Trees of the genus Taxus are native to the northern hemisphere.
They are non-coniferous evergreens.
Despite belonging to the coniferous group, they do not have true cones but fruits containing a single seed surrounded by a red-coloured, mucilaginous and sweet aril.
They are slow-growing and long-lived trees (up to 1000 years), yew trees are dioecious, there are male and female trees that will only bear fruit if they are not too far from the male trees.
Most yews are highly poisonous, all parts of the tree (leaves, seeds, roots, wood) are poisonous except the coloured aril surrounding the seed.
For a long time yew trees were only known for their high toxicity and longevity and were associated with magical rituals, often mortuary.
Since the 1970s-1980s these plants have become important as a source of very powerful anti-cancer drugs.
Common yew, which can grow to a large size, was widespread in Europe in ancient times, but was gradually destroyed because of its toxicity to animals (especially horses) and for its wood (industrial manufacture of large bows and crossbows in the Middle Ages).
The North American yews (brevifolia and canadensis) are more discreet and less widespread; the Amerindians also used their wood (bows, tools, paddles).
The Japanese yew is also found in south-eastern Russia, northern China and Korea, it is a tree that can reach a large size (15 to 18 metres), it is not rare as it is an ornamental tree.
Two other Asian yews are being studied: Taxus chinensis and Taxus wallichiana, some botanists consider them to be varieties or subspecies of Taxus baccata.
Western yew bark was the first source of PACLITAXEL (TAXOL) and from young twigs and leaves of European (or common) yew, French chemists extracted a chemical precursor to DOCETAXEL
PACLITAXEL and DOCETAXEL are widely used in cancer chemotherapy, with worldwide demand for these products increasing steadily.
CHEMICAL COMPOSITION AND PROPERTIES
The yew fruit usually has a red coloured fleshy part, the aril, which surrounds a single seed.
The aril is edible and attracts fruit-eating birds, which release the intact seed into the wild, thus helping the yew to spread.
The aril contains sugars, proteins, a small amount of lipids including unsaturated fatty acids, minerals and vitamins.
The seed is dangerous, containing highly toxic (but very bitter) compounds that are released when crushed.
LEAVES, WOOD, BARK, ROOTS, SEEDS OF TAXUS TREES
All parts of yew trees (except the aforementioned aril) are toxic.
Toxicity varies according to the species of yew.
Taxus baccata (European yew), Taxus brevifolia (Western American yew), Taxus cuspidata (Japanese yew) are the most toxic.
Two types of compounds are mainly responsible for this toxicity:
These are alkaloids (numerous, the most important are taxine A and B) which inactivate the ion channels (calcium and sodium), particularly in the heart muscle cells, causing irregularity in its functioning until the heart stops.
The symptoms are :
- irregular heartbeat (arrhythmia),
- low blood pressure (hypotension),
- dizziness and headaches,
- there are associated digestive signs; abdominal pain, diarrhoea, vomiting,
- in the absence of emergency care in an intensive care unit: loss of consciousness, cardiac and respiratory failure.
Poisoning usually occurs by eating yew leaves or leaf extracts, but taxins can also penetrate through the skin (be careful when maintaining a yew hedge) or by breathing (e.g. wood dust).
There are no antidotes except perhaps antidigoxin antibodies (antidigitalin).
Curiously, herbivorous animals that avoid eating toxic plants are tempted by yew foliage, which remains toxic even when dry and on the ground.
The minimum lethal dose varies according to the animal: horses are very sensitive (1 to 2 mg per kg), pigs too (3.5 mg per kg), goats much less so (60 mg per kg) as well as deer and roe deer. Birds (including chickens) are fairly resistant (80 mg per kg).
To my knowledge, there is no use of these alkaloid taxins in medicine except for medical research.
Taxanes have a chemical structure very similar to taxins but are not considered alkaloids by chemists.
It is some of these taxanes that are used directly or after chemical modification in oncology.
The action sought by pharmacologists is to block cell multiplication (mitosis) in cancer cells.
At present, 3 taxanes are used massively in cancer chemotherapy:
- PACLITAXEL or TAXOL (1992) which is a natural substance,
- DOCETAXEL or TAXOTERE (1996) which is a chemical transformation (hemisynthesis) of a natural compound,
- and more recently CABAZITAXEL or JEVTANA (2010) which is also semi-synthetic.
These substances are poisons of the mitotic spindle, a temporary organ in the cell that controls the movement of chromosomes in cell reproduction (mitosis), mitosis is blocked.
These anti-cancer drugs are used to control mainly ovarian, lung, breast, prostate and oropharyngeal cancers.
These taxanes are very toxic substances but exist in small quantities in the leaves or bark of yew trees.
ORIGIN OF TAXANES
The worldwide demand for these anti-cancer drugs is increasing to such an extent that harvesting the leaves or bark of yew trees is no longer sufficient and results in the massive destruction of these slow-growing trees.
Pharmacologists currently believe that paclitaxel is probably synthesised in yew trees mainly by endophytic fungi (which live in the tree's tissues).
Indeed, some of these endophytic fungi, when isolated and grown, produce paclitaxel, which may solve the problem of supply of this very important drug.
YEW TREE CULTIVATION
Yews are very hardy and undemanding trees.
They can be propagated by seedling or by cuttings (in which case you know what sex the tree will be).
Yew trees have very dense foliage and grow slowly, which means that they can be pruned to create bushy hedges or trees of various shapes (a bit like boxwood).
Several hundred tons of young yew twigs are needed each year in France alone to manufacture anti-cancer drugs.
There are collection centres for yew prunings and also small companies that manage this collection (such as EVOGREEN in the east of France).
Yew leaves are very toxic. Do not try ever to treat yourself with yew leaves.
Anti-cancer products exist in very small quantities in these leaves and are extracted, purified and dosed by complex chemical techniques.
These are very toxic substances, the dosage of which in cancer therapy is very precise.
YEW or TAXUS, poisonous trees but source of anti-cancer drugs
YEWS are found in the northern hemisphere. They are non-coniferous conifers that are very toxic but contain small quantities of substances that are very useful in the fight against many cancers (ovary, lung, breast, prostate, mouth and throat). They are easy to grow, their fruit is partly edible but the seed is toxic.
Copyright 2022 : Dr Jean-Michel Hurtel