2,3,7,8-Tetrachlorodibenzodioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a polychlorinated dibenzo-p-dioxin (in short but inaccurately also called dioxin). It is the most potent compound (congener) of the series and became known as a contaminant in Agent Orange, a herbicide used in the Vietnam War, as well as the Seveso disaster. It is a persistent environmental contaminant usually present in a complex mixture of dioxin-like compounds.

Mechanism of action
TCDD and dioxin-like compounds act via a specific receptor present in all cells: the aryl hydrocarbon (AH) receptor. This receptor is a transcription factor which is involved in expression of genes; in fact it has been shown that high doses of TCDD either increase or decrease the expression of several hundred genes in rats. Genes of enzymes activating the breakdown of foreign and often toxic compounds are classic examples of such genes. TCDD increases the enzymes breaking down, e.g., carcinogenic polycyclic hydrocarbons such as benzo(a)pyrene.

Also these polycyclic hydrocarbons activate the AH receptor, but less than TCDD and only temporarily. Even many natural compounds present in vegetables cause some activation of the AH receptor. This phenomenon can be viewed as adaptive and beneficial, because it protects the organism from toxic and carcinogenic substances. Excessive and persistent stimulation of AH receptor, however, leads to a multitude of adverse effects.

Scientists have searched for the physiological functions of the AH receptor for years, and one obvious function is to increase the activity of enzymes breaking down foreign chemicals or normal chemicals of the body as needed. There may be other functions, however, related to growth of various organs or other regulatory functions. The AH receptor is phylogenetically highly conserved transcription factor with a history of at least 500 million years, and found in all vertebrates, and its ancient analogs are important regulatory proteins even in more primitive species. In fact, knock-out animals with no AH receptor are quite sick and develop poorly. All this implies that a certain level of AH receptor activation is physiological and necessary for the body.

TCDD is not mutagenic and not directly genotoxic. Its main action in causing cancer is cancer promotion; it promotes the carcinogenicity initiated by other compounds. Very high doses may, in addition, cause cancer indirectly; one of the proposed mechanisms is oxidative stress and the subsequent oxygen damage to DNA. There are other explanations such as endocrine disruption or altered signal transduction. The endocrine disrupting activities seem to be dependent on life stage, being anti-estrogenic when estrogen is present (or in high concentration) in the body, and estrogenic in the absence of estrogen.

Toxic effects in animal studies
By far most information on toxicity of dioxin-like chemicals is based on animal studies utilizing 2,3,7,8-TCDD. There is barely any organ without some effects by high doses of TCDD. In short-term toxicity studies in animals the typical effects are anorexia and wasting, and even after a huge dose animals die only 1 to 6 weeks after the TCDD administration. Seemingly similar species are very differently sensitive to acute effects: lethal dose for a guinea pig is about 1 µg/kg, but to a hamster it is more than 1,000 µg/kg. A similar difference can be seen even between two different rat strains. Various hyperplastic (overgrowth) or atrophic (wasting away) responses are seen in different organs, thymus atrophy is very typical in several animal species. TCDD also affects the balance of several hormones. In some species, but not in all, severe liver toxicity is seen. Considering the low doses of dioxins in the present human population, only two types of toxic effects seem to be really important: developmental effects and cancer.

Developmental effects
Developmental effects occur at very low doses in animals. They include frank teratogenicity such as cleft palate and hydronephrosis. Development of some organs may be even more sensitive: very low doses perturb the development of sexual organs in rodents, and the development of teeth in rats. The latter is important in that tooth deformities were also seen after the Seveso accident and possibly after a long breast-feeding of babies in 1970s and 1980s when the dioxin concentrations in Europe were about ten times higher than at present.

Cancer
Cancers can be induced in animals at many sites. At sufficiently high doses TCDD has caused cancer in all animals tested. The most sensitive is liver cancer in female rats, and this has long been a basis for risk assessment. Dose-response of TCDD in causing cancer does not seem to be linear, and there is a threshold below which it seems to cause no cancer. TCDD is not mutagenic or genotoxic, in other words, it is not able to initiate cancer, and the cancer risk is based on promotion of cancer initiated by other compounds or on indirect effects such as disturbing defense mechanisms of the body e.g. by preventing apoptosis or programmed death of altered cells. Carcinogenicity is associated with tissue damage, and it is often viewed now as secondary to tissue damage.

TCDD may in some conditions potentiate the carcinogenic effects of other compounds. An example is benzo(a)pyrene that is metabolized in two steps, oxidation and conjugation. Oxidation produces epoxide carcinogens that are rapidly detoxified by conjugation, but some molecules may escape to the nucleus of the cell and bind to DNA causing a mutation, resulting cancer initiation. When TCDD increases the activity of oxidative enzymes more than conjugation enzymes, the epoxide intermediates may increase, increasing the possibility of cancer initiation. Thus a beneficial activation of detoxifying enzymes may lead to deleterious side effects.

Cases of poisoning
There have been a number of accidents where people have been exposed to high doses of dioxin-like chemicals (see Dioxins and dioxin-like compounds), but there are three historical cases of poisoning where the exposure has been to TCDD itself.


 * In 1976, thousands of inhabitants of Seveso, Italy were exposed to TCDD after an accidental release of several kilograms of TCDD from a pressure tank. A number of animals died, and high concentrations of TCDD, up to 56,000 pg/g of fat, were noted especially in children playing outside and eating local food. The acute effects were limited to about 200 cases of chloracne. Long-term effects seem to include a slight excess of multiple myeloma and myeloid leukaemia, as well as some developmental effects such as disturbed development of teeth and excess of girls born to fathers who were exposed as children. Several other long-term effects have been suspected, but the evidence is not very strong.


 * In Vienna, two women were poisoned at their workplace in 1998, and the measured concentrations in one of them were the highest ever measured in a human being, 144,000 pg/g of fat. This is about hundred thousandfold compared with TCDD concentrations in most people today, and about ten thousandfold compared with the sum of all dioxin-like compounds in young people today. She survived but suffered from difficult chloracne for several years. Aside from malaise and amenorrhea there were surprisingly few other symptoms or abnormal laboratory findings.


 * In 2004, then-presidential candidate Victor Yushchenko of Ukraine was poisoned with a large dose of TCDD. His blood TCDD concentration was measured 108,000 pg/g of fat, which is the second highest ever measured. This concentration implies a dose exceeding 2 mg, or 25 μg/kg body weight. Also he suffered from chloracne for many years, but again after initial malaise, other symptoms or abnormal laboratory findings were few.

Long-term effects in humans
The Expert Group of the World Health Organization considered developmental toxicity as the most pertinent risk of dioxins to human beings. Because people are usually exposed simultaneously to a number of dioxin-like chemicals, more detailed account is given at dioxins and dioxin-like compounds.

Cancer in humans
TCDD was classified in 1997 by the International Agency for Research on Cancer as a carcinogen for humans (group 1). In the occupational cohort studies available for the classification, the risk, even at very high exposures, was weak and borderline detectable. Therefore human data were not deemed sufficient, and the classification was, in essence, based on animal experiments and mechanistic considerations. This has been criticized as a deviation from IARC classification rules. It is much debated, whether TCDD is carcinogenic only at high doses which also cause toxic damage of tissues. Moreover, a recent review concludes that, after 1997, further studies do not support an association between TCDD exposure and cancer risk. New studies include the update of Vietnam veteran studies from Ranch Hand operation, which concluded that after 30 years the results do not provide evidence of disease.

There is also direct epidemiological evidence that TCDD is not carcinogenic at low doses, and in some studies cancer risk has even decreased. This is called a J-shape dose-response, low doses decrease the risk, and only higher doses increase the risk.