Selective retention of toxic polychlorinated dibenzo-p-dioxins and dibenzofurans in the liver of the rat after intravenous administration of a mixture

The 2022 world health organization reevaluation of human and mammalian toxic equivalency factors for polychlorinated dioxins, dibenzofurans and biphenyls

In October 2022, the World Health Organization (WHO) convened an expert panel in Lisbon, Portugal in which the 2005 WHO TEFs for chlorinated dioxin-like compounds were reevaluated. In contrast to earlier panels that employed expert judgement and consensus-based assignment of TEF values, the present effort employed an update to the 2006 REP database, a consensus-based weighting scheme, a Bayesian dose response modeling and meta-analysis to derive "Best-Estimate" TEFs. The updated database contains almost double the number of datasets from the earlier version and includes metadata that informs the weighting scheme. The Bayesian analysis of this dataset results in an unbiased quantitative assessment of the congener-specific potencies with uncertainty estimates. The "Best-Estimate" TEF derived from the model was used to assign 2022 WHO-TEFs for almost all congeners and these values were not rounded to half-logs as was done previously. The exception was for the mono-ortho PCBs, for which the panel agreed to retain their 2005 WHO-TEFs due to limited and heterogenous data available for these compounds. Applying these new TEFs to a limited set of dioxin-like chemical concentrations measured in human milk and seafood indicates that the total toxic equivalents will tend to be lower than when using the 2005 TEFs.

Assessment of physical traits of rat offspring derived from mothers exposed to dioxin

Negative effects of dioxin action are associated with limited abilities of their bio-degradation along with continuously increasing production and long-term bio-accumulation of those toxins in living organisms. Dioxins penetrate through placenta to fetus indicating indirect toxic effects on offspring of mothers exposed to the action of these toxins. During lactation a significant part of dioxins is excreted from organism with milk, which contributes to further accumulation of those compounds and multiple exceeding of maximal permissible dose of dioxins in newborns feeding with mother's milk. The aim of the study was to determine how a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administered to females 3 weeks before getting pregnant affects the parturition duration, labour delay, number and weight of offspring. The studies were performed on rat offspring deriving from primigravida females from the Buffalo strain, which 3 weeks before getting pregnant were administered with a single dose of TCDD. The obtained results revealed that labours in females exposed to dioxin effects were characterized by significant temporal range and their end occurred 3 weeks later compared to females from the control group, which gave birth within a very narrow temporal range ending within 2 days. Offspring obtained from females exposed to the TCDD action was smaller in number and was characterized by smaller rearing and smaller birth weight even after the first month of life; however weight gain in both groups was similar and it was twelve-fold increase.

Bioassay-derived dioxin equivalent concentrations in gonads and livers of the Atlantic cod females from the Baltic Sea

The DR-H4IIE.Luc bioassay is based on the ability of dioxin and dioxin-like contaminants to activate the AhR and its signal transduction pathway, a mechanism through which these contaminants elicit their toxic effects. The bioassay was used to examine the total dioxin-equivalent (TEQ) toxicity in gonads and livers of cod females from the southern Baltic Sea. The bioassay-derived TEQ-luc was measured after 24-h and 48-h exposure periods. Mean concentrations in the 24-h bioassay were 95 and 35 pg TEQ-luc g(-1) lipid in gonads and livers, respectively, and 58 and 38 pg TEQ-luc g(-1) lipid in the 48-h bioassay, respectively. The 48-h TEQ-luc levels displayed significant relationships with ΣPCB(7) and selected PCB congeners but not with the TEQ(DLPCB-REP). Levels in gonads approached 10% of the LC50 for developing larvae of other marine fish, yet the impact on survival of the cod during its early life remains to be assessed in a future.

The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

This article reviews the present state of the art regarding the toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). The absorption, body distribution, and metabolism can vary greatly between species and also may depend on the congener and dose. In biota, the 2,3,7,8-substituted PCDDs and PCDFs are almost exclusively retained in all tissue types, preferably liver and fat. This selective tissue retention and bioaccumulation are caused by a reduced rate of biotransformation and subsequent elimination of congeners with chlorine substitution at the 2,3,7, and 8 positions. 2,3,7,8-Substituted PCDDs and PCDFs also have the greatest toxic and biological activity and affinity for the cytosolic arylhydrocarbon (Ah)-receptor protein. The parent compound is the causal agent for Ah-receptor-mediated toxic and biological effects, with metabolism and subsequent elimination of 2,3,7,8- substituted congeners representing a detoxification process. Congener-specific affinity of PCDDs and PCDFs for the Ah-receptor, the genetic events following receptor binding, and toxicokinetics are factors that contribute to the relative in vivo potency of an individual PCDD or PCDF in a given species. Limited human data indicate that marked species differences exist in the toxicokinetics of these compounds. Thus, human risk assessment for PCDDs and PCDFs needs to consider species-, congener-, and dose-specific toxicokinetic data. In addition, exposure to complex mixtures, including PCBs, has the potential to alter the toxicokinetics of individual compounds. These alterations in toxicokinetics may be involved in some of the nonadditive toxic or biological effects that are observed after exposure to mixtures of PCDDs or PCDFs with PCBs.

Liver accumulation of 2,3,7,8-tetrachloro-[3H]dibenzofuran in mice: modulation by treatments with polychlorinated biphenyls

The distribution of 2,3,7,8-tetrachloro-[3H]dibenzofuran ([3H]TCDF; 40 micrograms/kg) resembled that earlier reported for 2,3,7,8-tetrachlorodibenzo-p-dioxin, with a strong accumulation in the liver and a selective uptake in the nasal olfactory mucosa of adult and fetal mice. Pretreatments with a series of selected congeners of polychlorinated biphenyls (PCBs), i.e.. I (IUPAC)-77, I-105, I-118, I-126, I-153, I-156, I-169, and a commercial preparation, Aroclor 1254 (25-100 mg/kg body wt. i.p.), were found to modulate the hepatic uptake of [3H]TCDF (24 h post-3H-injection). At a short pretreatment time (4 h), non-ortho-chlorinated congeners decreased the uptake of [3H]TCDF equivalents in the liver (e.g., I-126 = 3,3',4,4',5-pentachlorobiphenyl: 34% of control), while several mono- and di-ortho PCB congeners and Aroclor 1254 increased the hepatic uptake of [3H]TCDF (e.g., I-156 = 2,3,3',4,4',5-hexachlorobiphenyl: 183% of control). At a longer pretreatment time (48 h), both a non-ortho (I-169 = 3,3',4,4',5,5'-hexachlorobiphenyl) and mono-ortho PCB congener(s) (e.g. I-156) markedly increased the hepatic 3H-uptake (190%), a probable effect of an induction of hepatic binding sites for TCDF. Ethoxyresorufin-O-deethylase activities, regarded to mirror the metabolic activity of cytochrome P-450 IA1 (CYP IA1), were strongly and time-dependently induced after I-169, but not after I-156, pretreatment (25 mg/kg). The initial liver concentrations of the two PCB congeners were similar and increased for I-169 but not for I-156 at later time points. In conclusion, the results show a selective uptake of [3H]TCDF in the mouse liver and nasal olfactory mucosa of both dam and fetus. The uptake of [3H]TCDF in the liver is influenced both by dose and pre-exposure with PCBs. The presence of a PCB-sensitive, but CYP IA1-independent, hepatic binding site for TCDF is suggested. Consequently, pharmacokinetic interactions with PCBs complicate the toxicity assessment of TCDF in complex mixtures.

Toxicokinetic mixture interactions between chlorinated aromatic hydrocarbons in the liver of the C57BL/6J mouse: 2. Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and biphenyls (PCBs)

Six groups of C57BL/6J mice received single oral doses of 1.5-10.6 nmol/kg 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PnCDD), 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (HxCDD) or 2,3,4,7,8-pentachlorodibenzofuran (PnCDF) as single compounds or in combination with 300 mumol/kg 2,2',4,4',5,5'-hexachlorobiphenyl (HxCB). Two other groups of mice received a mixture of the first three compounds, either with or without HxCB. The hepatic deposition and elimination of the compounds and their CYP1a dependent 7-ethoxyresorufin-O-deethylation (EROD) activity were studied until day 175. Interactive effects on the hepatic deposition of PnCDD were observed in most of the mixed dose groups. For HxCDD and PnCDF interactive effects were either very small or absent. No interactive effects were observed on hepatic elimination rates of PnCDD, HxCDD or PnCDF. No evidence was found for the influence of HxCB cotreatment on the hepatic concentration-response curves of the three compounds or their mixture. Based on the results from the present study it is concluded that PCDDs, PCDFs and PCBs may influence each other's, toxicokinetics when administered in mixtures.

Biological activity of 2,4,8-trichlorodibenzofuran: promotion of rat liver foci and induction of cytochrome P-450-dependent monooxygenases

The biological activity of 2,4,8-trichlorodibenzofuran (2,4,8-TCDF) was tested using 2 endpoints: (a) the promotion of enzyme-altered, preneoplastic foci initiated by diethylnitrosamine (DEN) in livers of weanling female Sprague-Dawley rats; and (b) the induction of aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH), a marker for cytochrome P-4501 activity, in livers of adult female Sprague-Dawley rats and in H4IIEC3 rat hepatoma cells. When animals were treated with 200 or 500 mg/kg 2,4,8-TCDF 5 X weekly over 10 weeks after a single application of 10 mg/kg DNA, the higher dose of 2,4,8-TCDF had a promoting effect on the appearance of preneoplastic foci. Thus number and total area of foci deficient in adenosine-5'-triphosphatase were significantly increased by a factor of 1.6. 2,4,8-TCDF induced AHH-activities in 9000 X g supernatants of liver 2-3-fold, when rats were treated with 100-1000 mg/kg/day for 5 days and monooxygenase activities determined after another 3 days. The amounts of 2,4,8-TCDF required for inducing AHH activity in H4IIEC3 cells were 7 orders of magnitude higher than those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). the results indicate that the 2,4,8-TCDF has a biological activity which is extremely low compared to that of 2,3,7,8-TCDD.