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Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans
Regulatory Toxicology and Pharmacology 30, S63–S68 (1999) Article ID rtph.1999.1328, available online at http://www.idealibrary.com on
Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans 1 Frances Pollitt 2 Department of Health, Skipton House, 80 London Road, SE1 6LW London, United Kingdom Received May 25, 1999
Polychlorinated dibenzodioxins and polychlorinated dibenzofurans (dioxins) are contaminants with long biological half-lives. The most toxic dioxin, 2,3,7,8-tetrachlorodibenzodioxin (TCDD), has a halflife in humans of 9 years. A tolerable daily intake (TDI) of 10 pg/kg body wt/day has been recommended, which was derived from steady-state concentrations of TCDD at the no-observed-adverse-effect level in animal studies. Intakes of dioxins by breast-fed babies can exceed the TDI by almost 20-fold. However, assuming a half-life of 9 years for all dioxins, it can be shown that the steady-state body burden is not increased by the short period of high intake during breast feeding, compared to that resulting from ingestion of the TDI daily from birth. Therefore, the TDI appears to accommodate the high intakes of dioxins by breast-fed babies. For dioxins with a significantly shorter half-life than TCDD, it can be shown that breast feeding will lead to higher body burdens in early life than would have been reached by ingestion of the TDI daily from birth. However, these peak body burdens will still be below the steady-state body burden achieved by ingestion of 10 pg TCDD/kg body wt/day from birth.
INTRODUCTION
Polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) form two groups of structurally and toxicologically similar, persistent, chlorinated hydrocarbons which are distributed widely in the environment. These compounds have long halflives and bioaccumulate in mammalian tissues. Therefore, they are interesting examples to consider in the context of a discussion of the significance of excursions of intake above the tolerable daily intake (TDI). This paper will provide some background information on the toxicity and pharmacokinetics of PCDDs and PCDFs and will discuss the implications of exceeding the TDI, 1 The views expressed in this paper are those of the author and do not necessarily represent those of the Department of Health. 2 Fax: 00.44/171.972.5167. E-mail: [email protected].
particularly in the context of high intakes by breast-fed infants. BACKGROUND INFORMATION
Toxic Equivalent Factors The chemical structures of PCDDs and PCDFs are given in Fig. 1. These compounds elicit a broad range of toxicological effects, many of which are species and tissue specific. Although there are a total of 210 different PCDD and PCDF congeners, there are only 17 toxicologically significant congeners, which are those substituted with chlorine at the 2, 3, 7, and 8 positions on the molecule. The most potent and extensively studied congener is the 2,3,7,8-tetrachloro-substituted dioxin (TCDD). It is widely accepted that the toxicologically active PCDDs and PCDFs exert their effects by a common mechanism involving binding to a cytoplasmic receptor protein called the Ah (aryl hydrocarbon) receptor. Certain polychlorinated biphenyl (PCB) congeners also bind to the Ah receptor and can exhibit similar toxicological effects to the 2,3,7,8-substituted PCDDs and PCDFs. Since these compounds are considered to act by a common mechanism, and because they occur as mixtures in the environment, in food, and in human tissues, they are commonly assessed and regulated as a class. However, detailed toxicological information is available only for TCDD. Therefore, the concept of “TCDD equivalents” or “toxic equivalents” has been introduced to enable the assessment of the toxicity of mixtures of these compounds and to enable risk assessments of these mixtures to be carried out. This concept uses the available toxicological and in vitro biological data, and knowledge of structural similarities among the 2,3,7,8-substituted PCDD and PCDF congeners and the 13 “dioxin-like” PCB congeners (hereafter collectively termed “dioxins”), to generate a set of weighting factors or “toxic equivalency factors” (TEFs), each of which expresses the toxicity of a particular congener in terms of an equivalent amount of TCDD. Multiplication of the concentration of the congener by its TEF gives a TCDD toxic equivalent (TEQ). The toxicity of
S63 © 1999 International Life Sciences Institute, Washington, DC All rights reserved.
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FIG. 1. Structures of dibenzo-para-dioxin and dibenzofuran.
any mixture, relative to TCDD, is thus taken to be the sum of the individual TEQs (Department of the Environment, 1989). There are obviously a number of assumptions implicit in the toxic equivalents approach. It assumes that the effects of the different congeners are additive, whereas antagonistic or even synergistic interactions are possible. Also, where the TEFs are derived from in vitro data, the assumption is made that the pharmacokinetic behavior in vivo is equivalent to that of TCDD. Pharmacokinetics The pharmacokinetics of dioxins are congener, dose, and species specific. As with toxicological data, most information is available for TCDD. The pharmacokinetics of dioxins have been reviewed by Olson (1994). For humans, the main route of exposure to TCDD is the diet, particularly fatty food. The limited data available suggest that TCDD is effectively absorbed following oral exposure in an oil vehicle. There is some evidence from the rat that absorption may be dosedependent, with increased absorption occurring at lower doses. Although TCDD is initially distributed after absorption to all organs, within several hours the liver and adipose tissue become the primary sites of deposition. Disposition of TCDD is dose-dependent in rats and humans. In chronic studies in rats, at higher dose levels (above 1 ng/kg body wt/day), TCDD is preferentially deposited in the liver but, at lower dose levels, steady-state levels in liver and adipose tissue are similar. There is evidence that TCDD can be metabolized by a range of mammalian species to polar metabolites which are readily excreted in the urine and bile. Metabolism is also generally considered a detoxification process. TCDD is excreted slowly from all species tested, with elimination half-lives of approximately 11 days in the hamster (Olson et al., 1980), 12–31 days in the rat (Pohjanvirta et al., 1990; Rose et al., 1976), and 94 days in the guinea pig (Olson, 1986). Estimates of half-life in human have been derived from blood levels of TCDD measured over time in a cohort of potentially heavily exposed Vietnam veterans (the “Ranch Hand” cohort). The most recently reported estimate of half-life in this cohort, derived from multiple measurements from serum collected over 10 years from 213 individu-
als, is 8.7 years (95% confidence interval, 8.0 –9.5 years) (Michalek et al., 1996). The few existing data on other congeners indicate that there can be marked differences in pharmacokinetic parameters from those for TCDD. For example, octachlorodibenzodioxin (OCDD) exhibits only limited absorption (2–15%) in the rat. The whole body half-life of OCDD in the rat is between 3 and 5 months (Birnbaum and Couture, 1988). However, in a study in humans, the median half-life for OCDD was found to be 6.7 years, compared to 6.9 years for TCDD (FleschJanys et al., 1994). The longest half-life found in this study was 19.6 years, for 2,3,4,7,8-penta-CDF, although this figure was derived from only five subjects. There also appear to be differences in the relative disposition of the higher chlorinated PCDD and PCDF congeners in the liver and in adipose tissue, compared to TCDD (Thoma et al., 1990). Derivation of the TDI There is a vast literature on the toxicology of dioxins and, in recent years, a large number of mechanistic studies have been published. No attempt will be made here to present a comprehensive review of the toxicity of these chemicals. However, in order to inform the later discussion, the derivation of the UK TDI for dioxins will be described briefly. This TDI was recommended by a WHO Regional Office for Europe Expert Group in 1990 and, subsequently, was endorsed by the UK Department of Health’s Committee on the Toxicity of Chemicals in Food, Consumer Products, and the Environment (COT) (Ministry of Agriculture, Fisheries, and Food, 1992). The only congener on which it is possible to carry out a comprehensive hazard assessment is TCDD. This compound produces a wide range of toxic effects in laboratory animals, and there are interspecies differences in sensitivity and in the type of response seen. In chronic toxicity studies, the toxic effects caused by TCDD include thymic atrophy, hepatotoxicity, immunotoxicity, effects on reproduction and development, and carcinogenicity. Both the WHO Europe Expert Group and the COT identified carcinogenicity and effects on reproduction and development as critical effects in laboratory animals and used the no-observedadverse-effect levels (NOAELs) for these effects in animal studies as the basis of the TDI. Since TCDD is negative in mutagenicity tests, it was considered that it was unlikely that its carcinogenic action was due to genotoxicity and that it was appropriate, therefore, to set a TDI based on the carcinogenicity data. The critical study used for identification of a NOAEL for carcinogenicity was a 2-year study in which groups of rats received TCDD in the diet at levels of 1, 10, and 100 ng/kg body wt/day from 6 to 7 weeks of age for 2 years (Kociba et al., 1978). The critical end point was
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FIG. 2. Accumulated body burden for a dioxin congener with a half-life of 9 years, following ingestion of 10 pg/kg body wt/day from birth (“no breast feeding”) or following 6 months breast feeding and ingestion of 10 pg/kg body wt/day thereafter.
liver neoplasms, and the NOAEL was 1 ng/kg body wt/day. However, in view of the long half-life of TCDD, the critical parameter was considered to be not the ingested dose, but the steady-state liver concentration of TCDD at this dose (540 ppt). The daily dose in humans which would give rise to this liver concentration was estimated, and a safety factor of 10 was applied to this figure to allow for any remaining differences in susceptibility between rat and human. This gave a TDI of 10 pg TCDD/kg body wt/day. A similar approach was used to derive a TDI from the reproductive toxicity data. However, since in this case the target organ concentration of TCDD at the NOAEL was not known, the anticipated adipose tissue concentration at this dose was used as a surrogate. A rat teratology study was used as the critical study and a NOAEL of 30 ng TCDD/kg body wt/day was identified (Sparschu et al., 1971). Again, a TDI of approximately 10 pg/kg body wt/day was derived. The COT recommended that, when considering mixtures of dioxins, the TDI can be regarded as 10 pg TCDD equivalents/kg body wt/day. INTAKES OF DIOXINS FROM FOOD AND HUMAN MILK
The most recent estimates of intakes of dioxins from the diet in the United Kingdom were reported by the Ministry of Agriculture, Fisheries, and Food (MAFF) in 1997 (MAFF, 1997). The data are derived from analyses of total diet study samples collected in 1992 and are based on food intakes by an average UK adult consumer. They are presented as the TEQ of all congeners present. The TEQs were calculated using the Interna-
tional TEFs for PCDDs and PCDFs (Department of the Environment, 1989) and the TEFs for PCBs proposed by Ahlborg et al. (1994). The upper bound dietary intake of dioxins by the average UK adult consumer was estimated as 2.4 pg TEQ/kg body wt/day or 144 pg TEQ/day for a 60-kg adult (the upper bound estimate is calculated using the assumption that where the levels of the individual congeners are below the limit of detection, they are present at the limit of detection). This figure is well below the TDI. However, MAFF also reported estimates of intakes of dioxins by breast-fed babies, using analytical data obtained from three pooled human milk samples taken in 1993 or 1994. Intakes were estimated as 170 pg TEQ/kg body wt/day at 2 months of age, falling to 39 pg TEQ/kg body wt/day at 10 months of age, i.e., they can exceed the TDI by almost 20-fold. It is interesting to consider the consequences of these high intakes in early life for the body burden of dioxins in humans and for the risk assessment of dioxins. BODY BURDENS OF DIOXINS IN BREAST-FED AND NON-BREAST-FED INDIVIDUALS
For most chemicals with a half-life of a few hours, intakes up to 20-fold above the TDI by young babies would cause considerable concern. However, is this the case with long half-life chemicals such as TCDD? Figure 2 shows the increase in the body burden of dioxins with age either following ingestion daily of the TDI of 10 pg TEQ/kg body wt/day from birth or following 6 months breast feeding and ingestion of the TDI daily thereafter. The body burden has been calculated using the assumptions that the half-life of all congeners con-
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tributing to the TEQ is 9 years, that there is 100% absorption of all congeners, and that the body burden at birth is zero. The intakes from breast feeding are those estimated by MAFF (MAFF, 1997). It can be seen that breast feeding increases the body burden in early life but the steady-state level, achieved at about 40 –50 years of age, is not increased. The steady-state body burden at the TDI is estimated to be approximately 45 ng TEQ/kg body wt. For comparison, it is interesting to note that the body burden of dioxins in the general population, calculated from average background serum lipid concentrations, has been estimated as 13 ng TEQ/kg body wt (De Vito et al., 1995). Thoma et al. (1990) have reported levels of dioxins (PCDD and PCDF congeners only) in adipose tissue from eight infants aged 2–13 months and from 28 adults aged 31– 80 years. As anticipated from Fig. 2, the levels were lower in infants than in adults for all congeners except OCDF. Beck et al. (1990) measured the levels of dioxins (PCDD and PCDF congeners only) in adipose tissue in a sudden infant death baby (9.3 months old) who had been breast fed for about 80 days. A level of 3.4 ng TEQ/kg body wt was found. In comparison, the levels in two non-breast-fed infants of 3.8 and 4.8 months of age were 2.8 and 2.1 ng TEQ/kg body wt, respectively. Although the number of subjects is small, the difference between the breast-fed and nonbreast-fed infants is lower than would be expected from Fig. 2. This cannot be explained by low absorption of dioxins in the breast-fed infant, since the bioavailability of dioxins from human milk has been shown to be about 95% (Korner et al., 1993; Pluim et al., 1993). TOXICOLOGICAL IMPLICATIONS
Is the higher body burden of dioxins achieved in early life by breast-fed babies likely to be toxicologically significant? Do the studies on which the TDI is based accommodate differences in the rate at which the steady-state body burden is reached in rats and in humans? The TDI is based on two end points—carcinogenicity and toxicity to the fetus. In the Kociba carcinogenicity study, dosing of the rats commenced at 6–7 weeks of age. Assuming a half-life for TCDD in the rat of 24 days (De Vito et al., 1995) and an initial body burden of zero, steady-state body burdens would have been achieved at 20–24 weeks of age, approximately 20–25% of the animal’s life span. In humans, steady-state body burdens are not achieved until 40–50 years of age, 55–70% of an average human’s life span. This appears reassuring, since it implies that the rats were exposed to the maximum, steady-state tissue level of TCDD for a longer proportion of their life span than humans would be. Also, since it has been assumed that the carcinogenic effect is thresholded and that a certain tissue concentration must be achieved before there is an increased risk of cancer, the fact that the period of breast feeding does not lead to an increased
body burden overall should also provide reassurance. It may be worth noting, however, that (as in most carcinogenicity studies) dosing with TCDD in the Kociba study did not begin until the rats had reached puberty. Therefore, if there is increased susceptibility to the carcinogenic effect of TCDD in early life, this has not been taken into account in deriving the TDI. This may be particularly relevant in the context of high intakes of dioxins by breast-fed babies. The other study used to set the TDI was a teratology study in the rat. Theoretically, a woman of reproductive age who was breast fed as a baby will have a higher body burden than one who was not breast fed (see Fig. 2). If the exposure of the fetus is proportional to the mother’s body burden, this implies that the fetus of a breast-fed woman may be at increased risk compared to that of a non-breast-fed woman. However, since the TDI was derived from the adipose tissue concentration of TCDD at the NOAEL in the rat, rather than the ingested dose, there should still be an adequate safety margin to protect the fetus. Despite the fact that the TDI of 10 pg/kg body wt/day, based on studies in adult rats, appears to accommodate the high intakes of dioxins by breast-fed babies, it might be advisable to reevaluate the toxicological database on TCDD to ensure that there are no implications for early postnatal development. For example, Bjerke and Peterson (1994), using a cross-fostering design, have reported that lactational exposure alone to TCDD, administered to pregnant female rats as a single oral dose of 1 mg/kg body wt on gestational day 15, caused decreased plasma testosterone levels, decreased weights of secondary sex organs, decreased epididymal sperm reserves, and feminization of behavior in male rat pups suckled by the dams. It is not known how the exposure of the rat pups in this study compares with that of breast-fed babies. IMPLICATIONS OF SHORTER HALF-LIFE
What would be the effect of breast feeding on the body burden of a congener with a shorter half-life than TCDD? Figures 3 and 4 show the accumulated body burden following 6 months breast feeding and ingestion of 10 pg TEQ/kg body wt/day thereafter, for congeners with half-lives of 6 months, 1 year, and 3 years. The same intake estimates and assumptions are used as before. In the case of the shorter half-life congeners, breast feeding leads to a body burden at 6 months of age which is higher than that which would be achieved by ingestion of the TDI alone from birth, but which then falls to the steady-state level. However, in no case does the peak body burden of these congeners reach the steady-state body burden of TCDD (half-life of 9 years). Therefore, the use of the toxic equivalent approach to estimate the intake of mixtures of dioxins provides protection in the case of shorter half-life congeners, since it predicts greater accumulation, and hence
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FIG. 3. Accumulated body burden for dioxin congeners with half-lives of 6 months, 1 year, 3 years, and 9 years, following 6 months breast feeding and ingestion of 10 pg/kg body wt/day thereafter.
higher steady-state body burdens, than will actually be achieved by these congeners. In the case of congeners with longer half-lives than TCDD, it underpredicts the degree of accumulation. However, the data currently available indicate that only a few congeners have halflives longer than TCDD (Flesch-Janys et al., 1994). CONCLUSIONS
A number of conclusions can be proposed based on the considerations discussed above:
1. If the assumption is made that the half-life of all dioxin congeners in human milk is identical to that of TCDD (9 years), breast feeding is predicted to result in a higher body burden of dioxins in early life, but not to result in an increased steady-state body burden, compared to that resulting from ingestion of the TDI of 10 pg/kg body wt/day from birth. 2. For congeners with a significantly lower half-life, e.g., 1–3 years, breast feeding will lead to higher body burdens in early life than would have been reached by ingestion of the TDI from birth. However, these peak body burdens will still be below the steady-state body
FIG. 4. Accumulated body burden for dioxin congeners with half-lives of 1, 3, and 9 years, following 6 months breast feeding and ingestion of 10 pg/kg body wt/day thereafter.
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burden achieved by ingestion of 10 pg TCDD/kg body wt/day from birth. 3. The use of the toxic equivalent concept to estimate intakes of mixtures of dioxins is conservative when applied to congeners with half-lives shorter than that of TCDD, since it overestimates the likely accumulation of these congeners in human tissues. 4. The carcinogenicity study on which the TDI is based does not take account of any potential increased susceptibility to dioxins in early life. Otherwise, the TDI appears to accommodate the high intakes of dioxins by breast-fed babies, at least in relation to the end points on which it is based— carcinogenicity, fetotoxicity, and teratogenicity. 5. It is suggested that the toxicological database on TCDD is reexamined to assess whether postnatal development could be adversely affected by the high intakes of dioxins by infants during breast feeding. 6. More work is needed to assess whether breast-fed babies do achieve higher body burdens of dioxins than non-breast-fed babies, as predicted. ACKNOWLEDGMENT I thank Professor A. G. Renwick for the figures showing accumulated body burdens of dioxins over time.
REFERENCES Ahlborg, U. G., Becking, G. C., Birnbaum, L. S., Brouwer, A., Derks, H. J. G. M., Feeley, M., Golor, G., Hannberg, A., Larsen, J. C., Liem, A. K. D., Safe, S. H., Schlatter, C., Waern, F., Younes, M., and Yrjanheikki, E. (1994). Toxic equivalency factors for dioxinlike PCBs. Chemosphere 28, 1049 –1067. Beck, H., Drob, A., Kleeman, W. J., and Mathar, W. (1990). PCDD and PCDF concentrations in different organs from infants. Chemosphere 20(7–9), 903–910. Birnbaum, L. S., and Couture, L. A. (1988). Disposition of octachlorodibenzo-p-dioxin (OCDD) in male rats. Toxicol. Appl. Pharmacol. 93, 22–30. Bjerke, D. L., and Peterson, R. E. (1994). Reproductive toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in male rats: Different effects of in utero versus lactational exposure. Toxicol. Appl. Pharmacol. 127, 241–249. Department of the Environment (1989). Dioxins in the environment. Pollution Paper No. 27. HMSO, London, UK. De Vito, M. J., Birnbaum, L. S., Farland, W. H., and Gasiewicz, T. A. (1995). Comparisons of estimated human body burdens of dioxinlike chemicals and TCDD body burdens in experimentally exposed animals. Environ. Health Perspect. 103(9), 820 – 831.
Flesch-Janys, D., Gurn, P., Jung, D., Konietzko, J., Manz, A., and Papke, A. (1994). First results of an investigation of the elimination of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in occupationally exposed persons. Organohal. Cmpds. 21, 93–99. Kociba, R. J., Keyes, D. G., Beyer, J. E., Carreon, R. M., Wade, C. E., Dittenber, D. A., Kalnins, R. P., Frauson, L. E., Park, C. N., Barnard, S. D., Hummel, R. A., and Humiston, C. G. (1978). Results of a two-year chronic toxicity and oncogenicity study of 2,3,7,8-tetrachlorodibenzo-p-dioxin in rats. Toxicol. Appl. Pharmacol. 46, 279 –303. Korner, W., Dawidowsky, N., and Hagenmaier, H. (1992). Fecal excretion rates of PCDDs and PCDFs in two breast-fed infants. Chemosphere 27(1–3), 157–162. Michalek, J. E., Pirkle, J. L., Caudill, S. P., Tripath, R. C., Patterson, D. G., Jr., and Needham, L. L. (1996). Pharmacokinetics of TCDD in veterans of Operation Ranch Hand: Ten-year follow-up. J. Toxicol. Environ. Health 47, 209 –220. Ministry of Agriculture, Fisheries, and Food (1992). Dioxins in Food. Food Surveillance Paper No. 31, pp. 46 – 49. HMSO, London, UK. Ministry of Agriculture, Fisheries, and Food (1997). Dioxins and polychlorinated biphenyls in foods and human milk. Food Surveillance Information Sheet No. 105, June 1997. MAFF Food Safety Directorate, London, UK. Olson, J. R., Gasiewicz, T. A., and Neal, R. A. (1980). Tissue distribution, excretion, and metabolism of 2,3,7,8-tetrachlorodibenzo-pdioxin (TCDD) in the golden Syrian hamster. Toxicol. Appl. Pharmacol. 56, 78 – 85. Olson, J. R. (1986). Metabolism and disposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin in guinea pigs. Toxicol. Appl. Pharmacol. 85, 263–273. Olson, J. R. (1994). Pharmacokinetics of dioxins and related compounds. In Dioxins and Health (A. Schecter, Ed.), pp. 163–197. Plenum Press, New York and London. Pluim, H. J., Wever, J., Koppe, J. G., van der Slikke, J. W., and Olie, K. (1993). Intake and faecal excretion of chlorinated dioxins and dibenzofurans in breast-fed infants at different ages. Chemosphere 26, 1947–1952. Pohjanvirta, R., Vartiainen, T., Uusi-rauva, A., Monkkonen, J., and Tuomisto, J. (1990). Tissue distribution, metabolism, and excretion of [ 14C]-TCDD in a TCDD-susceptible and a TCDD-resistant rat strain. Pharmacol. Toxicol. 66, 93–100. Rose, J. Q., Ramsey, J. C., Wentzler, T. H., Hummel, R. A., and Gehring, P. J. (1976). The fate of 2,3,7,8-tetrachlorodibenzo-pdioxin following single and repeated oral doses to the rat. Toxicol. Appl. Pharmacol. 36, 209 –226. Sparschu, G. L., Dunn, F. L., and Rowe, V. K. (1971). Study of the teratogenicity of 2,3,7,8-TCDD in the rat. Food Cosmet. Toxicol. 9, 405– 412. Thoma, H., Mucke, W., and Kauert, G. (1990). Comparison of the polychlorinated dibenzo-p-dioxin and dibenzofuran in human tissue and human liver. Chemosphere 20, 433– 442.
Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans 1 Frances Pollitt 2 Department of Health, Skipton House, 80 London Road, SE1 6LW London, United Kingdom Received May 25, 1999
Polychlorinated dibenzodioxins and polychlorinated dibenzofurans (dioxins) are contaminants with long biological half-lives. The most toxic dioxin, 2,3,7,8-tetrachlorodibenzodioxin (TCDD), has a halflife in humans of 9 years. A tolerable daily intake (TDI) of 10 pg/kg body wt/day has been recommended, which was derived from steady-state concentrations of TCDD at the no-observed-adverse-effect level in animal studies. Intakes of dioxins by breast-fed babies can exceed the TDI by almost 20-fold. However, assuming a half-life of 9 years for all dioxins, it can be shown that the steady-state body burden is not increased by the short period of high intake during breast feeding, compared to that resulting from ingestion of the TDI daily from birth. Therefore, the TDI appears to accommodate the high intakes of dioxins by breast-fed babies. For dioxins with a significantly shorter half-life than TCDD, it can be shown that breast feeding will lead to higher body burdens in early life than would have been reached by ingestion of the TDI daily from birth. However, these peak body burdens will still be below the steady-state body burden achieved by ingestion of 10 pg TCDD/kg body wt/day from birth.
INTRODUCTION
Polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) form two groups of structurally and toxicologically similar, persistent, chlorinated hydrocarbons which are distributed widely in the environment. These compounds have long halflives and bioaccumulate in mammalian tissues. Therefore, they are interesting examples to consider in the context of a discussion of the significance of excursions of intake above the tolerable daily intake (TDI). This paper will provide some background information on the toxicity and pharmacokinetics of PCDDs and PCDFs and will discuss the implications of exceeding the TDI, 1 The views expressed in this paper are those of the author and do not necessarily represent those of the Department of Health. 2 Fax: 00.44/171.972.5167. E-mail: [email protected].
particularly in the context of high intakes by breast-fed infants. BACKGROUND INFORMATION
Toxic Equivalent Factors The chemical structures of PCDDs and PCDFs are given in Fig. 1. These compounds elicit a broad range of toxicological effects, many of which are species and tissue specific. Although there are a total of 210 different PCDD and PCDF congeners, there are only 17 toxicologically significant congeners, which are those substituted with chlorine at the 2, 3, 7, and 8 positions on the molecule. The most potent and extensively studied congener is the 2,3,7,8-tetrachloro-substituted dioxin (TCDD). It is widely accepted that the toxicologically active PCDDs and PCDFs exert their effects by a common mechanism involving binding to a cytoplasmic receptor protein called the Ah (aryl hydrocarbon) receptor. Certain polychlorinated biphenyl (PCB) congeners also bind to the Ah receptor and can exhibit similar toxicological effects to the 2,3,7,8-substituted PCDDs and PCDFs. Since these compounds are considered to act by a common mechanism, and because they occur as mixtures in the environment, in food, and in human tissues, they are commonly assessed and regulated as a class. However, detailed toxicological information is available only for TCDD. Therefore, the concept of “TCDD equivalents” or “toxic equivalents” has been introduced to enable the assessment of the toxicity of mixtures of these compounds and to enable risk assessments of these mixtures to be carried out. This concept uses the available toxicological and in vitro biological data, and knowledge of structural similarities among the 2,3,7,8-substituted PCDD and PCDF congeners and the 13 “dioxin-like” PCB congeners (hereafter collectively termed “dioxins”), to generate a set of weighting factors or “toxic equivalency factors” (TEFs), each of which expresses the toxicity of a particular congener in terms of an equivalent amount of TCDD. Multiplication of the concentration of the congener by its TEF gives a TCDD toxic equivalent (TEQ). The toxicity of
S63 © 1999 International Life Sciences Institute, Washington, DC All rights reserved.
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FIG. 1. Structures of dibenzo-para-dioxin and dibenzofuran.
any mixture, relative to TCDD, is thus taken to be the sum of the individual TEQs (Department of the Environment, 1989). There are obviously a number of assumptions implicit in the toxic equivalents approach. It assumes that the effects of the different congeners are additive, whereas antagonistic or even synergistic interactions are possible. Also, where the TEFs are derived from in vitro data, the assumption is made that the pharmacokinetic behavior in vivo is equivalent to that of TCDD. Pharmacokinetics The pharmacokinetics of dioxins are congener, dose, and species specific. As with toxicological data, most information is available for TCDD. The pharmacokinetics of dioxins have been reviewed by Olson (1994). For humans, the main route of exposure to TCDD is the diet, particularly fatty food. The limited data available suggest that TCDD is effectively absorbed following oral exposure in an oil vehicle. There is some evidence from the rat that absorption may be dosedependent, with increased absorption occurring at lower doses. Although TCDD is initially distributed after absorption to all organs, within several hours the liver and adipose tissue become the primary sites of deposition. Disposition of TCDD is dose-dependent in rats and humans. In chronic studies in rats, at higher dose levels (above 1 ng/kg body wt/day), TCDD is preferentially deposited in the liver but, at lower dose levels, steady-state levels in liver and adipose tissue are similar. There is evidence that TCDD can be metabolized by a range of mammalian species to polar metabolites which are readily excreted in the urine and bile. Metabolism is also generally considered a detoxification process. TCDD is excreted slowly from all species tested, with elimination half-lives of approximately 11 days in the hamster (Olson et al., 1980), 12–31 days in the rat (Pohjanvirta et al., 1990; Rose et al., 1976), and 94 days in the guinea pig (Olson, 1986). Estimates of half-life in human have been derived from blood levels of TCDD measured over time in a cohort of potentially heavily exposed Vietnam veterans (the “Ranch Hand” cohort). The most recently reported estimate of half-life in this cohort, derived from multiple measurements from serum collected over 10 years from 213 individu-
als, is 8.7 years (95% confidence interval, 8.0 –9.5 years) (Michalek et al., 1996). The few existing data on other congeners indicate that there can be marked differences in pharmacokinetic parameters from those for TCDD. For example, octachlorodibenzodioxin (OCDD) exhibits only limited absorption (2–15%) in the rat. The whole body half-life of OCDD in the rat is between 3 and 5 months (Birnbaum and Couture, 1988). However, in a study in humans, the median half-life for OCDD was found to be 6.7 years, compared to 6.9 years for TCDD (FleschJanys et al., 1994). The longest half-life found in this study was 19.6 years, for 2,3,4,7,8-penta-CDF, although this figure was derived from only five subjects. There also appear to be differences in the relative disposition of the higher chlorinated PCDD and PCDF congeners in the liver and in adipose tissue, compared to TCDD (Thoma et al., 1990). Derivation of the TDI There is a vast literature on the toxicology of dioxins and, in recent years, a large number of mechanistic studies have been published. No attempt will be made here to present a comprehensive review of the toxicity of these chemicals. However, in order to inform the later discussion, the derivation of the UK TDI for dioxins will be described briefly. This TDI was recommended by a WHO Regional Office for Europe Expert Group in 1990 and, subsequently, was endorsed by the UK Department of Health’s Committee on the Toxicity of Chemicals in Food, Consumer Products, and the Environment (COT) (Ministry of Agriculture, Fisheries, and Food, 1992). The only congener on which it is possible to carry out a comprehensive hazard assessment is TCDD. This compound produces a wide range of toxic effects in laboratory animals, and there are interspecies differences in sensitivity and in the type of response seen. In chronic toxicity studies, the toxic effects caused by TCDD include thymic atrophy, hepatotoxicity, immunotoxicity, effects on reproduction and development, and carcinogenicity. Both the WHO Europe Expert Group and the COT identified carcinogenicity and effects on reproduction and development as critical effects in laboratory animals and used the no-observedadverse-effect levels (NOAELs) for these effects in animal studies as the basis of the TDI. Since TCDD is negative in mutagenicity tests, it was considered that it was unlikely that its carcinogenic action was due to genotoxicity and that it was appropriate, therefore, to set a TDI based on the carcinogenicity data. The critical study used for identification of a NOAEL for carcinogenicity was a 2-year study in which groups of rats received TCDD in the diet at levels of 1, 10, and 100 ng/kg body wt/day from 6 to 7 weeks of age for 2 years (Kociba et al., 1978). The critical end point was
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FIG. 2. Accumulated body burden for a dioxin congener with a half-life of 9 years, following ingestion of 10 pg/kg body wt/day from birth (“no breast feeding”) or following 6 months breast feeding and ingestion of 10 pg/kg body wt/day thereafter.
liver neoplasms, and the NOAEL was 1 ng/kg body wt/day. However, in view of the long half-life of TCDD, the critical parameter was considered to be not the ingested dose, but the steady-state liver concentration of TCDD at this dose (540 ppt). The daily dose in humans which would give rise to this liver concentration was estimated, and a safety factor of 10 was applied to this figure to allow for any remaining differences in susceptibility between rat and human. This gave a TDI of 10 pg TCDD/kg body wt/day. A similar approach was used to derive a TDI from the reproductive toxicity data. However, since in this case the target organ concentration of TCDD at the NOAEL was not known, the anticipated adipose tissue concentration at this dose was used as a surrogate. A rat teratology study was used as the critical study and a NOAEL of 30 ng TCDD/kg body wt/day was identified (Sparschu et al., 1971). Again, a TDI of approximately 10 pg/kg body wt/day was derived. The COT recommended that, when considering mixtures of dioxins, the TDI can be regarded as 10 pg TCDD equivalents/kg body wt/day. INTAKES OF DIOXINS FROM FOOD AND HUMAN MILK
The most recent estimates of intakes of dioxins from the diet in the United Kingdom were reported by the Ministry of Agriculture, Fisheries, and Food (MAFF) in 1997 (MAFF, 1997). The data are derived from analyses of total diet study samples collected in 1992 and are based on food intakes by an average UK adult consumer. They are presented as the TEQ of all congeners present. The TEQs were calculated using the Interna-
tional TEFs for PCDDs and PCDFs (Department of the Environment, 1989) and the TEFs for PCBs proposed by Ahlborg et al. (1994). The upper bound dietary intake of dioxins by the average UK adult consumer was estimated as 2.4 pg TEQ/kg body wt/day or 144 pg TEQ/day for a 60-kg adult (the upper bound estimate is calculated using the assumption that where the levels of the individual congeners are below the limit of detection, they are present at the limit of detection). This figure is well below the TDI. However, MAFF also reported estimates of intakes of dioxins by breast-fed babies, using analytical data obtained from three pooled human milk samples taken in 1993 or 1994. Intakes were estimated as 170 pg TEQ/kg body wt/day at 2 months of age, falling to 39 pg TEQ/kg body wt/day at 10 months of age, i.e., they can exceed the TDI by almost 20-fold. It is interesting to consider the consequences of these high intakes in early life for the body burden of dioxins in humans and for the risk assessment of dioxins. BODY BURDENS OF DIOXINS IN BREAST-FED AND NON-BREAST-FED INDIVIDUALS
For most chemicals with a half-life of a few hours, intakes up to 20-fold above the TDI by young babies would cause considerable concern. However, is this the case with long half-life chemicals such as TCDD? Figure 2 shows the increase in the body burden of dioxins with age either following ingestion daily of the TDI of 10 pg TEQ/kg body wt/day from birth or following 6 months breast feeding and ingestion of the TDI daily thereafter. The body burden has been calculated using the assumptions that the half-life of all congeners con-
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tributing to the TEQ is 9 years, that there is 100% absorption of all congeners, and that the body burden at birth is zero. The intakes from breast feeding are those estimated by MAFF (MAFF, 1997). It can be seen that breast feeding increases the body burden in early life but the steady-state level, achieved at about 40 –50 years of age, is not increased. The steady-state body burden at the TDI is estimated to be approximately 45 ng TEQ/kg body wt. For comparison, it is interesting to note that the body burden of dioxins in the general population, calculated from average background serum lipid concentrations, has been estimated as 13 ng TEQ/kg body wt (De Vito et al., 1995). Thoma et al. (1990) have reported levels of dioxins (PCDD and PCDF congeners only) in adipose tissue from eight infants aged 2–13 months and from 28 adults aged 31– 80 years. As anticipated from Fig. 2, the levels were lower in infants than in adults for all congeners except OCDF. Beck et al. (1990) measured the levels of dioxins (PCDD and PCDF congeners only) in adipose tissue in a sudden infant death baby (9.3 months old) who had been breast fed for about 80 days. A level of 3.4 ng TEQ/kg body wt was found. In comparison, the levels in two non-breast-fed infants of 3.8 and 4.8 months of age were 2.8 and 2.1 ng TEQ/kg body wt, respectively. Although the number of subjects is small, the difference between the breast-fed and nonbreast-fed infants is lower than would be expected from Fig. 2. This cannot be explained by low absorption of dioxins in the breast-fed infant, since the bioavailability of dioxins from human milk has been shown to be about 95% (Korner et al., 1993; Pluim et al., 1993). TOXICOLOGICAL IMPLICATIONS
Is the higher body burden of dioxins achieved in early life by breast-fed babies likely to be toxicologically significant? Do the studies on which the TDI is based accommodate differences in the rate at which the steady-state body burden is reached in rats and in humans? The TDI is based on two end points—carcinogenicity and toxicity to the fetus. In the Kociba carcinogenicity study, dosing of the rats commenced at 6–7 weeks of age. Assuming a half-life for TCDD in the rat of 24 days (De Vito et al., 1995) and an initial body burden of zero, steady-state body burdens would have been achieved at 20–24 weeks of age, approximately 20–25% of the animal’s life span. In humans, steady-state body burdens are not achieved until 40–50 years of age, 55–70% of an average human’s life span. This appears reassuring, since it implies that the rats were exposed to the maximum, steady-state tissue level of TCDD for a longer proportion of their life span than humans would be. Also, since it has been assumed that the carcinogenic effect is thresholded and that a certain tissue concentration must be achieved before there is an increased risk of cancer, the fact that the period of breast feeding does not lead to an increased
body burden overall should also provide reassurance. It may be worth noting, however, that (as in most carcinogenicity studies) dosing with TCDD in the Kociba study did not begin until the rats had reached puberty. Therefore, if there is increased susceptibility to the carcinogenic effect of TCDD in early life, this has not been taken into account in deriving the TDI. This may be particularly relevant in the context of high intakes of dioxins by breast-fed babies. The other study used to set the TDI was a teratology study in the rat. Theoretically, a woman of reproductive age who was breast fed as a baby will have a higher body burden than one who was not breast fed (see Fig. 2). If the exposure of the fetus is proportional to the mother’s body burden, this implies that the fetus of a breast-fed woman may be at increased risk compared to that of a non-breast-fed woman. However, since the TDI was derived from the adipose tissue concentration of TCDD at the NOAEL in the rat, rather than the ingested dose, there should still be an adequate safety margin to protect the fetus. Despite the fact that the TDI of 10 pg/kg body wt/day, based on studies in adult rats, appears to accommodate the high intakes of dioxins by breast-fed babies, it might be advisable to reevaluate the toxicological database on TCDD to ensure that there are no implications for early postnatal development. For example, Bjerke and Peterson (1994), using a cross-fostering design, have reported that lactational exposure alone to TCDD, administered to pregnant female rats as a single oral dose of 1 mg/kg body wt on gestational day 15, caused decreased plasma testosterone levels, decreased weights of secondary sex organs, decreased epididymal sperm reserves, and feminization of behavior in male rat pups suckled by the dams. It is not known how the exposure of the rat pups in this study compares with that of breast-fed babies. IMPLICATIONS OF SHORTER HALF-LIFE
What would be the effect of breast feeding on the body burden of a congener with a shorter half-life than TCDD? Figures 3 and 4 show the accumulated body burden following 6 months breast feeding and ingestion of 10 pg TEQ/kg body wt/day thereafter, for congeners with half-lives of 6 months, 1 year, and 3 years. The same intake estimates and assumptions are used as before. In the case of the shorter half-life congeners, breast feeding leads to a body burden at 6 months of age which is higher than that which would be achieved by ingestion of the TDI alone from birth, but which then falls to the steady-state level. However, in no case does the peak body burden of these congeners reach the steady-state body burden of TCDD (half-life of 9 years). Therefore, the use of the toxic equivalent approach to estimate the intake of mixtures of dioxins provides protection in the case of shorter half-life congeners, since it predicts greater accumulation, and hence
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FIG. 3. Accumulated body burden for dioxin congeners with half-lives of 6 months, 1 year, 3 years, and 9 years, following 6 months breast feeding and ingestion of 10 pg/kg body wt/day thereafter.
higher steady-state body burdens, than will actually be achieved by these congeners. In the case of congeners with longer half-lives than TCDD, it underpredicts the degree of accumulation. However, the data currently available indicate that only a few congeners have halflives longer than TCDD (Flesch-Janys et al., 1994). CONCLUSIONS
A number of conclusions can be proposed based on the considerations discussed above:
1. If the assumption is made that the half-life of all dioxin congeners in human milk is identical to that of TCDD (9 years), breast feeding is predicted to result in a higher body burden of dioxins in early life, but not to result in an increased steady-state body burden, compared to that resulting from ingestion of the TDI of 10 pg/kg body wt/day from birth. 2. For congeners with a significantly lower half-life, e.g., 1–3 years, breast feeding will lead to higher body burdens in early life than would have been reached by ingestion of the TDI from birth. However, these peak body burdens will still be below the steady-state body
FIG. 4. Accumulated body burden for dioxin congeners with half-lives of 1, 3, and 9 years, following 6 months breast feeding and ingestion of 10 pg/kg body wt/day thereafter.
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burden achieved by ingestion of 10 pg TCDD/kg body wt/day from birth. 3. The use of the toxic equivalent concept to estimate intakes of mixtures of dioxins is conservative when applied to congeners with half-lives shorter than that of TCDD, since it overestimates the likely accumulation of these congeners in human tissues. 4. The carcinogenicity study on which the TDI is based does not take account of any potential increased susceptibility to dioxins in early life. Otherwise, the TDI appears to accommodate the high intakes of dioxins by breast-fed babies, at least in relation to the end points on which it is based— carcinogenicity, fetotoxicity, and teratogenicity. 5. It is suggested that the toxicological database on TCDD is reexamined to assess whether postnatal development could be adversely affected by the high intakes of dioxins by infants during breast feeding. 6. More work is needed to assess whether breast-fed babies do achieve higher body burdens of dioxins than non-breast-fed babies, as predicted. ACKNOWLEDGMENT I thank Professor A. G. Renwick for the figures showing accumulated body burdens of dioxins over time.
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