Dose-response assessment of nephrotoxicity from a twenty-eight-day combined-exposure to melamine and cyanuric acid in F344 rats.

The adulteration of pet food with melamine and derivatives, including cyanuric acid, has been implicated in the kidney failure and death of cats and dogs in the USA and other countries. In a previous 7-day dietary study in F344 rats, we established a no-observed-adverse-effect level (NOAEL) for a co-exposure to melamine and cyanuric acid of 8.6 mg/kg bw/day of each compound, and a benchmark dose lower confidence limit (BMDL) of 8.4-10.9 mg/kg bw/day of each compound. To ascertain the role played by the duration of exposure, we treated F344 rats for 28 days. Groups of male and female rats were fed diet containing 0 (control), 30, 60, 120, 180, 240, or 360 ppm of both melamine and cyanuric acid. The lowest dose that produced histopathological alterations in the kidney was 120 ppm, versus 229 ppm in the 7-day study. Wet-mount analysis of kidney sections demonstrated the formation of melamine cyanurate spherulites in one male and two female rats at the 60 ppm dose and in one female rat at the 30 ppm dose, establishing a NOAEL of 2.1mg/kg bw/day for males and <2.6 mg/kg bw/day for females, and BMDL values as low as 1.6 mg/kg bw/day for both sexes. These data demonstrate that the length of exposure is an important component in the threshold of toxicity from a co-exposure to these compounds and suggest that the current risk assessments based on exposures to melamine alone may not reflect sufficiently the risk of a co-exposure to melamine and cyanuric acid.

Dose-response assessment of nephrotoxicity from a 7-day combined exposure to melamine and cyanuric acid in F344 rats.

The intentional adulteration of pet food with melamine and derivatives, including cyanuric acid, has been implicated in the kidney failure and death of a large number of cats and dogs in the United States. Although individually these compounds present low toxicity, coexposure can lead to the formation of melamine cyanurate crystals in the nephrons and eventual kidney failure. To determine the dose-response for nephrotoxicity upon coadministration of melamine and cyanuric acid, groups of male and female F344 rats (six animals per sex per group) were fed 0 (control), 7, 23, 69, 229, or 694 ppm of both melamine and cyanuric acid; 1388 ppm melamine; or 1388 ppm cyanuric acid in the diet for 7 days. No toxicity was observed in the rats exposed to the individual compounds, whereas anorexia and a statistically significant increase in blood urea nitrogen and serum creatinine levels was observed in the animals treated with 229 and 694 ppm melamine and cyanuric acid. The kidneys of these animals were grossly enlarged and pale yellow. Large numbers of crystalline structures deposited in the tubules were seen on sections in kidneys from all rats in these treatment groups. No significant changes were detected in the remaining treatment groups exposed to both melamine and cyanuric acid. In the melamine-only treatment group, 5 of 12 rats had scattered crystals present in renal tubules when examined by wet mount. These were not observed by histopathology. The observed adverse effect level (8.6 mg/kg bw [body weight]/day) and benchmark dose modeling data (8.4-10.9 mg/kg bw/day) determined in this study suggest that the tolerable daily intake values derived from studies conducted with melamine alone may underestimate the risk from coexposures to melamine and cyanuric acid.

Current developments in food additive toxicology in the USA.

A recently published proposal (Fed. Reg. 66 (2001) 4706) for mandatory submission of information on all plant-derived bioengineered foods fed to humans or animals will be reviewed. Under this proposal, information such as data on identity, level and function of the introduced substance(s); an estimate of dietary exposure; allergenic potential of the protein; data relevant to other safety issues that may be associated with the substance; selection of a comparable food; historic uses of comparable food; composition and characteristics of bioengineered food versus those of the comparable food should be provided. In addition, characterization of the parent plant; construction of the transformation vector and introduced genetic material along with number of insertion sites and genes; data on the genetic material and any newly inserted genes for antibiotic resistance should be submitted with the notification. The Interagency Coordinating Committee for Validation of Alternative Methods (ICCVAM) was identified by the U.S. Congress as the organization to review and validate new alternative toxicological test methods for 14 U.S. government agencies. Validated and accepted alternative toxicity tests will be incorporated into toxicity testing recommendations for regulatory agencies.