Sunday, November 18, 2012

Taxol in plant form

Species name: Taxus canadensis

Common name: Canada Yew

Location: Ontario

The Canada yew is an incredibly important shrub to people all over the world, and most don't even realize it. While technically not a tree, most of its closest relatives can be up to 25 meters high. The Canada yew has no main trunk, and so just kind of flops onto the ground. It was originally thought to be a subspecies of the English yew, Taxus baccata, but was shown to be a distinct species through DNA sequencing and actually more closely related to the Western yew than the English yew! Just goes to show that plants that appear to be closely related based on morphology alone might be misleading you when it comes to their genetic relatedness.

The Pacific or Western yew has just recently been downgraded from threatened to nearly threatened on the IUCN's Red List of endangered and threatened species worldwide. Its eastern relative, the Canada yew, has now replaced the Western yew as a commercial shrub. So why the need to produce it commercially? Well, it is an incredibly attractive plant with its red arils and soft green needles. The arils of these plants, contrary to what they look like, are not fruits! Yes, they're soft and fleshy and contain a seed. But because conifers cannot produce fruits because they do not have flowers, the red arils can't possibly be fruits. Instead they're just highly modified seed coats, which have been modified to store sugars and other tasty things that are attractive to birds and small mammals. Unfortunately, these red arils are also attractive to children and lead to quite a few deaths or hospitalizations every year since the seeds are incredibly toxic. The animals that consume the yew arils only eat the fleshy outsides, and either swallow the seeds whole and pass them right through their digestive tract, or leave the black seeds behind.

Sure, the ornamental use of this plant is quite important in Canadian industry. But that's certainly not the most important use of this plant worldwide. In the late 1980s and early 1990s, this plant was discovered to have a unique set of chemicals called taxanes with an incredibly important role in human health: they kill cancer cells. The chemicals were first isolated in 1992, and lead to the widespread population decline of the Pacific yew in western Canada. The Canada yew in the east was then discovered to have just as high a concentration of taxanes that were just as easily purified from the bark, and this species is much more abundant. It can also be sustainably harvested every 5 or so years, and so the purification of taxanes was switched to the bark of the Canada yew. Once the process was perfected and clinical trials were completed, drugs were officially put into production; worldwide they're called paclitaxel (the generic name) or taxol (the brand name). These two drugs are now the most widely prescribed anti-cancer drugs around the world. The other reason why this is so significant? The research was done at the university that I currently attend, The University of Western Ontario in London. Hometown pride! While there are still some stands of the Canada yew being used for taxane production, some of these chemicals have been successfully synthesized in the lab. It's only a matter of time before the entire spectrum of taxanes can be artificially lab-created and so wild populations of both species can be left to their own devices.

The medicinal use of the Canada yew doesn't stop there, but any medicinal use of this plant is incredibly dangerous because it is deadly toxic. Native North Americans used the young tips of this plant in very small amounts steeped in tea and consumed to alleviate rheumatism. There is some clinical evidence that consumption of this plant is effective in this way for rheumatism, but it is a very fine line between an appropriate medical dose and a lethal or very toxic one. For this reason, most doctors (and even herbalists) prefer other routes of relief.