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Yew - Medicinal Uses Of Yews

bark taxol western treatment

Yews have widely been recognized as toxic to livestock and humans. In smaller doses, yew has been used as a minor folk medicine in some parts of its range. Example of the medicinal uses of yews include the induction of menstruation, and the treatment of arthritis, kidney disease, scurvy, tuberculosis, and other ailments. However, in recent years, yews have become famous for their use in the treatment of several deadly cancers.

In particular, the dark-brown or purple bark of western yew has been found to contain relatively large concentrations of an alkaloid known as taxol. Taxol has been demonstrated as being an effective treatment against advanced cases of ovarian and breast cancer, two deadly diseases. The use of taxol for these purposes is now approved by the U.S. Food and Drug Administration and by regulatory agencies in other countries. This recently developed use of western yew has led to an essentially insatiable demand for its bark.

The concentrations of taxol in bark are variable, ranging from only one part per million (ppm) to 690 ppm. The concentration of taxol in foliage of western yew is generally smaller, ranging from 12 to 80 ppm. At the present time, taxol is most efficiently extracted from yew bark. Unfortunately, it takes about 14 kg of yew bark to yield only 1 g of taxol, equivalent to three to 12 yew trees per cancer patient treated. Clearly, very large amounts of yew bark must be collected in order to have sufficient material to satisfy the medical and research demands.

Yew is not a commercial species in forestry, and up until about 1989 the species was routinely trashed and usually burned after logging. Today, however, the bark of western yew is routinely stripped from the survivors of the logging aftermath. In addition, the bark is now widely stripped from living western yews in unlogged forests, including old-growth rainforests of the Pacific coast, where this species tends to be most abundant. Sometimes, this process is rather inefficient, and in some cases of poaching, the bark has only been taken from the lower part of taller yew trees, where it can be easily reached. Unfortunately, the removal of its bark usually kills the yews by destroying its vascular system, so bark stripping is a rather wasteful use of the plant. A system has been designed and implemented to regulate the harvest of yew bark, but it is difficult to enforce over extensive areas, and a great deal of poaching occurs.

The enthusiastic collection of western yew bark, while contributing to the successful treatment of some cancer cases, threatens to exhaust the very resource upon which this treatment depends, as it is extremely wasteful and does not aim to conserve the species for future use. This dilemma may be resolved if pharmaceutical biochemists develop an economical method of synthesizing taxol in the laboratory, making the extraction of the alkaloid from western yew bark unnecessary. Alternatively, means could be found to economically extract the taxol found in relatively small concentrations in other parts of the yew, especially the foliage. Another possibility is to establish managed plantations of western yew for the specific purpose of obtaining taxol. This is actually being done, but the western yew is slow growing, and it will take some years before the plantations can be economically harvested.

So far, these relatively sustainable approaches have not been developed to the point where pressure on wild western yews can be relieved, and this plant is being rapidly mined from its natural habitats. Because the western yew is not naturally abundant, its populations will rapidly become depleted, and the species will become endangered in the wild.


Resources

Books

Brockman, C.F. Trees of North America. Golden Press, New York: 1968.

Periodicals

Daly, D. "Tree of Life." Audubon 70 (1992): 76-85.

Joyce, C. "Taxol: Search for a Cancer Drug." BioScience 43 (1993): 133-36.

Bill Freedman

KEY TERMS

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Aril

—A fleshy, often brightly colored covering that partially encases a seed. The aril is edible and is intended to encourage an animal to eat the fruit and thereby disperse the seed.

Cultivar

—A distinct variety of a plant that has been bred for particular, agricultural or culinary attributes. Cultivars are not sufficiently distinct in the genetic sense to be considered to be subspecies.

Part per million (ppm)

—A unit of concentration, equivalent to 0.0001%, or one milligram in a kilogram, or one milliliter in a liter.

Sunfleck

—A transient patch of sunlight that travels over the forest floor as the Sun arcs overhead during the day.

Taxol

—An alkaloid chemical that can be extracted from the bark and other tissues of yews and is active in the treatment of human ovarian and breast cancers.

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