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.