 1,190, or 2,570 mg/kg to females) in feed for 5 weeks. All mice survived to the end of the study. Mean body weights of exposed groups were similar to those of the controls. No differences in feed consumption were noted between exposed and control groups. Significantly increased incidences of epithelial hyperplasia of the cecum occurred in males exposed to 10,000 ppm and females exposed to 5,000 or 10,000 ppm; significantly increased incidences of epithelial hyperplasia of the colon occurred in males and females exposed to 5,000 or 10,000 ppm.
40-Week Study in Heterozygous F1 P53+/− Mice
Groups of 25 male and 25 female mice were exposed to 0, 100, 300, 1,000, 3,000, or 10,000 ppm senna (equivalent to average daily doses of approximately 12, 36, 120, 365, or 1,260 mg/kg to males and 14, 42, 140, 435, or 1,520 mg/kg to females) in feed for 40 weeks. Mean body weights of exposed male and female mice were within 10% of those of the controls throughout the study. Feed consumption by exposed mice was generally similar to that by the controls.
Significant increases in the incidences of epithelial hyperplasia of the colon and cecum occurred in 10,000 ppm males and females, and the incidence of epithelial hyperplasia of the colon was significantly increased in 3,000 ppm females.
Genetic Toxicology
Four different samples of senna, including three samples of the same lot that was used in the 40-week study were tested for mutagenicity in bacterial test systems. In two samples, no evidence of mutagenicity was seen in several strains of S. typhimurium and E. coli, with or without exogenous metabolic activation. In the other two samples, mutagenic activity was seen in S. typhimurium strains TA98 and TA100, with variable requirements for exogenous metabolic activation.
In addition to senna, the NTP also tested rhein (a component of senna) for mutagenicity in S. typhimurium strains TA98 and TA100; dose-related increases in mutant colonies were seen with both strains in the presence of rat or hamster liver S9, at lower concentrations than were required for the positive responses seen with senna samples. Another component of senna, chrysophanic acid, was tested for mutagenicity in TA98, TA100, and TA1535; weak and inconsistent responses were seen in TA100 with rat and hamster liver S9. Sennosides A and B were also tested for mutagenicity in bacterial test systems; neither compound was mutagenic, with or without S9 metabolic activation.
In vivo, no increases in the frequencies of micronucleated erythrocytes were seen in male mice exposed for 40 weeks to senna via dosed feed. No significant changes in the percentage of reticulocytes among erythrocytes were observed in male mice, suggesting that exposure to senna did not induce bone marrow toxicity.
Conclusions
Under the conditions of this 40-week feed study, there was no evidence of carcinogenic activity of senna in male or female C3B6.129F1/Tac-Trp53tm1Brd N12 haploinsufficient mice exposed to 100, 300, 1,000, 3,000, or 10,000 ppm.
Senna induced epithelial hyperplasia of the large intestine (colon and cecum) in male and female mice.