Editor's Highlight: Comparative Toxicity of Organophosphate Flame Retardants and Polybrominated Diphenyl Ethers to Caenorhabditis elegans.

With the phasing-out of the polybrominated diphenyl ether (PBDE) flame retardants due to concerns regarding their potential developmental toxicity, the use of replacement compounds such as organophosphate flame retardants (OPFRs) has increased. Limited toxicity data are currently available to estimate the potential adverse health effects of the OPFRs. The toxicological effects of 4 brominated flame retardants, including 3 PBDEs and 3,3',5,5'-tetrabromobisphenol A, were compared with 6 aromatic OPFRs and 2 aliphatic OPFRs. The effects of these chemicals were determined using 3 biological endpoints in the nematode Caenorhabditis elegans (feeding, larval development, and reproduction). Because C. elegans development was previously reported to be sensitive to mitochondrial function, results were compared with those from an in vitro mitochondrial membrane permeabilization (MMP) assay. Overall 11 of the 12 flame retardants were active in 1 or more C. elegans biological endpoints, with only tris(2-chloroethyl) phosphate inactive across all endpoints including the in vitro MMP assay. For 2 of the C. elegans endpoints, at least 1 OPFR had similar toxicity to the PBDEs: triphenyl phosphate (TPHP) inhibited larval development at levels comparable to the 3 PBDEs; whereas TPHP and isopropylated phenol phosphate (IPP) affected C. elegans reproduction at levels similar to the PBDE commercial mixture, DE-71. The PBDEs reduced C. elegans feeding at lower concentrations than any OPFR. In addition, 9 of the 11 chemicals that inhibited C. elegans larval development also caused significant mitochondrial toxicity. These results suggest that some of the replacement aromatic OPFRs may have levels of toxicity comparable to PBDEs.

Ozone exposure-response model for lung function changes: an alternate variability structure.

CONTEXT: A statistical model that accurately predicts human forced expiratory volume in one second (FEV1) response to ozone exposure has been identified and proposed as the foundation for future risk assessments for ambient ozone. We believe that the assumptions about intra-subject variability in the published model can be improved and hypothesize that more realistic assumptions will improve the fit of the model and the accuracy of risk assessments based on the model. OBJECTIVE: Identify alternate assumptions about intra-subject variability and compare goodness-of-fit for models with various variability structures. MATERIALS AND METHODS: Models were fit to an existing data set using a statistical program for fitting nonlinear mixed models. Goodness-of-fit was assessed using Akaike's Information Criteria (AIC) and visual examination of graphical figures showing observed and predicted values. RESULTS: The AIC indicated that a model that assumed intra-subject variability was related to the magnitude of individual response fit the data better than a model that assumes intra-subject variability is constant across individuals and exposures (the original model). This finding was consistent with the variability of observed responses for filtered air exposures and for exposures predicted to be below the threshold for response. CONCLUSION: An ozone exposure-response model that assumes intra-subject variability increases with individual mean FEV1 response appears to fit the data better than one that assumes constant variability.

Prediction of lung function response for populations exposed to a wide range of ozone conditions.

CONTEXT: A human exposure-response (E-R) model previously demonstrated to accurately predict population mean FEV1 response to ozone exposure has been proposed as the foundation for future risk assessments for ambient ozone. OBJECTIVE: Fit the original and related models to a larger data set with a wider range of exposure conditions and assess agreement between observed and population mean predicted values. MATERIALS AND METHODS: Existing individual E-R data for 23 human controlled ozone exposure studies with a wide range of concentrations, activity levels, and exposure patterns have been obtained. The data were fit to the original model and to a version of the model that contains a threshold below which no response occurs using a statistical program for fitting nonlinear mixed models. RESULTS: Mean predicted FEV1 responses and the predicted proportions of individuals experiencing FEV1 responses greater than 10, 15, and 20% were found to be in agreement with observed responses across a wide range of exposure conditions for both models. The threshold model, however, provided a better fit to the data than the original, particularly at the lowest levels of exposure. CONCLUSION: The models identified in this manuscript predict population FEV1 response characteristics for 18-35-year-old individuals exposed to ozone over a wide range of conditions and represent a substantial improvement over earlier E-R models. Because of its better fit to the data, particularly at low levels of exposure, the threshold model is likely to provide more accurate estimates of risk in future risk assessments of ozone-induced FEV1 effects.

Prediction of ozone-induced lung function responses in humans.

The main purpose of this study was to evaluate the ability of a human exposure-response model, which describes ozone-induced changes in forced expiratory volume in 1 second (FEV1) across a wide range of dynamic exposure conditions, to predict responses in independent data. We first conducted an n-fold cross-validation of the model using samples of the original EPA data from which the model was developed. We then identified seven more recently published studies with controlled exposures to a wide range of ozone exposure patterns relevant to the current ambient ozone health standard and used the model to calculate the mean predicted responses for the exposure conditions of the individual studies that we compared to the mean observed responses reported in these studies. The n-fold cross-validation indicated good internal agreement between mean predicted and mean observed responses in the original data used to develop the model. The model accurately captured the patterns of response in each of the seven independent studies with a tendency to overpredict the observed responses by about 1 percentage point of FEV1 decrement on average. We conclude that the model is currently capable of predicting human FEV1 responses across a wide range of dynamic exposure conditions and anticipate further improvements in predictions with the addition of low-concentration exposure data.

Absolute estimation of initial concentrations of amplicon in a real-time RT-PCR process.

BACKGROUND: Since real time PCR was first developed, several approaches to estimating the initial quantity of template in an RT-PCR reaction have been tried. While initially only the early thermal cycles corresponding to exponential duplication were used, lately there has been an effort to use all of the cycles in a PCR. The efforts have included both fitting empirical sigmoid curves and more elaborate mechanistic models that explore the chemical reactions taking place during each cycle. The more elaborate mechanistic models require many more parameters than can be fit from a single amplification, while the empirical models provide little insight and are difficult to tailor to specific reactants. RESULTS: We directly estimate the initial amount of amplicon using a simplified mechanistic model based on chemical reactions in the annealing step of the PCR. The basic model includes the duplication of DNA with the digestion of Taqman probe and the re-annealing between previously synthesized DNA strands of opposite orientation. By modelling the amount of Taqman probe digested and matching that with the observed fluorescence, the conversion factor between the number of fluorescing dye molecules and observed fluorescent emission can be estimated, along with the absolute initial amount of amplicon and the rate parameter for re-annealing. The model is applied to several PCR reactions with known amounts of amplicon and is shown to work reasonably well. An expanded version of the model allows duplication of amplicon without release of fluorescent dye, by adding 1 more parameter to the model. The additional process is helpful in most cases where the initial primer concentration exceeds the initial probe concentration. Software for applying the algorithm to data may be downloaded at http://www.niehs.nih.gov/research/resources/software/pcranalyzer/ CONCLUSION: We present proof of the principle that a mechanistically based model can be fit to observations from a single PCR amplification. Initial amounts of amplicon are well estimated without using a standard solution. Using the ratio of the predicted initial amounts of amplicon from 2 PCRs is shown to work well even when the absolute amounts of amplicon are underestimated in the individual PCRs.

The temporal dynamics of ozone-induced FEV1 changes in humans: an exposure-response model.

Although ozone is known to induce reversible decrements in forced expiratory volume in 1 s (FEV1), no exposure-response model has been identified that accurately describes the dynamics of response to the changing concentrations and activity patterns of normal ambient human exposure. The purpose of the current analysis was to identify and evaluate a dynamic model of FEV1 response using a large existing data set (541 volunteers, 864 exposures, 3485 FEV1 measures) with a wide range of exposure conditions (ozone = 0.0 to 0.4 ppm, activity level = rest to heavy exercise, duration = 1 to 7.6 h), including recovery in clean air. A previously described model containing a differential equation and a logistic function was modified to include a new between-subjects variance structure and was fitted to the data. The model described well the mean observed response data across the range of exposure conditions, including the periods of recovery in clean air. Predicted values of individual responses were distributed lognormally and appeared to accurately describe the distribution of observed responses. We observed that responsiveness to ozone decreased with age, that response was weakly related to body size, and that response was marginally more sensitive to changes in ozone concentration than to changes in minute ventilation. In summary, we have identified a dynamic ozone exposure-response model that accurately describes the temporal pattern of FEV1 response to a wide range of changing exposure conditions and that may have utility for predicting population responses to ambient exposures.

Application of a statistical dynamic model investigating the short-term cellular kinetics induced by riddelliine, a hepatic endothelial carcinogen.

In recent studies, riddelliine, a pyrrolizidine alkaloid, was found to increase rates of replication and apoptosis and induce hemangiosarcoma in the liver of rats and mice. To analyze DNA replication and apoptosis data taken from the same animals, we have developed a predictive mathematical model for describing BrdU labeling and apoptotic processes. The model allows the incorporation of simple diurnal patterns in cellular kinetics and is applied to data on hepatocytes and endothelial cells taken from riddelliine exposed rats. Predictions from the model were used with multivariable nonlinear regression techniques to estimate replication and apoptotic rate constants for both cell types and all treatment groups. Hypothesis tests were used with the predicted rates to separate the competing effects of riddelliine on replication and apoptosis of hepatocytes and endothelial cells as well as compare replication rates between cell types. That estimated replication rates were found to be significantly higher for endothelial cells supports the supposition of induction of hemangiosarcoma by riddelliine in the liver.