Octamethylcyclotetrasiloxane (D) is a volatile cyclic siloxane used primarily as a monomer or intermediate in the production of some silicon-based polymers widely used in industrial and consumer applications and may be present as a residual impurity in a variety of consumer products. A robust toxicological data set exists for D Treatment-related results from a chronic inhalation study conducted in rats are limited to mild effects on the respiratory tract, increases in liver weight, increases in the incidence of uterine endometrial epithelial hyperplasia, and a dose-related trend in the incidence of endometrial adenomas. The observed increases in liver weight appear to be related to the induction of hepatic metabolizing enzymes, similar to those that are induced in the presence of phenobarbital. D is not mutagenic or genotoxic in standard in vitro and in vivo tests; therefore, the benign uterine tumors observed likely occur by a non-genotoxic mechanism. Results from mechanistic studies suggest that D has very weak estrogenic and antiestrogenic activity, as well as dopamine agonist-like activity. In rats, D exposure delays ovulation and hypothesized to prolong exposure of the uterine endometrium to endogenous estrogen. Though this mode of action may play a role in the development of benign uterine tumors in the rat, it is considered unlikely to occur in the human due to the marked differences in cycle regulatory mechanisms. Reproductive effects were observed following D exposure in female rats. These effects appear to be related to a delay of the luteinizing hormone (LH) surge, which fails to induce complete ovulation in the rat. However, based on differences in ovulatory control in rats and humans, it appears these effects may be species-specific with no risk or relevance to human health. Results from pharmacokinetic studies indicate that dermal absorption of D is limited, due to its high volatility and, if absorbed via dermal, oral or inhalation exposure, the majority of D is rapidly cleared from the body, indicating bioaccumulation is unlikely.

No abstract

Methyl salicylate is the predominant constituent of oil of wintergreen and is used as a pesticide, a denaturant, an external analgesic, a fragrance ingredient, and a flavoring agent in products such as chewing gum, baked goods, syrups, candy, beverages, ice cream, and tobacco products; and it occurs naturally in some vegetables and berries. Methyl salicylate is of interest to the tobacco industry as oil of wintergreen is used as a flavorant in tobacco products. The purpose of this investigation was to conduct a critical review of the available literature for oral exposure to methyl salicylate, incorporating an analysis of the quality of the studies available and the current understanding of the mode of action. Following a review of all of the available literature, the most appropriate data sets for dose-response modeling were reported by Gulati et al. in which significant changes in reproductive/development endpoints were reported to occur after exposure to 500 mg/kg/d of methyl salicylate in male and female mice. Benchmark dose modeling was performed and the most sensitive endpoint, the number of litters per mating pair, was associated with a BMDL of 220 mg/kg/d. This BMDL was chosen as the point of departure and adjusted by a body weight scaling factor to derive a human equivalent dose. Based on the uncertainty factor analysis, the POD for methyl salicylate was adjusted by a UF of 3 for interspecies uncertainty to derive an allowable daily intake of 11 mg/kg/d.

Formaldehyde is one of the most comprehensively studied chemicals, with over 30 years of research focused on understanding the development of cancer following inhalation. The causal conclusions regarding the potential for leukemia are largely based on the epidemiological literature, with little consideration of cancer bioassays, dosimetry studies, and mechanistic research, which challenge the biological plausibility of the disease. Recent reanalyzes of the epidemiological literature have also raised significant questions related to the purported associations between formaldehyde and leukemia. Because of this, considerable scientific debate and uncertainty remain on whether there is a causal association between formaldehyde inhalation exposure and leukemia. Further complexity in evaluating this association is related to the endogenous production of formaldehyde. Multiple modes of action (MOA) have been postulated for the development of leukemia following formaldehyde inhalation that includes unsupported hypotheses of direct or indirect toxicity to the target cell population. Herein, the available evidence relevant to evaluating the postulated MOAs for leukemia following formaldehyde inhalation exposure is organized in the IPCS MOA Framework. The integration of all the available evidence clearly highlights the limited amount of data that support any of the postulated MOAs and demonstrates a significant amount of research supporting the null hypothesis that there is no causal association between formaldehyde inhalation exposure and leukemia. These analyses result in a lack of confidence in any of the postulated MOAs, increasing confidence in the conclusion that there is a lack of biological plausibility for a causal association between formaldehyde inhalation exposure and leukemia.

Benzo[a]pyrene (BaP) is a by-product of incomplete combustion of fossil fuels and plant/wood products, including tobacco. A physiologically based pharmacokinetic (PBPK) model for BaP for the rat was extended to simulate inhalation exposures to BaP in rats and humans including particle deposition and dissolution of absorbed BaP and renal elimination of 3-hydroxy benzo[a]pyrene (3-OH BaP) in humans. The clearance of particle-associated BaP from lung based on existing data in rats and dogs suggest that the process is bi-phasic. An initial rapid clearance was represented by BaP released from particles followed by a slower first-order clearance that follows particle kinetics. Parameter values for BaP-particle dissociation were estimated using inhalation data from isolated/ventilated/perfused rat lungs and optimized in the extended inhalation model using available rat data. Simulations of acute inhalation exposures in rats identified specific data needs including systemic elimination of BaP metabolites, diffusion-limited transfer rates of BaP from lung tissue to blood and the quantitative role of macrophage-mediated and ciliated clearance mechanisms. The updated BaP model provides very good prediction of the urinary 3-OH BaP concentrations and the relative difference between measured 3-OH BaP in nonsmokers versus smokers. This PBPK model for inhaled BaP is a preliminary tool for quantifying lung BaP dosimetry in rat and humans and was used to prioritize data needs that would provide significant model refinement and robust internal dosimetry capabilities.

Objective: To develop a physiologically based pharmacokinetic (PBPK) model for chloroprene in the mouse, rat and human, relying only on in vitro data to estimate tissue metabolism rates and partitioning, and to apply the model to calculate an inhalation unit risk (IUR) for chloroprene. Materials and methods: Female B6C3F1 mice were the most sensitive species/gender for lung tumors in the 2-year bioassay conducted with chloroprene. The PBPK model included tissue metabolism rate constants for chloroprene estimated from results of in vitro gas uptake studies using liver and lung microsomes. To assess the validity of the PBPK model, a 6-hr, nose-only chloroprene inhalation study was conducted with female B6C3F1 mice in which both chloroprene blood concentrations and ventilation rates were measured. The PBPK model was then used to predict dose measuresamounts of chloroprene metabolized in lungs per unit timein mice and humans. Results: The mouse PBPK model accurately predicted in vivo pharmacokinetic data from the 6-hr, nose-only chloroprene inhalation study. The PBPK model was used to conduct a cancer risk assessment based on metabolism of chloroprene to reactive epoxides in the lung, the target tissue in mice. The IUR was over100-fold lower than the IUR from the EPA Integrated Risk Information System (IRIS), which was based on inhaled chloroprene concentration. The different result from the PBPK model risk assessment arises from use of the more relevant tissue dose metric, amount metabolized, rather than inhaled concentration Discussion and conclusions: The revised chloroprene PBPK model is based on the best available science, including new test animal in vivo validation, updated literature review and a Markov-Chain Monte Carlo analysis to assess parameter uncertainty. Relying on both mouse and human metabolism data also provides an important advancement in the use of quantitative in vitro to in vivo extrapolation (QIVIVE). Inclusion of the best available science is especially important when deriving a toxicity value based on species extrapolation for the potential carcinogenicity of a reactive metabolite.

Decamethylcyclopentasiloxane (D5) is a low-molecular-weight cyclic siloxane used primarily as an intermediate in the production of several widely-used industrial and consumer products and intentionally added to consumer products, personal products and some dry cleaning solvents. The global use requires consideration of consumer use information and risk assessment requirements from various sources and authoritative bodies. A global "harmonized" risk assessment was conducted to meet requirements for substance-specific risk assessments conducted by regulatory agencies such as USEPA's Integrated Risk Information System (IRIS), Health Canada and various independent scientific committees of the European Commission, as well as provide guidance for chemical safety assessments under REACH in Europe, and other relevant authoritative bodies. This risk assessment incorporates global exposure information combined with a Monte Carlo analysis to determine the most significant routes of exposure, utilization of a multi-species, multi-route physiologically based pharmacokinetic (PBPK) model to estimate internal dose metrics, benchmark modeling to determine a point of departure (POD), and a margin of safety (MOS) evaluation to compare the estimates of intake with the POD. Because of the specific pharmacokinetic behaviors of D5 including high lipophilicity, high volatility with low blood-to-air partition coefficients and extensive metabolic clearance that regulate tissue dose after exposure, the use of a PBPK model was essential to provide a comparison of a dose metric that reflects these processes. The characterization of the potential for adverse effects after exposure to D5 using a MOS approach based on an internal dose metric removes the subjective application of uncertainty factors that may be applied across various regulatory agencies and allows examination of the differences between internal dose metrics associated with exposure and those associated with adverse effects.

No abstract