Dose-Response Modeling with Summary Data from Developmental Toxicity Studies.

Dose-response analysis of binary developmental data (e.g., implant loss, fetal abnormalities) is best done using individual fetus data (identified to litter) or litter-specific statistics such as number of offspring per litter and proportion abnormal. However, such data are not often available to risk assessors. Scientific articles usually present only dose-group summaries for the number or average proportion abnormal and the total number of fetuses. Without litter-specific data, it is not possible to estimate variances correctly (often characterized as a problem of overdispersion, intralitter correlation, or "litter effect"). However, it is possible to use group summary data when the design effect has been estimated for each dose group. Previous studies have demonstrated useful dose-response and trend test analyses based on design effect estimates using litter-specific data from the same study. This simplifies the analysis but does not help when litter-specific data are unavailable. In the present study, we show that summary data on fetal malformations can be adjusted satisfactorily using estimates of the design effect based on historical data. When adjusted data are then analyzed with models designed for binomial responses, the resulting benchmark doses are similar to those obtained from analyzing litter-level data with nested dichotomous models.

Comparison of false-positive rates of 2 hypothesis-test approaches in relation to laboratory toxicity test performance.

We compared 2 statistical hypothesis-test approaches (no-observed-effect concentration [NOEC] and test of significant toxicity [TST]) to determine the influence of laboratory test performance on the false-positive error rate using the US Environmental Protection Agency's Ceriodaphnia dubia reproduction whole-effluent toxicity (WET) test endpoint. Simulation and power calculations were used to determine error rates based on observed control coefficients of variation (CV) for 8 laboratories over a range of effect levels. Average C. dubia control reproduction among laboratories was 20 to 40 offspring per female, and the 75th percentile CV was 0.10 to 0.31, reflecting a range in laboratory performance. The 2 approaches behave similarly for CVs of 0.2 to 0.3. At effects <10%, as CV decreases, TST is less likely to declare toxicity and NOEC is more likely to do so. Laboratory performance affects whether a sample is declared toxic and influences the probability of false-positive (and -negative) error rates using either approach. At the 75th percentile control CV observed for each laboratory, 4 laboratories would achieve approximately a 5% false-positive rate using 13 or fewer replicates for this test method. For the remaining 4 laboratories, more replicates would be needed to achieve a 5% false-positive rate. The present analyses demonstrate how false-positive rates are influenced by laboratory performance and WET test design. Environ Toxicol Chem 2019;38:511-523. Published 2019 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Sensitivity analysis of biologically motivated model for formaldehyde-induced respiratory cancer in humans.

Conolly et al. (2003, 2004) developed biologically motivated models of formaldehyde carcinogenicity in F344 rats and humans based on a two-stage clonal expansion model of cancer. Based on the human model, Conolly et al. (2004) claimed that cancer risks associated with inhaled formaldehyde are deminimis at relevant human exposure levels. However, they did not conduct a sensitivity analysis to evaluate the robustness of this conclusion. Here, we present a limited sensitivity analysis of the formaldehyde human model. We show that when the control animals from the National Toxicology Program (NTP) studies are replaced with control animals only from NTP inhalation studies, estimates of human risk are increased by 50-fold. When only concurrent control rats are used, the model does not provide any upper bound (UB) to human risk. No data went into the model on the effect of formaldehyde on the division rates and death rates of initiated cells. We show that slight numerical perturbations to the Conolly et al. assumptions regarding these rates can be made that are equally consistent with the underlying data used to construct the model, but produce estimates of human risk ranging anywhere from negative up to 10,000 times higher than those deemed by Conolly et al. to be 'conservative'. Thus, we conclude that estimates of human risk by Conolly et al. (2004) are extremely sensitive to modeling assumptions. This calls into question the basis for the Conolly et al. claim of de minimis human risk and suggests caution in using the model to derive human exposure standards for formaldehyde.

Uncertainties in biologically-based modeling of formaldehyde-induced respiratory cancer risk: identification of key issues.

In a series of articles and a health-risk assessment report, scientists at the CIIT Hamner Institutes developed a model (CIIT model) for estimating respiratory cancer risk due to inhaled formaldehyde within a conceptual framework incorporating extensive mechanistic information and advanced computational methods at the toxicokinetic and toxicodynamic levels. Several regulatory bodies have utilized predictions from this model; on the other hand, upon detailed evaluation the California EPA has decided against doing so. In this article, we study the CIIT model to identify key biological and statistical uncertainties that need careful evaluation if such two-stage clonal expansion models are to be used for extrapolation of cancer risk from animal bioassays to human exposure. Broadly, these issues pertain to the use and interpretation of experimental labeling index and tumor data, the evaluation and biological interpretation of estimated parameters, and uncertainties in model specification, in particular that of initiated cells. We also identify key uncertainties in the scale-up of the CIIT model to humans, focusing on assumptions underlying model parameters for cell replication rates and formaldehyde-induced mutation. We discuss uncertainties in identifying parameter values in the model used to estimate and extrapolate DNA protein cross-link levels. The authors of the CIIT modeling endeavor characterized their human risk estimates as "conservative in the face of modeling uncertainties." The uncertainties discussed in this article indicate that such a claim is premature.

Pollen limitation of reproductive effort in willows.

Pollen limitation of seed set differs from resource limitation in its implications for the evolution of floral traits. Willow flowers attract insects, but also abundantly produce wind-dispersed pollen. I demonstrated pollen limitation in single branches bearing 2-4 inflorescences (catkins) in a field experiment with five species by artificially increasing or decreasing the pollen load. Because the responses by single branches might be explained by diversion of resources to better-pollinated branches within a plant, a second experiment with one species tested both pollen limitation of whole plants and the autonomy of catkins. Seed set of single willow catkins is unaffected by experimental alterations of seed set in other catkins on the same plant. Hand-pollination of single catkins and of whole plants increased seed set to the same degree, suggesting there is little or no competition for resources between catkins only 5-10 cm apart. Thus, seed set in willows appears to be pollen limited, favoring insect pollination and the evolution of entomophilous traits. The data support previous views that willows have a dual pollination system utilizing wind and insects.

Forest fires and the snowshoe hare-Canada lynx cycle.

This paper shows that there is a reasonable coincidence between the Canada lynx cycle and the occurrence of forest and brush fires. Fires set in motion plant succession, potentially leading to an increase in snowshoe hares (Grange, 1965). Snowfall is also correlated with the lynx cycle and tends to account for the variation not accounted for by fires. I conclude that Grange's (1949, 1965) hypothesis that fire and plant succession drive the snowshoe hare cycle deserves serious consideration, as do Butler's (1962) and Watt's (1968, 1973) suggestions that precipitation may drive or decisively modify furbearer 'cycles'. On this evidence, the snowshoe hare-Canada lynx cycle seems likely to be a forced oscillation rather than a predator-prey, parasitehost, or herbivore-vegetation limit cycle, as proposed by several authors. Fire clearly appear to be a periodic phenomenon, not-withstanding the difficulty of showing periodicity in simple weather time series. Fire is itself a meteorological phenomenon, although complexly related to simple weather variables.

Size and sex allocation in monoecious woody plants.

The female size advantage hypothesis predicts that the allocation ratio of female: male reproductive effort should increase with plant size (total reproductive effort). A male height advantage hypothesis has also been proposed, based on the supposed greater advantage of height to male reproductive success in wind-pollinated plants. These ideas were tested with data for wind-pollinated, monoecious trees and shrubs which exhibit a suitably large range of sizes. Number of male inflorescences increased faster with size than did number of female inflorescences in 2 of 9 species; in the remaining 7 species there was no significant difference. The male:female ratio of inflorescence numbers increased with height in 4 of 7 species and did not change significantly in the remaining 3 species, as shown by regression. Height and size are highly correlated and so their effects could not be distinguished. The fact that many conifers place the female cones uppermost in the crown suggests that size and not height favors increased allocation to male function, as does well-established theory connecting the existence of male versus female size advantage to pollen and seed dispersal chacteristics. Regression analysis of the relation between male and female reproductive effort should be done by reduced major axis regression; ordinary least squares regression underestimates slopes; in this study opposite conclusions could be drawn from ordinary least squares and reduced major axis regressions.

Instability of the snowshoe hare and woody plant interaction.

We show that Alaskan woody plants respond to browsing in two ways that might destabilize a plantherbivore interaction and account for snowshoe hare population 'cycles'. (1) Browse production of preferred, earliersuccessional woody plants increases in response to moderate levels of browsing. Such yield increases are potentially destabilizing. Later successional woody plants show decreases in yield after moderate browsing, which is consistent with the persistence of snowshoe hares in late successional 'refuge' habitats (Keith 1966, Wolff 1980). (2) Many woody plants are destructively overbrowsed or girdled at the peak of the snowshoe hare cycle. The more palatable and plastic, early to mid successional plants respond by sprouting accompanied by juvenile reversion. Sprouts are markedly less palatable than mature shoots. We show here that sprout palatability and twig biomass are restored in 2-3 years for earlier successional plants, but palatability may not recover for 4-10 years in sprouts of some mid to late successional plants. The decrease in palatability helps to account for the snowshoe hare 'crash' (assuming that damage to more palatable plants is widespread during the 'peak'), and the 2-3 year time lag for recovery of more palatable species could account for (May 1974) the observed 8-11 year period of the hare cycles. Browse yield increases acting during the snowshoe hare population nadir and increase, and sprouting with juvenile reversion acting during the hare peak and decline can in principle account for the oscillatory nature and the observed 8-11 year periodicity of the snowshoe hare cycle.

Lack of induced chemical defense in juvenile Alaskan woody plants in response to simulated browsing.

Juvenile individuals of five species of Alaskan trees and shrubs were clipped at six levels of intensity and sampled for resin, tannin, nutrient, and carbohydrate content. Clipping caused no induction of putative defensive compounds (resins or tannins), indicating that the high level of resin found in many Alaskan woody plants in response to browsing is best explained as a reversion to a juvenile growth stage rather than an induction of a specific chemical defense.

Enhancing toxicity test performance by using a statistical criterion.

Aquatic toxicity tests are laboratory experiments that measure the biological effect (e.g., growth, survival, reproduction) of effluents, receiving waters, or storm water on aquatic organisms. These toxicity tests must be performed using the best laboratory practices, and every effort must be made to enhance repeatability of the test method. We evaluated the generated reference toxicant test data for insurance of a level of quality assurance for tests over time within a laboratory and among laboratories. We recommend the reporting and evaluation of the percent minimum significant difference (PMSD) value for all toxicity test results. The minimum significant difference (MSD) represents the smallest difference between the control mean and a treatment mean that leads to the statistical rejection of the null hypothesis (i.e., no toxicity) at each concentration of the toxicity test dilution series. The MSD provides an indication of within-test variability, and smaller values of MSD are associated with increased power to detect a toxic effect. We recommend upper and lower PMSD bounds for each test method in order to minimize within-test variability and increase statistical power. To ensure that PMSD does not exceed an upper bound, testing laboratories may need to increase replication, decrease variability among replicates, or increase the control mean performance.

Human health effects of trichloroethylene: key findings and scientific issues.

BACKGROUND: In support of the Integrated Risk Information System (IRIS), the U.S. Environmental Protection Agency (EPA) completed a toxicological review of trichloroethylene (TCE) in September 2011, which was the result of an effort spanning > 20 years. OBJECTIVES: We summarized the key findings and scientific issues regarding the human health effects of TCE in the U.S. EPA's toxicological review. METHODS: In this assessment we synthesized and characterized thousands of epidemiologic, experimental animal, and mechanistic studies, and addressed several key scientific issues through modeling of TCE toxicokinetics, meta-analyses of epidemiologic studies, and analyses of mechanistic data. DISCUSSION: Toxicokinetic modeling aided in characterizing the toxicological role of the complex metabolism and multiple metabolites of TCE. Meta-analyses of the epidemiologic data strongly supported the conclusions that TCE causes kidney cancer in humans and that TCE may also cause liver cancer and non-Hodgkin lymphoma. Mechanistic analyses support a key role for mutagenicity in TCE-induced kidney carcinogenicity. Recent evidence from studies in both humans and experimental animals point to the involvement of TCE exposure in autoimmune disease and hypersensitivity. Recent avian and in vitro mechanistic studies provided biological plausibility that TCE plays a role in developmental cardiac toxicity, the subject of substantial debate due to mixed results from epidemiologic and rodent studies. CONCLUSIONS: TCE is carcinogenic to humans by all routes of exposure and poses a potential human health hazard for noncancer toxicity to the central nervous system, kidney, liver, immune system, male reproductive system, and the developing embryo/fetus.