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BIOLOGICALLY-BASED DOSE-RESPONSE MODELING IN DEVELOPMENTAL TOXICOLOGY: BIOCHEMICAL AND CELLULAR SEQUELAE OF 5-FLUOROURACIL EXPOSURE IN THE DEVELOPING RAT
Citation:
Shuey, D., C. Lau, T. Logsdon, R. Zucker, K. Elstein, M. Narotsky, R. Setzer, R. Kavlock, AND J. Rogers. BIOLOGICALLY-BASED DOSE-RESPONSE MODELING IN DEVELOPMENTAL TOXICOLOGY: BIOCHEMICAL AND CELLULAR SEQUELAE OF 5-FLUOROURACIL EXPOSURE IN THE DEVELOPING RAT. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/J-95/090, 1994.
Description:
Mechanistically-based dose-response models for developmental toxicity require elucidation of biological events that intervene between maternal exposure and adverse developmental outcome. We examined some of the major events in the rat embryo following subcutaneous injection of 5-fluorouracil (5-FU) on day 14 of gestation. This treatment resulted in a dose-dependent reduction in fetal weight at doses of 20 mg/kg and higher, generalized reduced ossification above 25 mg/kg and wavy ribs at 30 mg/kg and higher. Numerous malformations including cleft palate and hindlimb defects were substantially increased at 35 and 40 mg/kg. 5-FU inhibits thymidylate synthetase (TS) resulting in inhibited growth of rapidly dividing tissues. To identify early events in the pathogenesis of hindlimb defects, we examined the effects of 5-FU on TS activity, cell cycle, growth and morphology in the developing hindlimb as a function of dose and time. The rate of decline of TS activity following 5-FU exposure was dose-related, though maximal inhibition and recovery were similar at doses within (20 and 40 mg/kg) and below (10 mg/kg) the range of detectable developmental toxicity. Flow cytometric analysis of nuclei from embryonic hindlimbs revealed a transient increase in the percentage of cells in S-phase and decrease in G0/G1 8 hr after maternal injection of 20-40 mg 5-FU/kg. Reduction in growth and morphometric changes were observed only after exposure to 40 mg/kg. The tissue-specificity of these effects was examined by comparing the hindlimb with other embryonic tissues. There was also a dose-related decline of TS activity in the embryonic liver. However, the pattern of recovery of TS activity, and cell cycle alterations were different in the liver than in the hindlimb, probably reflecting a higher proliferative rate as compared to the hindlimb. We have derived a quantitative, empirical model for induction of hindlimb defects based on TS inhibition and subsequent cellular events following 5-FU exposure. The model predicted a dose-response similar to that of the observed data although the predicted curve was shifted towards lower doses, suggesting additional mechanisms of toxicity and/or repair. Biologically-based dose-response modeling provides a framework for testing mechanistic hypotheses, and developing such models should ultimately improve our ability to perform risk assessments.