Assessing Modes of Action, Measures of Tissue Dose and Human Relevance of Rodent Toxicity Endpoints with Octamethylcyclotetrasiloxane (D4)

Octamethylcyclotetrasiloxane (D4) lacks endocrine disruptive potential via estrogen pathways

Octamethylcyclotetrasiloxane (D4) is a volatile, highly lipophilic monomer used to produce silicone polymers found in many consumer products and used widely in industrial applications and processes. Many reviews of the toxicology of D4 conclude that its adverse effects on endocrine-sensitive endpoints occur by a MoA dependent on systemic toxicity rather than one mediated via endocrine activity, but others identify D4 as an estrogenic endocrine disruptive chemical (EDC) based on results of screening-level assays indicating that D4 interacts with ERα and at high doses, affects estrogen-sensitive endpoints in rodents. To resolve these divergent interpretations, we tested two specific hypotheses related to the interaction of D4 with estrogen receptor-alpha subtype (ERα) at the biochemical and molecular levels of biological organization and a third specific hypothesis related to estrogenic and anti-estrogenic pathways at the physiological level. At the physiological level, we used an established WoE methodology to evaluate all data relevant to estrogen agonist and antagonist activity of D4 by examining its effects on ERα-relevant endpoints in rodent toxicology studies. At the biochemical level, we calculated whether D4 could produce a functionally significant change in the ERα occupancy by 17β-estradiol (E2) using equations well-established in pharmacology. For these calculations, we used data on the potency and kinetics of D4 from studies in rats as well as published potency and affinity data on endogenous estrogens and their circulating concentrations in humans. At the molecular level, we used established molecular docking techniques to evaluate the potential for D4 and related chemicals to fit within and to activate or block the binding pocket of ERα. Our analyses indicate that the estrogenic effect of D4 is molecularly, biochemically, and physiologically implausible, which corroborates previous evaluations of D4 that concluded it is not an estrogenic endocrine disruptor. The claim that D4 exhibits estrogenic endocrine disruptive properties based on a presumed link between the results of screening-level assays (RUA and ERTA) and adverse effects is not supported by the data and relies on deficient evaluative and interpretative methods. Instead, a plausible mechanistic explanation for the various adverse effects of D4 observed in rodent studies, including its effects in reproduction studies, is that these are secondary to high-dose-dependent, physico-chemical effects that perturb cell membrane function and produce rodent-specific sensory irritation.

Kinetically-derived maximal dose (KMD) confirms lack of human relevance for high-dose effects of octamethylcyclotetrasiloxane (D4)

The kinetically-derived maximal dose (KMD) is defined as the maximum external dose at which kinetics are unchanged relative to lower doses, e.g., doses at which kinetic processes are not saturated. Toxicity produced at doses above the KMD can be qualitatively different from toxicity produced at lower doses. Here, we test the hypothesis that high-dose-dependent toxicological effects of octamethylcyclotetrasiloxane (D4) occur secondary to kinetic overload. Octamethylcyclotetrasiloxane (D4) is a volatile, highly lipophilic monomer used to produce silicone polymers, which are ingredients in many consumer products and used widely in industrial applications and processes. Chronic inhalation at D4 concentrations 104 times greater than human exposures produces mild effects in rat respiratory tract, liver weight increase and pigment accumulation, nephropathy, uterine endometrial epithelial hyperplasia, non-significant increased uterine endometrial adenomas, and reduced fertility secondary to inhibition of rat-specific luteinizing hormone (LH) surge. Mechanistic studies indicate a lack of human relevance for most of these effects. Respiratory tract effects arise in rats due to direct epithelial contact with mixed vapor/aerosols and increased liver weight is a rodent-specific adaptative induction of drug-metabolizing hepatic enzymes. D4 is not mutagenic or genotoxic, does not interact with dopamine receptors, and interacts at ERα with potency insufficient to cause uterine effects or to alter the LH surge in rats. These mechanistic findings suggest high-dose-dependence of the toxicological effects secondary to kinetic overload, a hypothesis that can be tested when appropriate kinetic data are available that can be probed for the existence of a KMD. We applied Bayesian analysis with differential equations to information from kinetic studies on D4 to build statistical distributions of plausible values of the Km and Vmax for D4 elimination. From those distributions of likely Km and Vmax values, a set of Michaelis-Menten equations were generated that are likely to represent the slope function for the relationship between D4 exposure and blood concentration. The resulting Michaelis-Menten functions were then investigated using a change-point methodology known as the "kneedle" algorithm to identify the probable KMD range. We validated our Km and Vmax using out of sample data. Analysis of the Michaelis-Menten elimination curve generated from those Vmax and Km values indicates a KMD with an interquartile range of 230.0-488.0 ppm [2790-5920 mg/m3; 9.41-19.96 µM]. The KMD determined here for D4 is consistent with prior work indicating saturation of D4 metabolism at approximately 300 ppm [3640 mg/m3; 12.27 µM] and supports the hypothesis that many adverse effects of D4 arise secondary to high-dose-dependent events, likely due to mechanisms of action that cannot occur at concentrations below the KMD. Regulatory methods to evaluate D4 for human health protection should avoid endpoint data from rodents exposed to D4 above the KMD range and future toxicological testing should focus on doses below the KMD range.

Empirical analysis of lead neurotoxicity mode of action and its application in health risk assessment

The mode of action (MOA) framework is proposed to inform a biological link between chemical exposures and adverse health effects. Despite a significant increase in knowledge and awareness, the application of MOA in human health risk assessment (RA) remains limited. This study aims to discuss the adoption of MOA for health RA within a regulatory context, taking our previously proposed but not yet validated MOA for lead neurotoxicity as an example. We first conducted a quantitative weight of evidence (qWOE) assessment, which revealed that the MOA has a moderate confidence. Then, targeted bioassays were performed within an in vitro blood-brain barrier (BBB) model to quantitatively validate the scientific validity of key events (KEs) in terms of essentiality and concordance of empirical support (dose/temporal concordance), which increases confidence in utilizing the MOA for RA. Building upon the quantitative validation data, we further conducted benchmark dose (BMD) analysis to map dose-response relationships for the critical toxicity pathways, and the lower limit of BMD at a 5% response (BMDL5) was identified as the point of departure (POD) value for adverse health effects. Notably, perturbation of the Aryl Hydrocarbon Receptor (AHR) signaling pathway exhibited the lowest POD value, measured at 0.0062 μM. Considering bioavailability, we further calculated a provisional health-based guidance value (HBGV) for children's lead intake, determining it to be 2.56 μg/day. Finally, the health risk associated with the HBGV was assessed using the hazard quotient (HQ) approach, which indicated that the HBGV established in this study is a relative safe reference value for lead intake. In summary, our study described the procedure for utilizing MOA in health RA and set an example for MOA-based human health risk regulation.