Toward a biologically based dose-response model for developmental toxicityof 5-fluorouracil in the rat: Acquisition of experimental data

Biologically based dose-response (BBDR) models represent an emerging approa ch to improving the current practice of human health-risk assessment. The c oncept of BBDR modeling is to incorporate mechanistic information about a c hemical that is relevant to the expression of its toxicity into descriptive mathematical terms, thereby providing a quantitative model that will enhan ce the ability for low-dose and cross-species extrapolation. Construction o f a BBDR model for developmental toxicity is particularly complicated by th e multitude of possible mechanisms. Thus, a few model assumptions were made . The current study illustrates the processes involved in selecting the rel evant information for BBDR modeling, using an established developmental tox icant, 5-fluorouracil (5-FU), as a prototypic example. The primary BBDR mod el for 5-FU is based on inhibition of thymidylate synthetase (TS) and resul tant changes in nucleotide pools, DNA synthesis, cell-cycle progression, an d somatic growth. A single subcutaneous injection of 5-FU at doses ranging from 1 to 40 mg/kg was given to pregnant Sprague-Dawley rats at gestational day 14; controls received saline. 5-FU was absorbed rapidly into the mater nal circulation, and AUC estimates were linear with administered doses. We found metabolites of 5-FU directly incorporated into embryonic nucleic acid s, although the levels of incorporation were low and lacked correlation wit h administered doses. On the other hand, 5-FU produced dose-dependent inhib ition of thymidylate synthetase in the whole embryo, and recovery from enzy me inhibition was also related to the administered dose. As a consequence o f TS inhibition, embryonic dTTP and dGTP were markedly reduced, while dCTP was profoundly elevated, perhaps due to feedback regulation of intracellula r nucleotide pools. The total contents of embryonic macromolecules (DNA and protein) were also reduced, most notably at the high doses. Correspondingl y, dose-related reductions of fetal weight were seen as early as GD 15, and these deficits persisted for the remainder of gestation. These detailed do se-response parameters involved in the expression of 5-FU developmental tox icity were incorporated into mathematical terms for BBDR modeling. Such qua ntitative models should be instrumental to the improvement of high-to-low d ose and cross-species extrapolation in health-risk assessment.