Urinary tract calculi and thresholds in carcinogenesis.

Area of Science:

• Toxicology
• Carcinogenesis
• Comparative Physiology

Background:

• High-dose chemical administration in rodents can lead to urinary tract calculi (stones).
• These calculi cause urothelial damage, hyperplasia, and subsequent tumor formation.
• This mode of action is considered a threshold phenomenon, more prevalent in rodents than primates.

Purpose of the Study:

• To evaluate the relevance of rodent urinary tract tumors to human cancer risk assessment.
• To understand the dose-dependent nature of chemical-induced calculi and subsequent tumorigenesis.
• To identify key differences between rodents and humans affecting risk extrapolation.

Main Methods:

• Review of existing toxicological data on chemicals inducing urinary tract calculi in rodents.
• Comparative analysis of rodent and human urinary tract anatomy and physiology.
• Assessment of human exposure levels in relation to calculi-forming doses.

Main Results:

• Chemicals inducing tumors via calculi formation in rodents operate at high doses.
• Rodents exhibit greater susceptibility to calculi formation compared to humans.
• Human exposure levels for most relevant chemicals are below the threshold for calculi induction.

Conclusions:

• Tumorigenesis via urinary tract calculi in rodents is a high-dose phenomenon with limited human relevance.
• Physiological differences between rodents and humans reduce the applicability of rodent bioassay findings.
• Current human exposure levels for such chemicals do not present a carcinogenic hazard through this mechanism.