Analysis for data-derived extrapolation factors for procymidone

The derivation of Chemical Specific Adjustment Factors (CSAFs) (IPCS, 2005; U.S. EPA, 2014) depends on the choice of appropriate dose metric. EPA and IPCS guidance was applied to derive a CSAF for developmental toxicity for procymidone (PCM). Although kinetic data were not available in humans at any dose, sufficient toxicokinetic data are available in a surrogate species, primates, and from chimeric mice with both rat and human liver cells to offer insights. Alternative approaches were explored in the derivation of the CSAG based on review of the available kinetic data. The most likely dosimetric adjustment is the Cmax based on the character of the critical effect - reduced anogenital distance and increased incidence of hypospadias in male rats, which likely occurs during a small window of time during development of the rat fetus. Cmax is also the default dosimeter from U.S. EPA (1991). However, in this case, the use of Cmax is also likely more conservative than the use of area under the curve (AUC), which otherwise is the default recommendation of the IPCS (2005). Despite human data, estimated tentative CSAF value is 0.48 (range, 0.22 to 0.74). The use of any of these values would be supported by the available data.

Data derived Extrapolation Factors for developmental toxicity: A preliminary research case study with perfluorooctanoate (PFOA)

Guidelines of the United States Environmental Protection Agency (EPA, 1991) and the International Programme on Chemical Safety (IPCS, 2005) suggest two different default positions for dosimetric extrapolation from experimental animals to humans when the dosimetry of the critical effect is not known. The default position of EPA (1991) for developmental toxicity is to use peak concentration (or Cmax) for this dosimetric extrapolation. In contrast, IPCS (2005, page 39) states its default position for dosimetric choice in the absence of data is to use the area under the curve (or AUC). The choice of the appropriate dose metric is important in the development of either a Chemical Specific Adjustment Factor (CSAF) of IPCS (2005) or a Data Derived Extrapolation Factor (DDEF) of EPA (2014). This research shows the derivation of a DDEF for developmental toxicity for perfluorooctanoate (PFOA), a chemical of current interest. Here, identification of the appropriate dosimetric adjustment from a review of developmental effects identified by EPA (2016) is attempted. Although some of these effects appear to be related to Cmax, most appear to be related to the average concentration or its AUC, but only during the critical period of development for a particular effect. A comparison was made of kinetic data from PFOA exposure in mice with newly available and carefully monitored kinetic data in humans after up to 36 weeks of PFOA exposure in a phase 1 clinical trial by Elcombe et al. (2013). Using the average concentration during the various exposure windows of concern, the DDEF for PFOA was determined to be 1.3 or 14. These values are significantly different than comparable extrapolations by several other authorities based on differences in PFOA half-life among species. Although current population exposures to PFOA are generally much lower than both the experimental animal data and the clinical human study, the development of these DDEFs is consistent with current guidelines of both EPA (2014) and IPCS (2005).

Application of the threshold of toxicological concern approach to ingredients in personal and household care products

In the absence of chemical-specific data, the threshold of toxicological concern (TTC) provides a method to determine a conservative estimate of a chronic oral exposure below which there is a very low probability of risk. The TTC approach was originally developed to support exposures to indirect food additives and was based on linear low-dose risk estimates to assure protection in the event that the chemical was later determined to be a carcinogen. Subsequently, TTC values based on noncancer endpoints were proposed for chemicals without structural alerts for genotoxicity. The original database supporting the TTC values for noncancer endpoints includes >600 structurally diverse chemicals. The objectives of this work were to evaluate the applicability of the TTC database to ingredients used in consumer products based on a comparison of the diversity of chemical structures with those in the original TTC database and to confirm that the range of NOELs for these ingredients is consistent with the range of NOELs in the original database. The results show good coverage of the product ingredient structures and confirm that the NOELs for the ingredient chemicals are similar in range to the original dataset, supporting the use of the TTC for ingredients in consumer products.

Combination of cancer data in quantitative risk assessments: case study using bromodichloromethane

There are often several data sets that may be used in developing a quantitative risk estimate for a carcinogen. These estimates are usually based, however, on the dose-response data for tumor incidences from a single sex/strain/species of animal. When appropriate, the use of more data should result in a higher level of confidence in the risk estimate. The decision to use more than one data set (e.g., representing different animal sexes, strains, species, or tumor sites) can be made following biological and statistical analyses of the compatibility of the these data sets. Biological analysis involves consideration of factors such as the relevance of the animal models, study design and execution, dose selection and route of administration, the mechanism of action of the agent, its pharmacokinetics, any species- and/or sex-specific effects, and tumor site specificity. If the biological analysis does not prohibit combining data sets, statistical compatibility of the data sets is then investigated. A generalized likelihood ratio test is proposed for determining the compatibility of different data sets with respect to a common dose-response model, such as the linearized multistage model. The biological and statistical factors influencing the decision to combine data sets are described, followed by a case study of bromodichloromethane.

Biological considerations for combining carcinogenicity data for quantitative risk assessment

Quantitative risk assessments for carcinogens conducted by the U.S. EPA have most frequently been based on results of a bioassay from a single sex/strain/species of animal. In some cases, more than one data set derived from different sexes, strains, or species of animals is suitable for quantitative risk assessment. When there is no apparent difference among the data sets in sensitivity to the carcinogen, use of more of the available data should result in a higher level of confidence in the risk estimate. Several biological factors must be considered before combining data from different animal sexes, strains, species, or tumor sites. The relevance of the animal models, study design and execution, dose selection, and route of administration are factors which influence whether data from separate studies should be combined. The decision to combine data sets is also based on what is known of the mechanism of action of the agent, its pharmacokinetics, any species/sex specificity of the effect, and considerations regarding tumor site specificity. Statistical analysis also indicates whether the data sets may be described by the same multistage model. The evaluation of these factors in the decision to combine or not combine data sets is discussed.

Simultaneous measurement of tyrosine, tryptophan and related monoamines for determination of neurotransmitter turnover in discrete rat brain regions by liquid chromatography with electrochemical detection

Concomitant measurement of monoamine neurotransmitter turnover in discrete rat brain areas with the use of radiolabeled amino acid precursors permits simultaneous evaluation of interacting transmitter systems. [3H]Tyrosine and [3H]tryptophan were administered via indwelling catheters to unrestrained rats. Content and specific activity of norepinephrine, dopamine, 5-hydroxytryptamine, and the metabolites dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid in addition to tyrosine and tryptophan were quantified by liquid chromatography with electrochemical detection and liquid scintillation counting. The method employs a simple extraction procedure without prior cleanup for chromatography. Neurotransmitter turnover rates that incorporated tyrosine- or tryptophan-specific activities were found to be two to four times greater than those that did not include them.

Male reproductive toxicity and recovery associated with acute ethoxyethanol exposure in rats

Effects of 2-ethoxyethanol (EE) on semen parameters in male rats were investigated employing an animal model that allowed assessment of toxicity and recovery in the same animal. Prior to exposure, 70-d-old Long-Evans hooded males were placed with ovariectomized, hormonally primed females on several occasions and their copulatory behaviors were monitored and scored. At 100 d of age, these males were mated with females that were sacrificed 15 min postejaculation. The semen sample was recovered from the female reproductive tract and scored for sperm motility, sperm count, and abnormal sperm morphology. Following this preexposure baseline assessment, the males were intubated with 0, 936, 1872, or 2808 mg EE/kg for 5 consecutive days. The males were mated weekly for the next 14 wk. Copulatory behaviors were monitored and ejaculated semen samples analyzed on wk 1, 4, 7, 10, and 14. The males were sacrificed at wk 16 and the testes and epididymides were processed for histological evaluation. Data analyses indicated that EE produced a rapid decline in sperm counts in the two highest groups, with most of the males becoming azoospermic by wk 7. The males in the low dose group also exhibited a significant decrease in sperm counts at this week. Additionally, there was a significant increase in abnormal sperm morphology at wk 7 in the low-dose males. Partial or complete recovery was apparent in the sperm parameters by wk 14, as evidenced by an increase in sperm counts and a decrease in abnormal morphology and further supported by epididymal and testicular histological assessment at wk 16. At sacrifice, there were no significant differences between groups on body weights, organ weights, or epididymal sperm counts, except for a significant depression of epididymal weight in the middle dose group. While high doses of EE produced a decline in sperm counts starting after the first week of exposure, the early spermatid-late spermatocyte stages, represented by mature spermatozoa in the wk 7 ejaculates, appeared to be particularly sensitive to this compound. Moreover, most of the males exhibited recovery following this acute dosing regimen.

Evaluation of propylene oxide for mutagenic activity in 3 in vivo test systems

Propylene oxide (CAS No. 75-56-9) was tested for mutagenic activity following vapor exposure using 3 in vivo test systems. Rat dominant lethal and mouse sperm-head morphology assays were conducted using males exposed to propylene oxide at 300 ppm in a dynamic exposure chamber for 7 h per day on 5 consecutive days. A sex-linked recessive lethal test in Drosophila melanogaster employed a 24-h static exposure to propylene oxide at 645 ppm. Male mice were killed 1, 3, 5, 7, and 9 weeks post-exposure for evaluation of sperm-head morphology. Propylene oxide exposure did not result in an increase in abnormal forms. Male rats were mated with 2 virgin females per week for 6 weeks following exposure. A statistically significant increase in preimplantation losses and a statistically significant reduction in the number of living implants in the first post-exposure week did not appear to be treatment related. A highly significant increase in sex-linked recessive lethal mutations was observed in two germ cell stages (mature sperm and developing spermatocytes). These results warrant continued caution in potential human exposure to propylene oxide.