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Yew Trees & Taxol© Yew Trees
Yew trees (the genus Taxus) orginated during the age of dinosaurs. At least seven species of yews now grow in the temperate forests of Asia, Asia Minor, India, Europe, North Africa, and North America, but yew species around the world can cross-pollinate and interbreed, making possible the large number of ornamental yew cultivars propagated today.
Yew trees have long been revered. Humans have used them for medicine, tools, weapons, and ceremonies for as long as we have lived near them. Their slow growth, sprouting, longevity, and dark evergreen needles evoked symbolism imbued with death, rebirth, and immortality. Some yew trees in the British Isles are estimated to be over 4,000 years old, the last extant remnants of native vegetation in Europe.
Yews tolerate full sunlight, but in the virgin forests where they evolved, they inhabit the shaded under-story and grow very slowly. In mature forests, they provide perches for birds of prey, fruits and seeds for squirrels and birds, browse for deer and elk, and soil stabilization along streams.
Genetically diverse populations of wild yews are difficult and expensive to artificially propagate. Seeds are cumbersome to collect and require over a year to germinate. Only about 50% of wild yews yield cuttings root readily in nurseries. Young plants grow slowly and are subject to browsing when out-planted. Yews readily sprout new branches when pruned but if mature trees are cut down, they can take centuries to regain their original stature.
In 1960, the National Cancer Institute (NCI) and US Department of Agriculture began a cooperative venture to screen plants for novel anti-cancer compounds. Yews in the Pacific Northwest were sampled, and analyses in 1967 identified taxol as a promising compound, especially for refractory ovarian cancer.
During the next two decades, sufficient needles, branches, and bark of the Pacific yew (Taxus brevifolia) were collected from National Forests in the Pacific Northwest to establish the chemistry, extraction, and refinement of taxol and related taxane compounds, and to complete very promising clinical trials. Unfortunately, yew bark was identified as the most concentrated source of taxol, and many mature yews were destroyed for these trials by stripping their bark to extract taxol.
Because the compound then named taxol was identified by a publicly funded agency, it could not be patented (although the name later was). In order to interest pharmaceutical companies in its development, the NCI, via a competitive process, awarded Bristol Myers Squibb (BMS) exclusive rights to harvest yew bark on federal forest lands in the Pacific Northwest.
BMS now produces TAXOL© by semi-synthesis with compounds extracted from cultivated yew species and hybrids. However, paclitaxel (a generic name for TAXOL©), and its close analogues have been such useful drugs that other companies around the world are now producing these and related pharmaceuticals. Wild yews in the forests of the Pacific Northwest, Asia Minor, the southern Himalayas, and southeast Asia continue to be a source material.