Elimination of various polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) in rat faeces

The Influence of Obesity on the Pharmacokinetics of Dioxin in Mice: An Assessment Using Classical and PBPK Modeling

The effects of body fat mass on the elimination of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was examined in mice. When male C57BL/6J mice are fed a high-fat, simple carbohydrate diet (HFD) for 13 weeks, they develop an obese phenotype. In contrast, A/J mice fed an HFD do not become obese. After 13 weeks on a normal diet (ND) or HFD, male C57BL/6J and A/J mice received a single dose by gavage of 0.1 or 5.0 µg of 2,3,7,8-tetrachloro[1,6-3H] dibenzo-p-dioxin per kg body weight. Using classical pharmacokinetics, the blood elimination half-life of TCDD was approximately 10 and 2 times longer in the C57BL/6J on the HFD compared with the mice on the ND at 0.1 and 5.0 μg/kg doses, respectively. The diet did not increase the blood half-life of TCDD in the A/J mice, which did not get obese. Using a physiologically based pharmacokinetic model for TCDD that incorporated experimentally derived percent body fat mass and tissue partition coefficients, as well as data on hepatic sequestration, did not provide accurate predictions to the data and could not explain the increase in half-life of TCDD in the HFD groups. This work demonstrates that obesity influences the half-life of TCDD, but other undetermined factors are involved in its elimination because the increase in body fat mass, decreases in cytochrome P4501A2, and altered partition coefficients could not completely explain the prolonged half-life.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) poisoning in Victor Yushchenko: identification and measurement of TCDD metabolites

BACKGROUND

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has a long half-life of 5-10 years in human beings as a result of its high lipophilicity, and little or no metabolism. We monitored TCDD, its form, distribution, and elimination in Victor Yushchenko after he presented with severe poisoning.

METHODS

In late December, 2004, a patient presented with TCDD poisoning; the levels in his blood serum (108000 pg/g lipid weight) were more than 50 000-fold greater than those in the general population. We identified TCDD and its metabolites, and monitored their levels for 3 years using gas chromatography and high-resolution mass spectrometry in samples of blood serum, adipose tissue, faeces, skin, urine, and sweat, after they were extracted and cleaned with different organic solvents.

FINDINGS

The amount of unmodified TCDD in the samples that were analysed accounted for about 60% of TCDD eliminated from the body during the same period. Two TCDD metabolites-2,3,7-trichloro-8-hydroxydibenzo-p-dioxin and 1,3,7,8-tetrachloro-2-hydroxydibenzo-p-dioxin-were identified in the faeces, blood serum, and urine. The faeces contained the highest concentration of TCDD metabolites, and were the main route of elimination. Altogether, the different routes of elimination of TCDD and its metabolites accounted for 98% of the loss of the toxin from the body. The half-life of TCDD in our patient was 15.4 months.

INTERPRETATION

This case of poisoning with TCDD suggests that the design of methods for routine assessment of TCDD metabolites in human beings should be a main aim of TCDD research in the metabolomic era.

FUNDING

University of Geneva Dermatology Fund, and Swiss Centre for Applied Human Toxicology.

Dioxin health risk to infants using simulated tissue concentrations

Dioxin concentrations in infant and child were simulated using physiologically based pharmacokinetic (PBPK) models developed for these groups. The infant model was validated by comparing the simulated concentration with the measured concentration from the literature, and they showed good agreement. Simulations with our PBPK model showed temporal patterns in concentrations in various tissues. For risk assessment, estimated concentrations of 29 dioxins in the liver were summed up in a toxic equivalency (TEQ) basis to be compared with actual 2,3,7,8-TCDD concentrations in rat liver associated with toxicity. Maximum liver concentrations in breast-fed and formula-fed infants were 16.8pg TEQ/g and 3.5pg TEQ/g, respectively. The level in breast-fed infant liver was approximately 1/300 of the level associated with hepatocellular carcinoma and 1/5 of the level found in maternal rat liver associated with alterations in reproductive organs in the next generation. Based on our analysis, the present contamination level is not safe enough, but further dose-response data is required for a quantitative risk assessment.

Clearance of PCDD/Fs via the gastrointestinal tract in occupationally exposed persons

A digestive tract mass balance was performed on six men with high body burdens of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Intake via food was measured by analyzing duplicate portions of the food consumed by the volunteers and excretion via feces was determined by quantitative collection and analysis of the feces. Blood samples were taken to determine the current body burden. The results showed that the quantity of non-metabolized chemical excreted in the feces clearly exceeded the uptake via food for all of the 2,3,7,8-substituted PCDDs and some of the PCDFs, indicating a significant clearance across the gastrointestinal tract. The concentrations of these PCDD/F congeners in blood and feces were highly correlated (r > 0.8), demonstrating that the fecal PCDD/F content was determined by the body burden. The half lives in the test persons due to fecal clearance of non-metabolized chemical were estimated from the excretion rate and the current body burden and ranged between 10 years (Cl8DD) and 33 years (2,3,4,7,8-Cl5DF). These were compared with the overall contaminant half-lives due to all clearance processes which were calculated from the body burden and the decrease in blood concentrations measured over several years. The fecal clearance of non-metabolized PCDD/F contributed on average between 37% (2,3,7,8-Cl4DD) and 90% (Cl8DD) to the total elimination. This indicates that the gastrointestinal pathway plays a decisive role in the clearance of most 2,3,7,8-subsituted PCDD/F congeners.

Intake, fecal excretion, and body burden of polychlorinated dibenzo-p-dioxins and dibenzofurans in breast-fed and formula-fed infants

To assess toxicokinetics of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), oral intake and fecal excretion were measured in two breast-fed infants and one formula-fed infant during the 1st y of life. The intake of these compounds was up to 50 times higher in the breast-fed infants. In these children, fecal excretion of the main tetra- to hexachlorinated congeners was less than 9% of the intake at age of 1 and 5 mo, indicating almost complete intestinal absorption during breast-feeding. In contrast, distinctly higher fecal excretion rates were observed for the hepta- and octachlorinated compounds. Despite much lower PCDD/PCDF intake after weaning, concentrations in stool fat did not decrease substantially. We conclude that concentrations in fecal fat more or less reflect those in body fat. Additionally, PCDD/PCDF concentrations were measured in blood fat of all infants (and in a second formula-fed baby) at the age of 11 mo. International toxicity equivalent (I-TEq) concentrations in the formula-fed infants were less than 25% of maternal values and about 10 times lower than in the infants breast-fed for 6-7 mo. In the latter, a distinct accumulation was found for the tetra- to hexachlorinated congeners compared with maternal concentrations. We conclude that accumulation of PCDDs and PCDFs in infants is as high as expected on the basis of intake data and assuming complete absorption and negligible elimination during the 1st y of life.

Intake and fecal excretion of PCDDs, PCDFs, HCB and PCBs (138, 153, 180) in a breast-fed and a formula-fed infant

Intake and fecal excretion of PCDDs, PCDFs, HCB and PCBs (IUPAC Nos. 138, 153, 180) were measured in a breast-fed and a formula-fed infant at the age of 1 and 5 months. As expected, the intake of these compounds was clearly higher in the breast-fed infant. In this baby an almost complete absorption was observed for lower chlorinated PCDDs and PCDFs and also for HCB and PCBs, whereas for hepta- and octachlorinated PCDDs and PCDFs fecal excretion was considerably higher (from 20% up to nearly 100% of the intake). Due to low concentrations in diet and feces of the formula-fed infant an evaluation was possible only for a few compounds at the age of 5 months. These values were in the same range when compared with those of the breast-fed infant. For collection of feces new cotton diapers were used which were pre-extracted in order to reduce the levels of polychlorinated compounds. Unexpectedly, after washing the tissue a much higher contamination was observed which made a calculation of fecal excretion rates in the formula-fed infant at the age of 1 month impossible.

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.

Impact of polychlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls on human and environmental health, with special emphasis on application of the toxic equivalency factor concept

A scientific evaluation was made of the mechanisms of action of polychlorinated dibenzo-p-dioxins, dibenzofurans and biphenyls. Distinction is made between the aryl-hydrocarbon (Ah) receptor-mediated and non-Ah receptor-mediated toxic responses. Special attention is paid to the applicability of the toxic equivalency factor (TEF) concept.

Transfer of various polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) via placenta and through milk in a marmoset monkey

A defined mixture of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) was subcutaneously administered to a pregnant marmoset monkey (Callithrix jacchus) 11 weeks prior to delivery. Transfer of PCDDs and PCDFs via placenta and mother's milk was investigated by measurement of concentrations in a newborn 1 day after birth and in an infant of the same litter after a lactation period of 33 days. Furthermore, comparative measurements were performed in different tissues of the mother at the end of the lactation period, and in addition, in two groups of four adult monkeys each 1 and 6 weeks after treatment. Deposition of the PCDDs and PCDFs into fetal liver was very low for most of the 2,3,7,8-substituted congeners. Highest deposition was observed for 2,3,7,8-T4CDD and 1,2,3,7,8-P5CDD. For all other compounds concentrations in the hepatic tissue of newborn shortly after birth were lower than one tenth of corresponding concentrations in adults. Especially for PCDFs, prenatal deposition in fetal liver was extremely low. Fetal liver is apparently largely unable to accumulate PCDDs/PCDFs. In contrast to liver, concentrations of 2,3,7,8-substituted PCDDs/PCDFs in adipose tissue of the newborn were at least one third of the levels in adults. However, concentrations of OCDD and OCDF were about three times higher in the newborn than in adult adipose tissue. Transfer of some of the 2,3,7,8-substituted PCDDs and PCDFs to the offspring via mother's milk was considerable, leading to hepatic concentrations in the suckled infant at the end of the 33-day nursing period well above corresponding concentrations in the dam. When hepatic concentrations in the infant and dam were compared 2- to 4-fold higher concentrations were found in the infant's liver for 2,3,7,8-T4CDD/F and for 1,2,3,7,8-P5CDD. In the case of the 2,3,7,8-substituted H6CDDs, P5CDFs, and most of the H6CDFs, hepatic concentrations in the infant and dam were in the same range at the end of the suckling period. In contrast to this, less than one tenth the concentration of OCDD was found in the infant's liver when compared with adult liver. A corresponding phenomenon was observed for PCDFs. At the maximum absorption, 1 week after injection, for almost all 2,3,7,8-substituted congeners highest concentrations were measured in hepatic tissue of adult monkeys. This is especially true for those substances with six and more chlorine atoms. Besides adipose tissue, comparatively high levels were found in thymus and also in lung tissue.(ABSTRACT TRUNCATED AT 400 WORDS)

Persistence of various polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) in hepatic and adipose tissue of marmoset monkeys

A defined mixture of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) was subcutaneously administered to marmoset monkeys (Callithrix jacchus). Tissue concentrations in hepatic and adipose tissue were measured at different times after treatment (1-28 weeks). One week after application high concentrations could be detected for the 2,3,7,8-substituted congeners only. The percent of the administered dose in whole liver differed for the various 2,3,7,8-substituted congeners, ranging from 24.5 +/- 4.5% for 2,3,7,8-TCDD to 74.1 +/- 4.9% for 2,3,4,6,7,8-H6CDF. Therefore, the concentration ratio (liver/adipose tissue) was also very different, ranging from about 1 (2,3,7,8-T4CDD or 2,3,7,8-T4CDF) to greater than 10 in the case of some higher chlorinated PCDDs and PCDFs. Half-lives of PCDDs and PCDFs were very different for the various 2,3,7,8-substituted congeners. For the most toxic compound (2,3,7,8-T4CDD) a t/2 of about 8 weeks in hepatic tissue and about 11 weeks in adipose tissue was found when calculated from data obtained later than 6 weeks after injection. For 2,3,7,8-T4CDD and 1,2,3,7,8-P5CDD the decreases in hepatic concentrations were much faster during the first 6 weeks after administration (t/2 of 4 weeks). This was apparently due to redistribution phenomena. Half-life increased with increasing degrees of chlorination. In some cases (e.g. OCDD, OCDF) no significant decrease in tissue concentrations could be observed after 28 weeks. The shortest t/2 was determined for 2,3,7,8-T4CDF: shorter than 6 days in hepatic tissue and about 10 days in adipose tissue. Calculation of the body burden of the non-2,3,7,8-substituted PCDDs/PCDFs 1 week after injection revealed that all groups of isomers were present at less than 5%. Consequences of these findings for the use of TCDD-toxic-equivalency factors are discussed and a change in strategy is suggested.

Absorption and tissue distribution of various polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) in the rat

A defined mixture of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) was parenterally administered to rats and absorption and tissue distribution were measured: 1) Toluene/DMSO (1 + 2; v/v) proved to be a convenient vehicle for the subcutaneous administration of the various PCDDs and PCDFs. Seven days after application the rate of absorption was 90% of the administered dose or even higher for almost all of the PCDDs/PCDFs in the mixture. In a few cases only (e.g. OCDD) the rate was found to be 84-89%; 2) Seven days after subcutaneous administration all 2378-substituted congeners were found in the liver, whereas only a few non-2378-substituted congeners could be measured in minor quantities. The 2378-substituted congeners also predominated in adipose tissue; however, most of the non-2378-substituted congeners were also detected; 3) The amount deposited within the liver as percentage of the administered dose differed for the various 2378-substituted PCDDs and PCDFs, ranging from less than 10% for OCDD or 2378-T4CDF, and between 60 and close to 100% for 12378-P5CDD or the H6CDDs. Therefore, the concentration ratio (liver/adipose tissue) was also found to be very different, ranging from less than 3 in the case of 2378-T4CDD or 2378-T4CDF to greater than 40 in the case of 1234678-H7CDD, 23478-P5CDF, 123678-H6CDF, or 1234678-H7CDF; 4) Studies performed at the time period of ongoing absorption (13-14 h after injection) provided the first evidence that some of the non-2378-substituted congeners do reach substantial concentrations in hepatic tissue shortly after administration; 5) Subsequent to intraperitoneal injection of the same PCDD/PCDF mixture the concentrations within the liver were found to be almost identical with that found after subcutaneous injection. In contrast, much higher concentrations of the congeners were found in (abdominal) adipose tissue; 6) In the liver of untreated rats of the same strain no T4CDDs/T4CDFs or P5CDDs/P5CDFs were detectable under our experimental conditions.