A minimal human physiologically based kinetic model of thyroid hormones and chemical disruption of plasma thyroid hormone binding proteins

Effects of pethoxamid treatment on the disposition of thyroxine in rats

Pethoxamid (PXA) is a chloroacetamide herbicide that works by inhibiting the germination of target weeds in crops. PXA is not a genotoxic agent, however, in a two-year chronic toxicity study, incidence of thyroid follicular cell hyperplasia was observed in male rats treated at a high dose. Many non-mutagenic chemicals, including agrochemicals are known to produce thyroid hyperplasia in rodents through a hepatic metabolizing enzyme induction mode of action (MoA). In this study, the effects of oral gavage PXA treatment at 300 mg/kg for 7 days on the disposition of intravenously (iv) administered radio-labeled thyroxine ([125I]-T4) was assessed in bile-duct cannulated (BDC) rats. Another group of animals were treated with phenobarbital (PB, 100 mg/kg), a known enzyme inducer, serving as a positive control. The results showed significant increase (p < 0.01) in the mean liver weights in the PB and PXA-treated groups relative to the control group. The serum total T4 radioactivity Cmax and AUC0-4 values for PB and PXA-treated groups were lower than for the control group, suggesting increased clearance from serum. The mean percentages of administered radioactivity excreted in bile were 7.96 ± 0.38%, 16.13 ± 5.46%, and 11.99 ± 2.80% for the control, PB and PXA groups, respectively, indicating increased clearance via the bile in the treated animals. These data indicate that PXA can perturb the thyroid hormone homeostasis in rats by increasing T4 elimination in bile, possibly through enzyme induction mechanism similar to PB. In contrast to humans, the lack of high affinity thyroid binding globulin (TBG) in rats perhaps results in enhanced metabolism of T4 by uridine diphosphate glucuronosyl transferase (UGT). Since this liver enzyme induction MoA for thyroid hyperplasia by PB is known to be rodent specific, PXA effects on thyroid can also be considered not relevant to humans. The data from this study also suggest that incorporating a BDC rat model to determine thyroid hormone disposition using [125I]-T4 is valuable in a thyroid mode of action analysis.

Spatially Dependent Tissue Distribution of Thyroid Hormones by Plasma Thyroid Hormone Binding Proteins

Plasma thyroid hormone (TH) binding proteins (THBPs), including thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin (ALB), carry THs to extrathyroidal sites, where THs are unloaded locally and then taken up via membrane transporters into the tissue proper. The respective roles of THBPs in supplying THs for tissue uptake are not completely understood. To investigate this, we developed a spatial human physiologically based kinetic (PBK) model of THs, which produces several novel findings. (1) Contrary to postulations that TTR and/or ALB are the major local T4 contributors, the three THBPs may unload comparable amounts of T4 in Liver, a rapidly perfused organ; however, their contributions in slowly perfused tissues follow the order of abundances of T4TBG, T4TTR, and T4ALB. The T3 amounts unloaded from or loaded onto THBPs in a tissue acting as a T3 sink or source respectively follow the order of abundance of T3TBG, T3ALB, and T3TTR regardless of perfusion rate. (2) Any THBP alone is sufficient to maintain spatially uniform TH tissue distributions. (3) The TH amounts unloaded by each THBP species are spatially dependent and nonlinear in a tissue, with ALB being the dominant contributor near the arterial end but conceding to TBG near the venous end. (4) Spatial gradients of TH transporters and metabolic enzymes may modulate these contributions, producing spatially invariant or heterogeneous TH tissue concentrations depending on whether the blood-tissue TH exchange operates in near-equilibrium mode. In summary, our modeling provides novel insights into the differential roles of THBPs in local TH tissue distribution.