Malachite green chloride is a triphenylmethane dye used in the fish industry as an antifungal agent. Leucomalachite green is formed by the reduction of malachite green chloride and persists in the tissues of exposed fish. These compounds were nominated for study by the United States Food and Drug Administration because of the potential for significant worker and consumer exposure, structural similarity to gentian violet, and lack of carcinogenicity data. After consideration of data obtained from a 28-day feeding study (NTP, 2004), female F344/N Nctr BR rats and B6C3F1/Nctr BR mice were exposed to malachite green chloride (approximately 87% pure) in feed for 2 years. Male and female F344/N Nctr BR rats and female B6C3F1/Nctr BR mice were exposed to leucomalachite green (99% pure) in feed for 2 years.
Two-year study in rats
Malachite Green Chloride
Groups of 48 female rats were fed diets containing 0, 100, 300, or 600 ppm malachite green chloride for 2 years (equivalent to average daily doses of approximately 0, 7, 21, and 43 mg malachite green chloride/kg body weight). Survival of exposed groups was similar to that of the control group. Mean body weights of the 300 and 600 ppm females were generally less than those of the controls. Feed consumption by exposed groups of rats was generally similar to that by the control group. The relative liver weight was significantly increased in the 600 ppm females.
Thyroid follicular cell adenomas and carcinomas occurred in female rats receiving 300 or 600 ppm malachite green chloride, and the combined incidence of adenomas and carcinomas exceeded the historical control range. A low incidence of cystic follicles was observed in the thyroid gland of exposed females. The increases were minimal and not statistically significant; however, due to the rarity of the tumor and related nonneoplastic changes, these increases may be biologically significant. Hepatocellular adenomas were minimally increased (not statistically significant) in the female rats exposed to 300 and 600 ppm malachite green and the incidence in all the exposed and control groups exceeded the historical control range. The incidence of eosinophilic foci in the liver was increased in the exposed groups of rats. The incidence of mononuclear cell leukemia decreased with an exposure-related trend in females; the incidences were significantly decreased in the 300 and 600 ppm females. Mammary gland carcinomas were increased (not statistically significant) in the female rats receiving 600 ppm and the incidence exceeded the historical control range. The reduction in body weight of the treated rats may have reduced the statistical power to demonstrate a more robust effect.
Leucomalachite Green
Groups of 48 male and female rats were fed diets containing 0, 91, 272, or 543 ppm leucomalachite green in feed for 2 years (equivalent to average daily doses of approximately 0, 5, 15, and 30 mg leucomalachite green/kg body weight to males and 0, 6, 17, and 35 mg/kg body weight to females). Survival of 272 ppm males was greater than that of the controls. Mean body weights of 543 ppm males and females and 272 ppm females were less than those of the controls throughout the study; mean body weights of 272 ppm males and 91 ppm females were less than those of the controls during year 2 of the study. Feed consumption by 543 ppm males and females was intermittently less than that by the controls throughout the study; feed consumption by 272 ppm females was intermittently less during year 2 of the study. Liver weights were significantly increased for 272 and 543 ppm males; relative liver weights were significantly increased for 272 and 543 ppm females. Relative thyroid gland weights of 543 ppm males and females were significantly increased.
Hepatocellular adenomas were minimally increased (not statistically significant) in female rats exposed to 91 or 543 ppm leucomalachite green, with the incidence exceeding the historical control range. Nonneoplastic liver lesions including eosinophilic focus, cystic degeneration, and cytoplasmic vacuolization were generally significantly increased in the exposed groups of male and female rats. Thyroid gland follicular cell adenomas or carcinomas (combined) and cysts were observed in exposed males and females. The increases were minimal and not statistically significant; however, the increases may be biologically significant due to the rarity of the neoplasm and related nonneoplastic changes. Testicular interstitial cell adenoma occurred with a positive trend in male rats, and the incidence was significantly increased in the 543 ppm group. The broad range of incidences in the historical control data and the uncertainty of the relationship between pituitary gland neoplasms and testicular adenomas may confound this result. The incidences of mononuclear cell leukemia were significantly decreased in exposed rats. The incidences of pituitary gland adenoma were significantly decreased in exposed male rats.
Two-year study in mice
Malachite Green Chloride
Groups of 48 female mice were fed diets containing 0, 100, 225, or 450 ppm malachite green chloride for 2 years (equivalent to average daily doses of approximately 0, 15, 33, and 67 mg malachite green chloride/kg body weight). Survival of exposed groups was similar to that of the controls. Mean body weights of exposed mice were generally similar to the control group throughout most of the study. Feed consumption by exposed groups was generally similar to that by the controls. Relative kidney weights of exposed groups of mice were generally less than those of the controls.
There were no increased incidences of neoplasms in exposed mice. Incidences of intracytoplasmic inclusions of the urinary bladder were significantly increased in exposed mice.
Leucomalachite Green
Groups of 48 female mice were fed diets containing 0, 91, 204, or 408 ppm leucomalachite green for 2 years (equivalent to average daily doses of approximately 0, 13, 31, and 63 mg leucomalachite green/kg body weight). Survival of exposed groups was similar to that of the controls. Mean body weights were generally similar to those of the controls. Feed consumption by exposed groups was generally similar to that by the controls. Relative kidney weights were significantly decreased in all dose groups.
The incidences of hepatocellular adenoma or carcinoma (combined) occurred with a positive trend and the incidence was significantly increased in 408 ppm mice. The incidences of hepatocellular adenoma were increased (not statistically significant) in exposed mice.
Conclusions
Under the conditions of these 2-year feed studies, there was equivocal evidence of carcinogenic activity of malachite green chloride in female F344/N rats based on the occurrence of thyroid gland follicular cell adenoma or carcinoma (combined) and marginal increases in hepatocellular adenoma and mammary gland carcinoma in exposed rats. There was no evidence of carcinogenic activity of malachite green chloride in female B6C3F1 mice exposed to 100, 225, or 450 ppm.
Under the conditions of these 2-year feed studies, there was equivocal evidence of carcinogenic activity of leucomalachite green in male F344/N rats based on an increase in interstitial cell adenoma of the testes and the occurrence of thyroid gland follicular cell adenoma or carcinoma (combined) in exposed rats. There was equivocal evidence of carcinogenic activity of leucomalachite green in female F344/N rats based on marginally increased incidences of hepatocellular adenoma and the occurrence of thyroid gland follicular cell adenoma or carcinoma (combined) in exposed rats. There was some evidence of carcinogenic activity of leucomalachite green in female B6C3F1 mice based on an increase in hepatocellular adenoma or carcinoma (combined).
Exposure to malachite green chloride in feed resulted in nonneoplastic lesions in the thyroid gland and liver of female rats and the urinary bladder of female mice. Exposure to leucomalachite green in feed resulted in nonneoplastic lesions in the thyroid gland and liver of male and female rats and the urinary bladder of female mice.