Biologically based dose-response (BBDR) models comprise one way to incorporate mechanistic information into a dose-response assessment to be used for risk assessments. The chemotherapeutic drug 5-fluorouracil (5-FU) has been used as a prototypic compound for the construction of a BBDR model for developmental toxicity. Previous work has provided data and a general mechanistic framework for the developmental toxicity of 5-FU when it was administered to pregnant rats subcutaneously on gestation day 14. In this paper, a mathematical model relating maternally administered treatment with 5-FU to embryonal thymidylate synthetase inhibition and thymidylate synthetase inhibition to various measures of deoxyribonucleotide triphosphate (dNTP) pool perturbation is developed, and parameters are estimated using the data collected. The strategy used was to develop semi-empirical submodels for each of the intervening steps, and to estimate model parameters from previously described data. The models developed predict that there is no practical threshold for dNTP pool perturbation; that is, even minimal doses of 5-FU should result in some perturbation of dNTP pools. In particular, the relationship between dNTP pool perturbation and fetal weight deficit suggests that if there is a biological threshold for the effect of 5-FU on fetal weight, the responsible repair or compensation mechanism must be downstream of dNTP pool perturbation, and saturable at 5-FU doses lower than 10 mg/kg (the lowest dose examined for developmental effects in these studies).

Yolk-sacs from untreated Sprague-Dawley rat conceptuses were removed on gestational days (GD) 9 to 18 and examined for gamma-glutamyl transferase (GGT), alkaline phosphatase (AP), lactate dehydrogenase (LDH), and glutamic-oxaloacetic transaminase (GOT) activities. All enzyme activities were found to increase through gestation in total activity as well as in specific activity. Protein (PRO) and urea nitrogen (UN) were also found to increase through gestation whereas triglyceride (TRI) increased steadily from GD 9 to 13 and then appeared to plateau through GD 19. Additional rats were treated on GD 8 with 75 mg trypan blue (TB)/kg body weight and yolk-sacs taken on GD 9 to 18. Yolk-sac GGT and GOT activities from TB-treated rats were significantly higher than the respective controls during early gestation but recovered to or were lower than control levels during midgestation. GGT activity in treated rats was significantly higher than the controls on GD 15 and 16, and both GGT and GOT were significantly lower than controls on GD 17 and 18. AP activity in the TB-treated yolk-sacs was significantly lower than that in controls during early and midgestation but was not significantly different from the control values late in gestation. Triglyceride concentration was not affected early in gestation but significantly decreased on GD 16 and 18. Thus, the yolk-sac enzymes monitored, which are associated with nutrition and normal growth, increased in activity through GD 18. The yolk-sac toxicant, trypan blue, significantly affected enzymatic activity at various time periods during gestation and resulted in significant changes in yolk-sac protein and triglyceride content.

Mechanistically based dose-response models for developmental toxicity require elucidation of critical biological events that intervene between maternal exposure and adverse developmental outcome. We have examined some of the major events in the rat embryo/fetus following a subcutaneous injection of 5-fluorouracil (5-FU; 0-40 mg/kg) to the dam on Day 14 of gestation. This treatment resulted in reduced fetal weight that was significant at doses of 20 mg/kg and higher, generalized reduced ossification at doses above 25 mg/kg, and wavy ribs at doses of 30 mg/kg and higher. Numerous malformations including cleft palate and hindlimb defects were substantially increased at doses of 35 and 40 mg/kg. 5-FU inhibits thymidylate synthetase (TS), resulting in inhibited growth of rapidly proliferating 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, although 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 phase 8 hr after maternal injection of 20-40 mg 5-FU/kg, but not at lower doses. Reduction in growth and morphometric changes of hindlimbs were observed only after maternal 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 the higher cell proliferative rate in the liver at this stage. 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 toward lower doses. These results suggest that while this model may not capture all of the critical events involved in the induction of hindlimb defects following maternal exposure to 5-FU, it does reflect a central mechanism of its developmental toxicity. 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.

Methanol has widespread applications in industry and manufacturing and is under consideration as an alternative automotive fuel. Human exposure to methanol would be expected to increase if applications expand in coming years. Methanol has been shown to be a reproductive and developmental toxicant in the rodent, producing cleft palate in the CD-1 mouse. Developmental toxicity has also been demonstrated in vitro for rat and mouse embryos in whole embryo culture. The present study examines the developmental toxicity of methanol in the palate using a serum-free organ culture model. Gestation day 12 CD-1 mouse embryos were dissected and mid-craniofacial tissues were cultured in BGJ medium at 37 degrees C for 4 days with medium changes at 24 hr intervals. Cultures were exposed to methanol from 0-20 mg/ml for 6 hr, 12 hr, 1 or 4 days. Some cultures were exposed to ethanol for 4 days at doses ranging from 0-15 mg/ml. All cultures were gassed with a 50% O2, 5% CO2, and 45% N2 upon addition of fresh medium and prior to the addition of alcohol. Following organ culture the craniofacial explants were examined for effects on morphology, fusion, proliferation, and growth. Incidence and completeness of palatal fusion decreased with increasing exposure. Depending on the concentration and duration of methanol exposure, the medial epithelium either degenerated completely or remained intact in unfused palates and either condition would interfere with fusion. Cellular proliferation appeared to be a specific and sensitive target for methanol as craniofacial tissues responded to methanol with reduction in total DNA content at an exposure that did not affect total protein. However, both DNA and protein decreased with increasing exposure to methanol. Incorporation of thymidine decreased significantly after 4 day exposure and autoradiography of 3H-thymidine (TdR) demonstrated exposure-dependent reduction in proliferation of palatal mesenchymal cells. Ethanol decreased fusion score, total protein, and DNA, but 3H-TdR/DNA was not significantly changed. In general the ethanol was more potent than methanol for inhibition of protein and DNA synthesis and palatal fusion. This study demonstrated that methanol can selectively affect specific sensitive cell populations and has effects on proliferation and cell fate.

The developmental toxicity of the alternative motor vehicle fuel methanol was assessed in mice by the inhalation route. Pregnant CD-1 mice were exposed to 1,000, 2,000, 5,000, 7,500, 10,000, or 15,000 ppm methanol for 7 hr/day on days 6-15 of gestation. Sham-exposed controls were exposed to filtered air under similar conditions. Additional control groups were left in their home cages either unhandled or food-deprived for 7 hr/day to match the food deprivation experienced by the exposed mice. Dams were observed twice daily and weighed on alternate days during the exposure period. Blood methanol concentrations were determined in some mice on gestation days 6, 10, and 15. On day 17, the remaining mice were weighed and killed and the gravid uteri removed. Implantation sites, live and dead fetuses and resorptions were counted, fetuses were examined externally and weighed as a litter. Half of each litter was examined for skeletal morphology and the other half of each litter was examined for internal soft tissue anomalies. One dam died in each of the 7,500, 10,000, and 15,000 ppm methanol exposure groups, but no dose-response relationship was evident for maternal death. The sham-exposed and food-deprived controls as well as all methanol exposed dams gained less weight than did unexposed dams fed ad libitum, but methanol did not exacerbate this effect. Significant increases in the incidence of exencephaly and cleft palate were observed at 5,000 ppm and above, increased embryo/fetal death at 7,500 ppm and above (including an increasing incidence of full-litter resorptions), and reduced fetal weight at 10,000 ppm and above. A dose-related increase in cervical ribs or ossification sites lateral to the seventh cervical vertebra was significant at 2,000 ppm and above. Thus, the NOAEL for the developmental toxicity in this study was 1,000 ppm. A log-logistic dose response model was applied to the incidence data for exencephaly, cleft palate, resorption and cervical rib, and maximum likelihood estimates (MLEs) and benchmark dosages (BDs, the lower 95% confidence interval of the MLEs) corresponding to 1% and 5% added risk above background were calculated. The MLE for 5% added combined risk of having either exencephaly or cleft palate or being resorbed was 3667 ppm, and the corresponding BD was 3,078 ppm. For cervical rib, the 5% added risk values for the MLE and BD were 824 and 305 ppm, respectively. The BDs for 1% added risk were 1915 ppm for exencephaly, cleft palate or resorption, and 58 ppm for cervical rib.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutathione conjugates of 2-bromohydroquinone (GSylBHQ) cause renal proximal tubular necrosis that is dependent upon the activity of γ-glutamyl transferase (GGT). GGT is present in embryonic yolk sac and its activity increases with gestational age, suggesting that the developing embryo might be at risk from maternal exposure to glutathione conjugates or compounds which are shown to form glutathione conjugates. Studies in pregnant rats exposed on Day 9 of gestation to 400 or 800 μmol/kg BHQ or 20 μmol/kg 2-Br-(di-GSyI)HQ and examined on Day 11 of gestation suggested that the parent compound (BHQ) or a metabolite was nephrotoxic in the adult and dysmorphogenic in the embryo and that 2-Br-(di-GSyJ)HQ was nephrotoxic but not dysmorphogenic at the dose tested (20 μmol/kg). We therefore exposed Day 9 rat embryos to BHQ, 2-Br-6-(GSyl)HQ, or 2-Br-(di-GSyI)HQ in vitro for 48 hr to determine the relative dysmorphogenic activity of the parent compound and the two conjugates. In vitro exposure to BHQ (0-40 μM) resulted in dose-related decreases in somite number (SN), total protein, and developmental score (DEVSC), with no effect on yolk sac diameter (YSD), crown rump length (CR), head length (HL), or percentage abnormal embryos (%AE); 60 μM BHQ was embryolethal. Embryos exposed to 2-Br-6-(GSyl)HQ (0-120 μM) were not affected at concentrations below 120 μM, at which dose there were significant effects on protein. YSD, CR, HL, DEVSC, SN, and %AE. Embryos exposed to 2-Br-(di-GSyl)HQ had a significantly lower DEVSC at the 80 μM concentration and significantly lower YSD, protein, and DEVSC and significantly higher %AE at the 10, 25, and 120 μM concentrations. CR, HL, and SN were not affected at any exposure level with this compound. In conclusion, BHQ was found to be developmentally toxic in vitro and in vivo at doses which also produced severe maternal renal necrosis. The doses of 2-Br-(di-GSyI)HQ in vivo which caused only mild maternal renal necrosis did not produce developmental toxicity. Conjugation of BHQ with either one or two molecules of GSH decreased the embryolethality of BHQ. The conjugates appeared to be of comparable toxicity as measured by the incidence of abnormal embryos in vitro. The role of maternal toxicity and GSH conjugation in 2-BHQ-mediated developmental toxicity remains to be determined.

5-Fluorouracil (5-FU) inhibits the enzyme thymidylate synthetase (TS) which results in inhibition of DNA synthesis. 5-FU is teratogenic in many species, inducing cleft palate, limb, and tail defects. In the present study, gestation day (GD) 14 embryonic rat craniofacial explants were exposed to 5-FU in organ culture with increasing concentrations and durations of exposure. Palates exposed to 5-FU were morphologically abnormal and craniofacial shape, size, and palatal fusion pattern were affected with the severity of effects dependent on concentration and duration of exposure. Cleft palate was induced in vitro as opposing palates overlapped in a narrowed oral cavity. Palates exposed to higher levels of 5-FU were growth inhibited, but fused even though proliferation ceased and few cells were available to participate in elevation and fusion. This was demonstrated as a biphasic concentration-response profile for palatal fusion in which 0.05 to 0.15 micrograms 5-FU/ml produced decreasing rates of palatal fusion, while exposure to 0.15 to 3.0 micrograms/ml resulted in progressively increasing rates of fusion. The effects of 5-FU were detected biochemically as a reduction in TS activity which was concentration and time dependent during the first 12 hours, with a return to control levels by 24 hours. During the first day, 5-FU did not alter protein levels, but DNA levels significantly decreased at the high concentration, 2.0 micrograms/ml. After 5 days in culture, both DNA and protein decreased with increasing 5-FU concentration and duration of exposure. Also by the end of the culture period, 3H-TdR incorporation had decreased in a concentration dependent manner. It is concluded that progressive inhibition of proliferation and growth in organ culture results in two different morphological outcomes: cleft palate resulting from a narrowed oral cavity and increased incidence of anterior palatal fusion under conditions of strong growth reduction. This study demonstrates that elevation and fusion can occur in the absence of growth and proliferation. Based on these observations, severe inhibition of growth or proliferation would not necessarily be sufficient to induce cleft palate.

The embryotoxicity of phenol and twelve para-substituted congeners on mid-gestation rat embryos was evaluated in vitro. Through application of correlative procedures and stepwise regression, equations describing the relationship between physical-chemical properties and various measures of activity were developed. Embryotoxicity was quantified by the log of the reciprocal of the potency estimates for reduction in selected growth parameters and induction of four morphological defects. In general, co-cultured hepatocytes ameliorated embryotoxicity; only phenol-induced embryotoxicity was enhanced by the presence of hepatocytes. In the absence of hepatocytes, measures of growth retardation were positively correlated with molar refractivity of the phenols. With hepatocytes, lipophilicity became important for describing the potential to induce growth deficits. The structural defects had varying correlation patterns in both culture systems. Potencies of these congeners in vitro were also compared to maternal and developmental potencies observed in vivo (Kavlock, Teratology, 41:43-59, '90). Two of the congeners were very toxic in both systems. For the remaining congeners, one maternal toxicity measure was found to be positively correlated with embryotoxicity for growth and development in vitro without hepatocytes. With hepatocytes, a broad spectrum of correlations, both positive and negative, were observed between in vivo developmental toxicity endpoints and in vitro embryotoxicity. Data from preliminary dosimetry studies suggest that phenol congeners may accumulate in embryos exposed in vitro more readily than with in vivo exposure. Potency calculations based on dosimetry information may demonstrate better correlations between data and allow additional relationships between chemical structure and activity to be developed.