Biomedical subjects

R J Kavlock

Publications and source records attributed to R J Kavlock.

At least 37 records · Page 2Linked to original sources

PubMed1996

The road to embryologically based dose-response models.

The goal of researchers working in the area of developmental toxicology is to prevent adverse reproductive outcomes (early pregnancy loss, birth defects, reduced birth weight, and altered functional development) in humans due to exposures to environmental contaminants, therapeutic drugs, and other factors. To best achieve that goal, it is important that relevant information be gathered and assimilated in the risk assessment process. One of the major challenges of improved risk assessment is to better use all pertinent biological and mechanistic information. This may be done qualitatively (e.g., demonstrating that the experimental model is not appropriate for extrapolation purposes); semiquantitatively (using information to reduce the degree of uncertainty present under default extrapolation procedures), or quantitatively (formally describing the relationships between exposure and adverse outcome in mathematical forms, including components that directly reflect individual steps in the overall progression of toxicity). In this paper we review the recent advances in the risk assessment process for developmental toxicants and hypothesize on future directions that may revolutionize our thinking in this area. The road to these changes sometimes appears to be a well-mapped course on a relatively smooth surface; at other times the path is bumpy and obscure, while at still other times it is only a wish in the eye of the engineer to cross an uncharted and rugged environment.

Abnormalities, Drug-Induced↗

PubMed1996

Research needs for the risk assessment of health and environmental effects of endocrine disruptors: a report of the U.S. EPA-sponsored workshop.

The hypothesis has been put forward that humans and wildlife species adverse suffered adverse health effects after exposure to endocrine-disrupting chemicals. Reported adverse effects include declines in populations, increases in cancers, and reduced reproductive function. The U.S. Environmental Protection Agency sponsored a workshop in April 1995 to bring together interested parties in an effort to identify research gaps related to this hypothesis and to establish priorities for future research activities. Approximately 90 invited participants were organized into work groups developed around the principal reported health effects-carcinogenesis, reproductive toxicity, neurotoxicity, and immunotoxicity-as well as along the risk assessment paradigm-hazard identification, dose-response assessment, exposure assessment, and risk characterization. Attention focused on both ecological and human health effects. In general, group felt that the hypothesis warranted a concerted research effort to evaluate its validity and that research should focus primarily on effects on development of reproductive capability, on improved exposure assessment, and on the effects of mixtures. This report summarizes the discussions of the work groups and details the recommendations for additional research.

Animals↗

PubMed1995

Biological modeling of 5-fluorouracil developmental toxicity.

A biologically-based dose-response (BBDR) model is a mathematical description of the biological events leading to expression of a toxic response. As an alternative to current approaches in non-cancer risk assessment, such models will reduce uncertainty in that they will provide a more comprehensive description of toxicity. We are involved in construction of a BBDR model for the developmental toxicity of 5-fluorouracil (5-FU) in the rat using multiple approaches. First, to identify critical events in the pathogenesis of 5-FU developmental toxicity, thymidylate synthetase (TS) inhibition and alterations in cell kinetics and growth were examined in embryos following maternal administration of 5-FU on day 14 of gestation. A dose-related decline in TS activity was observed within 1 h; however, maximal inhibition and recovery were similar at 10, 20 and 40 mg/kg. Dose-dependent cell cycle alterations were observed within 4 h after exposure and were maximal at 8 h. Hindlimb growth reduction was observed 24 h after exposure to 40 mg/kg, but not at lower doses. At term hindlimb defects were observed at doses above 30 mg/kg. An integrated dose-response model for hindlimb defects was derived from empirical relationships among these events. The resultant dose-response somewhat over-predicted the developmental toxicity of 5-FU, although results of a Monte Carlo simulation indicated that these data were not incompatible with model predictions. Overall, the results suggest that TS inhibition is a key component of the mechanism of 5-FU developmental toxicology, but the model does not capture all of the critical events in the induction of hindlimb defects. A preliminary mechanistic model for the inhibition of embryonic TS, DNA synthesis and cell cycle following maternal exposure to 5-FU, independently derived from literature data to further examine the potential role of this pathway in its developmental toxicity, predicted a dose-response for TS inhibition and DNA synthesis that closely reflected the observed patterns. These results further suggest that TS inhibition, resultant deficits in DNA synthesis and cell cycle perturbations represent a critical mechanistic pathway in the developmental toxicity of 5-FU.

Animals↗

PubMed1995

Profile of procarbazine-induced embryotoxicity in an embryo hepatocyte co-culture system and after in utero glutathione depletion.

Procarbazine (PCZ) is an antineoplastic agent useful in the treatment of Hodgkin's disease, brain tumors, and chronic leukemia. PCZ is dysmorphogenic to developing embryos exposed in vivo or cultured in the serum of PCZ-treated rats. However, embryos directly cultured with PCZ (up to 400 micrograms/ml) or PCZ plus S-9 liver fractions are unaffected. Since intact liver cells provide several advantages over hepatic subcellular fractions for in vitro bioactivation, we exposed rat embryos to PCZ in an embryo/hepatocyte co-culture system. Gestation day (GD) 9.5 rat embryos exposed to 0, 200, 300, or 400 micrograms PCZ/ml in the presence of untreated or phenobarbital induced male rat hepatocytes failed to display toxicity. However, in a companion study GD 9.5 rat embryos cultured in the serum from PCZ-treated rats exhibited developmental deficiencies. Studies have shown that the formation of toxic metabolites can result from glutathione (GSH) conjugation of toxicants in the liver. Therefore, in a second set of experiments, rat embryos were cultured in serum from rats pretreated with two GSH depleters (phorone and buthionine sulfoximine) and subsequently dosed with PCZ. Effects on development were enhanced when embryos were cultured in the serum from PCZ-treated/GSH depleted rats. These data indicate that PCZ requires in vivo activation to be dysmorphogenic and further suggest that the metabolite(s) responsible for procarbazine embryo-toxicity are formed readily under conditions of low GSH levels. This argues against a glutathione conjugate as the ultimate toxicant.

Abnormalities, Drug-Induced↗

PubMed1995

Developmental toxicity of formate and formic acid in whole embryo culture: a comparative study with mouse and rat embryos.

Acute methanol (MeOH) toxicity in primates is attributed to the conversion of MeOH to formate and the resulting acidosis. MeOH has been shown to be developmentally toxic in mice and rats both in vivo and in vitro, but rodents neither accumulate formate nor develop acidosis after MeOH exposure. To further assess the potential human developmental toxicity of MeOH exposure, we evaluated the developmental effects of sodium (Na) formate and formic acid in rodent whole embryo culture (WEC). Day 9 rat embryos were cultured for 24 or 48 hours and day 8 mouse embryos were cultured for 24 hours in the presence of Na-formate or formic acid. Rat and mouse embryos exposed to either agent for 24 hours exhibited a trend toward reduced growth and development and the number of abnormalities increased at the higher concentrations. Rat embryos exposed for 48 hours to either Na-formate or formic acid showed a trend toward reduced growth and development with increasing concentration. Embryo lethality and incidence of abnormal embryos were also increased at the higher concentrations. The anomalies observed in both species after exposure to either compound were primarily open anterior and posterior neuropore with less frequent incidence of rotational defects, tail anomalies, enlarged pericardium and delayed heart development. Exposure to Na-formate or formic acid for comparable periods of time results in comparable degrees of embryotoxicity at concentrations (mMolar) at least 4-fold lower than those previously reported for methanol exposure.

Abnormalities, Drug-Induced↗

PubMed1995

Dose-response assessments for developmental toxicity. IV. Benchmark doses for fetal weight changes.

Recently, most attention on the application of benchmark dose (BMD) techniques to toxicology data has focused on quantal measures of response. Before the advantages of the BMD approach can be exploited in the risk assessment process, it is important that continuous measures of response also be modeled appropriately. In this study, we examined a variety of approaches to estimating BMDs for a change in fetal weight following chemical exposure from a total of 85 developmental toxicity experiments. We modeled the change in the mean fetal weight of a litter in response to treatment using a continuous power model, as well as reductions in the weight of individual fetuses within litters (defined as falling below a preset level) using a log-logistic model which incorporates litter size as a covariable and considers intralitter correlations. For the litter-based approach, several methods of defining a benchmark effect (BME) were considered, including a percentage change in mean litter weight, a change in mean litter weight relative to variability in the control group, and a reduction in the mean litter weight to some point on the control group distribution curve. For the fetus-based approach, we examined several BME options on the cumulative frequency distribution of the control fetuses for defining a low weight fetus and calculated several levels of additional risk. BMDs for four litter-based BMEs (a difference of 5% in mean fetal weight, a decrease to the 25th percentile mean weight of control litters, a decrease in the mean weight by 2 standard errors, and a decrease of 0.5 standard deviation units) and two fetus-based BMEs (a 5% added risk of weighing less than the 5th percentile of control weights and a 10% added risk of weighing less than the 10th percentile) showed strong similarities to each other and to statistically derived NOAELs. In addition to providing comparison with the NOAEL as a reference value, these analyses provided confirmation of the advantages of the BMD approach over the NOAEL in terms of the influence of dose spacing and dose selection. Combined with our previous analyses of quantal endpoints of fetal effects, this information provides a firm basis upon which to implement the benchmark dose concept in developmental toxicity risk assessments.

Animals↗

PubMed1995

Nonadditive developmental toxicity in mixtures of trichloroethylene, Di(2-ethylhexyl) phthalate, and heptachlor in a 5 x 5 x 5 design.

In order to identify nonadditive effects on development, three compounds were combined using five dosages of each agent (a 5 x 5 x 5 full-bacterial design). Trichloroethylene (TCE), di(2-ethylhexyl) phthalate (DEHP), and heptachlor (HEPT), in corn oil, were administered by gavage to Fischer-344 rats on Gestation Days 6-15. Dose levels were 0, 10.1, 32, 101, and 320 mg/kg/day for TCE; 0, 24.7, 78, 247, and 780 mg/kg/day for DEHP; and 0, 0.25, 0.8, 2.5, and 8 mg/kg/day for HEPT. The dams were allowed to deliver and their pups were weighed and examined postnatally. Maternal death showed no main effects but DEHP and HEPT were synergistic. For maternal weight gain on Gestational Days 6-8, main effects for all three agents were observed, as well as 6-8 main effects for all three agents were observed, as well as TCE-DEHP synergism, and DEHP-HEPT antagonism. Maternal weight gain on Gestational Days 6-20 adjusted for litter weight showed main effects for TCE and HEPT, but no interactions. Main effects for all three agents were evident for full-litter resorptions and prenatal loss. The HEPT main effects were unexpected and were interpreted as reflecting potentiation by HEPT of the other agents. For full-litter loss, the TCE-HEPT and DEHP-HEPT interactions were antagonistic, perhaps due to a "ceiling" effect. For prenatal loss, the TCE-DEHP interaction was synergistic. Postnatal loss showed DEHP and HEPT main effects but no interaction. Analysis of pup weights on Day 1 revealed TCE and DEHP main effects and DEHP-HEPT antagonism; on Day 6, DEHP and HEPT main effects, DEHP-HEPT antagonism, and TCE-DEHP synergism were evident. Microphthalmia and anophthalmia incidences revealed TCE and DEHP main effects but no interactions. This extensive examination of a full-factorial design elucidates the complexities of studying and interpreting mixture toxicity. The data are available for further analysis.

Abnormalities, Drug-Induced↗

PubMed1995

Benchmark Dose Workshop: criteria for use of a benchmark dose to estimate a reference dose.

The purpose of the Benchmark Dose Workshop was to assess the feasibility and implications of replacing the no observed adverse effect level (NOAEL) with a benchmark dose (BMD) when deriving reference doses and concentrations (RfDs and RfCs). The workshop participants supported the use of the BMD method to remove many of the limitations inherent in using the NOAEL approach. Participants endorsed in general the use of a BMD for all quantal noncancer health effects and endorsed in particular the BMD for assessing developmental toxicity based on data presented at the workshop. The discussions of implementation recognized the need to demonstrate that changing from a NOAEL to a BMD gives the risk manager more certain information on which to base decisions. Most participants agreed that the current NOAEL-derived RfDs and RfCs are sufficiently protective and should only be changed as data become available for estimating a BMD. It was recognized that to achieve general acceptance of the BMD approach, it will have to be applied to a variety of endpoints.

Animals↗

PubMed1995

Validation of an in vitro teratology system using chiral substances: stereoselective teratogenicity of 4-yn-valproic acid in cultured mouse embryos.

In vitro systems are important for toxicity testing as well as for investigating the mechanism of action of xenobiotics. The validation of such in vitro systems is often incomplete and extrapolation to the in vivo situation is equivocal. In the present study, we studied the effects of enantiomers of an analogue of the antiepileptic drug valproic acid (VPA): R(+)- and S(-)-4-yn-VPA (R- and S-2-n-propyl-4-pentynoic acid), which have previously been shown to induce selective teratogenicity in mice after in vivo administration, in mouse whole-embryo culture (WEC). Aqueous solutions of the sodium salts of the pure R- and S-enantiomers as well as R,S-4-yn-VPA (racemic mixture) or VPA itself were added to the culture medium at 0, 0.075, 0.15, 0.3, 0.6, or 1.2 mmol/liter and embryos were evaluated 24 hr later. The S-4-yn-VPA enantiomer induced clear concentration-dependent dysmorphogenesis that was evident even at the lowest concentration. The primary anomalies were neural tube defects, erratic neural seams, blisters, and rotational defects. Embryolethality was observed at 1.2 mmol/liter. The R-4-yn-VPA enantiomer was neither embryotoxic nor dysmorphogenic at any tested concentration. The lack of biological activity over 24 hr in WEC with the R-enantiomer suggests also that, as previously shown in vivo, there was no racemization of this isomer to the more active S-enantiomer. The racemic mixture of R and S isomers appeared to be slightly more embryolethal and dysmorphogenic than VPA. Overall, the potency of the S-enantiomer was approximately four times that of VPA. Therefore, the rank order of the four chemicals tested was S(-) >> S(-), R(+) > VPA >>> R(+), which is in agreement with the effects observed in in vivo exposed mice. These data demonstrate a direct stereoselective effect of these compounds on the embryo. This is the first illustration of the stereoselectivity of a xenobiotic in the WEC in vitro test system. Pure and stable enantiomers, which induce stereoselective toxicity in vivo, are demonstrated to be valuable for validation of this in vitro system.

Animals↗

PubMed1995

The application of benchmark dose methodology to data from prenatal developmental toxicity studies.

The benchmark dose (BMD) concept was applied to 246 prenatal-developmental toxicity (DT) datasets from government, industry and commercial laboratories. Five modeling approaches were used, 2 generic and 3 specific to DT models. BMDs for both quantal and continuous data were compared with statistically derived no observed adverse effect levels (NOAELs) to determine similarities. Quantal (Q) endpoints included litter responses (e.g., one or more dead or malformed implants), and QBMDs were calculated using a Q Weibull (QW) model. Two types of continuous (C) data were modeled, the proportion of implants affected per litter, and the change in fetal weight (both mean and distribution); continuous power (CP) and DT models were used to calculate CBMDs. QBMDs for a 5% change in response (QBMD05) were 6-fold lower, on average, than the corresponding NOAEL. CBMD05s on average were similar to the corresponding NOAELs, and CBMD05s from different models were similar to each other. Including litter size but not threshold improved the fit of the DT models. For fetal weight data, specific cutoff values were used to calculate BMDs that were similar on average to the corresponding NOAELs: (1) changes from the control mean (5% of the mean, 25th percentile of the control distribution, or a decrease of 0.5 standard deviation), and (2) a 5 or 10% decrease in the proportion of fetuses below the 5th or 10th percentile, respectively, of the control distribution. These results support the use of BMDs as providing a more consistent basis for risk assessment than do NOAELs.

Animals↗

PubMed1995

A multidisciplinary approach to toxicological screening: II. Developmental toxicity.

As part of the validation of an integrated bioassay for systemic toxicity, neurotoxicity, and developmental toxicity, we evaluated the effects of four pesticides, four chlorinated solvents, and two other industrial chemicals in Fischer 344 rats. The pesticides included carbaryl, triadimefon, chlordane, and heptachlor; the solvents included dichloromethane (DCM), carbon tetrachloride, trichloroethylene (TCE), and tetrachloroethylene (perchloroethylene, PER); and the industrial chemicals were di(2-ethylhexyl)phthalate (DEHP) and phenol. In the developmental toxicity studies, timed-pregnant rats were treated by gavage with vehicle or 1 of 2 dose levels of each compound on gestation d 6-19. The dams were allowed to deliver and their litters were examined on postnatal d 1, 3, and 6. Litter weights were determined on postnatal d 1 and 6. Implants were also counted to determine prenatal loss. Maternal toxicity was evidenced by dose-related alterations in weight gain for all 10 compounds. Clinical signs of maternal toxicity were present for all chemicals except chlordane and heptachlor. DEHP exposure resulted in the most pronounced developmental toxicity (high levels of pre- and postnatal mortality), whereas chlordane induced extensive postnatal loss. Of the solvents, only DCM did not cause a high incidence of full-litter resorption. Phenol, heptachlor, triadimefon, and carbaryl showed only slight potential for developmental toxicity. Malformations suggestive of teratogenicity included kinked tail (phenol), microphthalmia (TCE, PER, DEHP), and cleft palate with renal agenesis (DEHP). Although several findings (eye defects caused by TCE and PER, full-litter resorption and delayed parturition caused by PER, and delayed parturition/dystocia associated with triadimefon) have not been previously reported, the results are generally consistent with previous reports and highlight the importance and relative ease of incorporation of developmental evaluations into a multidisciplinary screening battery.

Animals↗

PubMed1995

A multidisciplinary approach to toxicological screening: IV. Comparison of results.

Toxicity data collected under standardized test conditions may be of the utmost importance in health risk assessment, in which human exposure limits are often derived from laboratory experiments. A standardized approach to data collection is also important for evaluating the sensitivity and specificity of test methods used to determine toxic potential. Several experiments were undertaken to determine the effects of chemical exposures using a multidisciplinary screening battery, which included tests for systemic, neurological and developmental toxicity. The effects of 1- and 14-d exposures to 10 chemicals on systemic and neurological indices of toxicity were determined in female F344 rats using standardized test batteries. Parallel experiments determined chemical effects on prenatal and postnatal development following exposure of the dams for 14 d. The chemicals included four pesticides (carbaryl, triadimefon, chlordane, and heptachlor), four solvents (trichloroethylene, tetrachloroethylene, carbon tetrachloride, and dichloromethane), and two industrial compounds (phenol and diethylhexyl phthalate). The results showed that the chemicals produced markedly different qualitative patterns of effect on systemic, neurological, and developmental indices of toxicity. Differences in the pattern of systemic and neurological effects were also obtained that depended on dosing duration. Quantitative analyses indicated that the highest ineffective dose as well as the lowest effective dose could vary by as much as two orders of magnitude across the different indices of toxicity. These results clearly show that a test battery focused on a single endpoint of toxicity cannot be used to accurately predict either qualitatively or quantitatively a chemical's systemic, neurological, and developmental toxicity profile.

Analysis of Variance↗

PubMed1994

Developmental toxicity and structure-activity relationships of aliphatic acids, including dose-response assessment of valproic acid in mice and rats.

The anticonvulsant valproic acid (VPA), or 2-propylpentanoic acid, is a short-chain aliphatic acid that is teratogenic in humans and rodents. VPA and 14 related chemicals were screened for developmental toxicity using the Chernoff/Kavlock assay. Test agents, in corn oil, were administered by gavage to Sprague-Dawley rats once daily during organogenesis. The dams were allowed to deliver and the pups were examined postnatally. Segment II studies were also conducted using VPA and pentanoic acid in rats and with VPA in CD-1 mice. In both mice and rats, VPA caused transient maternal ataxia and developmental defects of the digits and, especially, the axial skeleton. Exencephaly, however, was seen only in mice. The screening protocol was effective in prioritizing agents within this class of compounds for more definitive developmental toxicity testing. All congeners tested induced maternal respiratory effects and six compounds caused motor depression. Only 2-ethylhexanoic (2EH) and 2-propylhexanoic (2PH) acid caused dramatic VPA-like effects on rat development (including mortality, extra presacral vertebrae, fused ribs, and delayed parturition), confirming the strict structural requirements for developmental toxicity previously reported for acute exposure in mice. The incorporation of skeletal examinations in the Chernoff/Kavlock assay enabled the detection of the sole developmental effect (increased incidence of lumbar ribs) of 2-butylhexanoic acid. VPA, 2EH, and 2PH were among the compounds that caused maternal motor depression. These data, consistent with previous reports, indicate a broader specificity for activity in the adult nervous system than that in the developing system and suggest differing mechanisms for the two effects.

Acids↗

PubMed1994

Dose-response assessment for developmental toxicity. I. Characterization of database and determination of no observed adverse effect levels.

Developmental toxicity risk assessment currently relies on the estimation of reference doses (RfDDTs) or reference concentrations (RfCDTS) based on the use of no observed adverse effect levels (NOAELs) and uncertainty factors. The benchmark dose (BMD) has been proposed as an alternative basis for reference value calculations. A large database of 246 developmental toxicity experiments (Segment II-type studies) representing 1825 data subsets for various endpoints was compiled for use in comparing NOAEL and BMD approaches to developmental toxicity risk assessment. This paper describes the characteristics of the database used and the estimation of NOAELs using several approaches. For each endpoint evaluated, two NOAELs were calculated using the NOSTASOT procedure (Tukey et al., 1985). The first NOAEL calculation, the QNOAEL, was based on a quantal response where a litter was defined as "affected" if one or more fetuses or implants in the litter had the endpoint of interest. The second NOAEL calculation, the CNOAEL, was based on the proportion of fetuses or implants affected within each litter and was treated as a continuous response variable. Fifty-seven percent of the 246 experiments had at least one endpoint that showed a significant trend with dose. A total of 386 data sets were significant with respect to both the quantal and continuous test of trend. An additional 44 data sets were identified with significant trend only by the quantal approach whereas 177 additional data sets were identified with significant trend tests only by the continuous approach. Thus, the continuous approach appeared to be more powerful in detecting dose-related toxicity, but the patterns detected by the two approaches differed.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Dose-response assessment for developmental toxicity. II. Comparison of generic benchmark dose estimates with no observed adverse effect levels.

Developmental toxicity risk assessment currently relies on the estimation of reference doses (RfDDTS) of reference concentrations (RfCDTS) based on the use of no observed adverse effect levels (NOAELS) divided by uncertainty factors (UFs). The benchmark dose (BMD) has been proposed as an alternative basis for reference value calculations. A large database of 246 developmental toxicity experiments representing 1825 endpoints related to dead implants or malformed fetuses has been compiled for use in evaluating alternative approaches to developmental toxicity risk assessment. Using this database we have compared two approaches for BMD estimation with each other and with corresponding statistically derived NOAELS. Comparisons have been based on proportion of affected litters (litters with one or more affected offspring, a quantal response variable) and on the proportion of affected offspring within each litter (a continuous response variable). A quantal Weibull model was used to calculate generic BMDs for the quantal response variable (QBMDs) and a continuous power model was used to calculate generic BMDs for the continuous response variable (CBMDs) at three levels of additional risk (10, 5, and 1%). CBMD05s (continuous benchmark doses for 5% risk) and CNOAELs (statistically derived NOAELs based on the continuous response variable) were similar, with over 98% of the data subsets having CBMD05 and CNOAEL values that differed by less than an order of magnitude. In contrast, QNOAELs tended to be greater than corresponding QBMD10s. The observed conservatism of the QBMD values relative to the corresponding CBMD values was attributed to two factors, lower maximum likelihood estimates for the quantal model and wider confidence intervals around the maximum likelihood estimates, compared to the continuous model.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Dose-response assessment for developmental toxicity. III. Statistical models.

Although quantitative modeling has been central to cancer risk assessment for years, the concept of dose-response modeling for developmental effects is relatively new. The benchmark dose (BMD) approach has been proposed for use with developmental (as well as other noncancer) endpoints for determining reference doses and reference concentrations. Statistical models appropriate for representing the unique features of developmental toxicity testing have been developed and applied (K. Rai and J. Van Ryzin, 1985, Biometrics 41, 1-9; L. Kupper, C. Portier, M. Hogan, and E. Yamamoto, 1986, Biometrics 42, 85-98; R. Kodell, R. Howe, J. Chen, and D. Gaylor, 1991, Risk Anal. 11, 583-590). Generalizations of those models (designated the RVR, LOG, and NCTR models, respectively) account for the correlations among observations in individual fetuses or implant within litters; the potential for variables other than dose, such as litter size, to affect the probability of adverse outcome; and the possibility of a threshold dose below which background response rates are unaltered. The generalized models were applied to a database of 607 endpoints with significant dose-related increases in response rate. It was determined that the models were generally capable of fitting the observed dose-response patterns, with the LOG model appearing to be superior with respect to fit. A significant contributor to the ability of the LOG model to fit the data was its flexibility with respect to the representation of the dependence of response probability on litter size, a trait not shared by the other two models. Litter size appeared to be a significant covariable for predicting response rates, even when intralitter correlation was accounted for by assuming a beta-binomial distribution for the observations among individual fetuses. In contrast, a threshold dose parameter did not appear to be necessary to adequately describe the observed dose-response patterns. BMD estimates (corresponding to 5% additional risk) from all three models were similar to one another and to BMDs estimated from other, generic dose-response models (not specifically designed for developmental toxicity testing) that modeled average proportion of fetuses affected. The BMDs at the 5% level of risk were similar to no observed adverse effect levels determined by statistical tests of trend. Greater emphasis on and further examination of dose-response modeling for developmental toxicity testing are needed; biologically based approaches that consider the continuum of developmental effects induced in such tests should be encouraged.

Animals↗

PubMed1994

Biologically based dose-response modeling in developmental toxicology: biochemical and cellular sequelae of 5-fluorouracil exposure in the developing rat.

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.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Developmental toxicity of methanol in whole embryo culture: a comparative study with mouse and rat embryos.

Methanol (MeOH), a widely used industrial solvent, has been proposed as an alternative motor vehicle fuel. Inhaled MeOH is developmentally toxic in both rats and mice but the mouse is more sensitive than is the rat. The contribution of the embryo to this differential sensitivity was studied in whole embryo culture (WEC) using equivalent stage rat (day 9) and mouse (day 8) embryos (plug day = day 0). Rat embryos were explanted and cultured in 0, 2, 4, 8, 12 or 16 mg MeOH/ml rat serum for 24 h and then transferred to rat serum alone for 24 h. Embryonic development of the 2 and 4 mg MeOH/ml groups was not significantly different from the controls whereas the higher concentrations resulted in a concentration related decrease in somite number, head length and developmental score. The 12 mg/ml dose resulted in some embryolethality as well as dysmorphogenesis, while the highest dose was embryolethal. MeOH was dysmorphogenic in vitro in rat embryos at a MeOH concentration comparable to that reported in maternal serum following teratogenic in vivo exposures. Day 8 mouse embryos were explanted and cultured in 0, 2, 4, 6 or 8 mg MeOH/ml culture medium (75% rat serum, 25% Tyrode's salt solution) for 24 h. Embryonic development in the 2 mg/ml MeOH group was not significantly different from the controls but all higher concentration groups had a significant decrease in developmental score and crown-rump length. The high concentration group also suffered 80% embryolethality. Thus, mouse embryos were affected at MeOH concentrations which were not dysmorphogenic or embryotoxic in the rat, suggesting that the higher sensitivity of the mouse to the developmental toxicity of inhaled methanol is due, at least in part, to greater intrinsic embryonal sensitivity of this species to methanol.

Animals↗