Cancer and birth defects
Tests for cancer and birth defects
Tests to see if chemicals cause cancer or birth defects are kinds of safety tests. Other kinds of safety tests are:
Toxicity testing, including LD50
Some chemicals cause abnormalities in babies when taken by pregnant women. The best known example is the morning sickness drug thalidomide, which caused shortened arms and legs in babies.
Currently chemicals are tested on animals to see if they will cause cancer or birth defects.
In
cancer tests
, rats and mice of both sexes are fed a chemical for 2 years. They are fed the maximum dose they can tolerate without dying. After 2 years all the survivors are killed and examined for tumours.
In
birth defect tests
, chemicals are fed to pregnant rabbits and rats or mice. Shortly before they are due to be born, the babies are removed from the womb and examined for abnormalities.
Many of the chemicals that cause cancer do so by damaging the genes or the chromosomes. There are several tests that look for this kind of damage.
Some chemicals that could cause cancer exist in the environment, for example, in waste water, around factories, or in the air. Plants have been used to test for such chemicals. For example, the Tradescantia plant has been used in Japan for many years to test for radioactivity around nuclear power plants. The flower is normally blue, but when a genetic mutation has taken place, the stamen hair cells will be pink. Broad bean or onion root tips are used to test for chromosome abnormalities caused by pollutants.
The thalidomide tragedy illustrates a major problem with animal tests, namely that each species reacts differently to drugs. Thalidomide was not tested on pregnant animals before going on the market. However, after deformed babies were born it was tested on many different species. Only in NZ White rabbits and primates did thalidomide produce birth defects like those in humans. Other species, such as guinea pigs, hamsters, rats and mice, are not affected in this way. Humans are particularly sensitive to thalidomide.
The opposite is also true -- sometimes humans are less sensitive than other species. Around 800 chemicals have produced birth defects in at least 1 species of mammal, but only around 25 are known to affect humans. For example, aspirin causes birth defects in rats, mice, guinea pigs, cats, dogs, and monkeys, but not when taken by humans.
Before a drug can affect an unborn baby, it has to cross from the mother's bloodstream through the placenta. The easiest way to study this process is to collect human placentas from maternity hospitals. They can be used to test if drugs cross the placenta, cause any damage to the placental blood circulation, or accumulate in the placental tissue.
With all safety tests it is important to remember two points:
If you would like to see a more detailed version of this document on tests for cancer and birth defects, including references, go to
More on Cancer and Birth Defects
.
If you would like to see detailed information on other safety tests, go to:
Skin and eye irritation tests, including Draize
Problems with cancer tests
Alternative cancer tests
Salmonella bacteria - the Ames test
This test uses a special strain of bacteria that contains a genetic mutation. If the test chemical alters the genes, the bacteria will return to normal and grown into healthy colonies. The bacteria are very sensitive to chemicals that cause genetic mutations, and this test is used in laboratories around the world.
Mammal cells - genetic mutations
Another test for genetic mutations uses cells in test tubes. If a mutation occurs when the test substance is added to the cell culture, the enzymes in the cells will no longer behave in the same way as in normal cells. Human white blood cells can be used for this test.
Mammal cells - DNA repair
Human liver cells from operations or organ donors can be used to look for signs of DNA repair when the test substance is added to the cell culture. DNA repair occurs after a chemical has caused damage to the genetic material.
Mammal cells - chromosome abnormalities
When cells divide, the chromosomes form two strands. Some chemicals cause breaks in these strands, or cause parts of one strand to break off and join the other strand in what is called sister chromatid exchange. Human white blood cells can be used for these tests.
Computer models
Computers are being programmed to recognise patterns in the molecular structure of chemicals already known to cause cancer. They use this information to predict whether a new chemical is likely to cause cancer. Computer models will become increasingly accurate as their database includes more and more chemicals.
Plant tests for environmental pollutants
Problems with birth defect tests
Alternative birth defect tests
Cell culture tests
Unfortunately many of these tests kill newborn or unborn animals to obtain cells to grow in test tubes. The aim is to see whether the cells will develop into normal tissue when the test chemical is added. However, there is now a permanent line of mouse embryonic stem cells that can be used for these tests. In a permanent line, the cells are reproduced continuously in the laboratory, and no further animals are killed.
Hydra test
The hydra is a very tiny and simple animal. If it is cut into pieces, each piece will grow into a new hydra. Chemicals that cause birth defects also alter the way these new hydras develop. The hydra test predicts the danger of thalidomide, whereas rat and mouse tests do not.
Drosophila eggs
Fruit fly eggs are homogenised when they are only a few hours old. The eggs contain cells that will grow into muscle and nerve tissue. A chemical that causes birth defects interferes with the development of these tissues.
Toxicity tests, including LD50