Toxicology – specification, history, toxicity assessment

Lecture No. 1

Copyright © Prof. MVDr. Zdeňka Svobodová, DrSc., Mgr. Zuzana Široká,

PhD.

Toxicology

Definition:• Toxicology is the study of the

adverse effects of chemical, physical or biological agents on living organisms and the ecosystem, including the prevention and amelioration of such adverse effects. (Society of Toxicology)

Historical background • Toxicology dates to the earliest humans• Poisons played an important role in the history

of mankind. In most cases, poisoning is caused by people's negative characteristics

• They may either be lack of information or ignorance, carelessness, untidiness, and, at worst, anger that may lead to cases of deliberate poisoning

• A category sui generis are cases of domestic pets poisoning (mainly dogs) in connection with a suicide of their owners (mainly by carbon dioxide)

Most known historical persons dealing with toxicology

- Dioscorides : first classification of poisons, use of emetics in treatment

- Paracelsus : „All substances are poisons; there is none which is not a poison. The right dose differentiates a poison from a remedy.“

- a physician – alchemist; set the basics of pharmacology, toxicology and

therapeutics; investigated the

dose-response relation

Famous poisonings and poisonersThe King Mithridates Eupator (1st century BC)• Is said to have taken small doses of 36 poisons to build tolerance

against them. When his son sent assassins to murder him, he attempted suicide by poison but the poison had no effect on him, and he had one of his servants kill him by the spear. This is where immunity from poison got its name of mithridatism.

- The principle of addiction:• The organism already responds during resorption, mainly at the

level of intestinal wall, and reduces the penetration of exogenous substances from the gastrointestinal tract to blood

• As part of adaptation, the organism enhances the capacity of detoxification processes

• The organism incorporates the poison into its biochemical processes, and the poison then becomes part of those processes. If the regular intake of the poison is interrupted,

the so-called withdrawal syndrome evolves• Examples of addictive substances are arsenic (arsenic used to be given to horses as a stimulant, but interrupting the supply may have led even to the death of the horse), NaCl, morphine, some plant toxins

Socrates• One of the greatest ancient philosophers was executed by a solution of the hemlock plant (Conium maculatum) in 399 BC. • Poison hemlock (Conium maculatum) grows on rubbish heaps. It contains the alkaloid coniine (most of the alkaloids are in fruit). The onset of toxic effects is in 20 – 30 minutes. The

death is most frequently caused by the cessation of breathing at full consciousness and before cardiac arrest.

• Coniine is a curare-like poison, such alkaloids block the transmission of stimuli from motor nerve endings to striated muscles, which subsequent leads to muscle paralysis. The paralysis progresses from the lower limbs upwards to respiratory muscles, and when these muscles are paralyzed, the victim dies of asphyxia. The brain is not affected and the victim retains consciousness.

• In the Middle Ages, many of the greatest poisoners were women

• The best known of them were Catherine de Medici and Lucrezia Borgia (a daughter of Pope Alexander VI.)

• Catherine had a cabinet full of different poisons at home, and she made the use of poisons a standard political tool. She was the wife of the French king Henry II, and later became Queen of France. She tried to prevent any weakening of the royal family's political power.

• In Italy, an infamous poisoner was a woman named Tofana, who made her mark by her arsenic-containing cosmetics ("Aqua Tofana").

Catherine de Medici

Lucrezia Borgia

• There are also many examples of the use of poison in more recent history

• In World War 2, German generals had a glass poison capsule (containing e.g. potassium cyanide) set in one of the tooth under a removable crown

• A case from a very recently past is that of the Ukrainian president Yushchenko, who was poisoned by dioxin (which left typical symptoms of acne on his face)

• Descriptions of animal poisoning also abound. • There have also been many cases of food poisoning: - Minamata Disease (Japan, 1950s – 1960s) This was a case of

methylmercury poisoning of both people and animals. A chemical factory dumped its mercury-containing waste to the Minamata Bay for dozens of years. Inorganic mercury in mercury compounds that accumulated in sediments on the bay bed began to transform to methylmercury (the most toxic form of mercury) under the action of bacteria. Methylmercury penetrated to the food chain of the aquatic environment, i.e. fish and subsequently to man. People (mostly fishermen) suffered of central nervous system disorders, loss of hearing and speech disturbance. Several cases of limb paralysis and severe mental disorders were also reported. Children were born with defects. Several dozen people die.

- Poisoning of people in Iraq in 1960s. Poisoning was caused because seed wheat was mistaken for food wheat. The wheat had been treated with phenylmercurychloride-based fungicidal agent. Although exported as seed, the wheat was eventually by mistake used as food wheat.

- Itai-Itai Disease (the ouch-ouch disease) In the 1950, Japan witnessed mass poisoned of its citizens with rice heavily contaminated with cadmium. The source of cadmium were ore dumps from which cadmium was washed by rainwater to the river. The name of the disease came from characteristic painful screams of the victims who suffered severe pain in the joints and the lower part of spine.

- The oil disease (Yusho) In 1968 about 1600 people in Japan were poisoned by rice oil contaminated with polychlorinated biphenyls (PCB) during the manufacturing process. The PCB leaked into edible oil from corroded pipes of the cooling system. The main symptoms of the poisoning were impaired immunity (people died of common infectious diseases – flu, pneumonia – rather than of PCB poisoning), damage to the nervous system, hyperkeratosis, hyperpigmentation, etc.

Methods to evaluate toxicity of chemicals

1) Based on cases of poisoning, cases studies (case histories)

2) Methods for toxicity prediction 3) Toxicity tests ad 1) It is not possible to perform toxicity

tests or to monitor effects of chemical substances on people and most species of animals. For that reason it is important to learn as much as possible from individual cases of poisoning, and that is also the reason why we so often return to the history of toxicology

ad 2) There is a relation between the chemical structure of substances and their biological characteristics. It cannot be assumed that knowledge of the molecule structure of a new xenobiotic alone will suffice for an unambiguous evaluation of biological properties. On the other hand, it will at least allow for a group classification. Toxicity prediction may be quite successful if we are dealing with a series of substances exhibiting chemical similarities. The relationship between the chemical structure and biological activity is expressed as Quantitative Structure – Activity Relationship (QSAR). Computer programmes are developed for computer-assisted toxicity predictions. But because these predictions may differ from reality, tests on experimental animals are necessary before a final decision can be taken. Thanks to prediction processes, however, the scope of these tests can be minimized (with a minimum number of experimental animals)

ad 3) Toxicity tests are performed at the level of• cells and tissues (in vitro tests)• organisms• Biocenoses

- For tests at the level of cells and tissues, primary cell cultures (higher sensitivity, lower reproducibility) or stable cell cultures (lower sensitivity, higher reproducibility) are used. Test can be evaluated directly (numbers of dead cells and extent of cytopathic effects) or indirectly (evaluations based on physiological reactions of the cells).

- One example is the Neutral Red Test (NRT). It uses the ability of undamaged lysosomes to incorporate and bind neutral red. After a 20-hour period of exposure to the test substance, a neutral red solution is added, left there to take effect, then it is drained off and lysing solution is added. Undamaged cells are lysed, and the neutral red released is measured photocolorimetrically.

- The advantage of in vitro tests is their speed, reproducibility, low financial and time demands. The disadvantage is that in vitro systems are no substitute for the enzymatic - immune system of the living organism. In spite of that, in vitro tests are suitable screening tests before animal experiments.

- The most frequently performed tests are those at the level of organisms. They must include all trophic levels, i.e. bacterias, invertebrates and vertebrates (experiments on fish, birds and mammals). Methods to be used in such experiments are unified world-wide by the Organisation for Economic Cooperation and Development (OECD) and the International Organization for Standardization (ISO).

- Tests at the level of biocenoses - these tests are very expensive and time consuming, they are used in special cases only. The preparation is applied over a defined area of land and its effects on soil microorganisms, earthworms, game, birds etc., are monitored, and its residual concentrations are studied. Similar methods are used for experiments in the aqueous environment.

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http://www.oecd.org/ http://www.iso.org