There’s a bizarre
mindset that divides medicine into “natural” (made from plants; untainted by
villainous pharmaceutical companies; delivered to your veins by forest animals)
and everything else (“man-made” pills fashioned from profits and poisons). The
reality, of course, is that many of the drugs used in our hospitals and
pharmacies come from plants. Willow bark contains salicylic acid, the main
ingredient in aspirin. Paclitaxel (taxol) was isolated from the bark of the
Pacific yew tree; today, it is used to stop cancer cells from dividing. The
rose periwinkle has given us vinblastine and vincristine, both used to treat
leukaemia.
These examples scratch the surface of what the botanical
world has given us, and what it might still offer. Of the tens of thousands of
plants used in “traditional medicine”, a piddling proportion has been tested
for chemicals with medical benefits. How do we find the rest? How do we go
about the business of “bioprospecting”?
One solution is to tap the knowledge of indigenous populations, who still rely
on plants for traditional medicine. When they get sick, how do they heal
themselves?
But this approach has problems. Traditional use doesn’t
always imply an actual medical benefit, and the chosen plants might not yield
interesting chemicals any more readily than the species around them. Many
attempts to follow such leads have ended in the drug-development
cul-de-sac. To make matters worse, collating traditional knowledge involves
fieldwork and training, and is both expensive and time-consuming.
Meanwhile, the tools of molecular biology have become faster
and cheaper. Companies can afford to gather large collections of plants, and
screen their constituent chemicals en masse. Why filter them any further when
you can test thousands of samples at once? But Haris
Saslis-Lagoudakisfrom Imperial College London thinks that this scattershot
approach to bioprospecting is a mistake. To him, traditional knowledge still
has great value in honing our search for tomorrow’s drugs.
He made his point by creating a family tree (a phylogeny) of
over 20,000 plant species from New Zealand, Nepal, and the Cape of South
Africa. Around 1,500 of these are used in traditional medicine and these,
rather than being spread out throughout the family tree, are actually clustered
in certain branches. The “hottest” branches contained 60 per cent more
traditionally used plants that you’d expect if they were distributed randomly.
As one example among many, rushfoil (Croton)
and physic nut (Jatropha) are
close relatives form the spurge family, and are both used to treat malaria in
Nepal. “We know that close relatives can share many of the chemical compounds
they produce,” says Saslis-Lagoudakis, “so our results suggest that the
use of Croton and Jatropha to treat malaria is due
to underlying shared chemistry between them.”
Saslis-Lagoudakis also found that people tend to use related
plants from the three continents to treat medical conditions that afflict the same
organs. For example, members from the soapberry family (Sapindaceae)
are used to treat digestive problems in New Zealand (Alectryon), Nepal (heartseed and Ceylon oak) and South
Africa (jacket plum). Since
these places are so distant, and their native floras are so radically
different, it’s likely the people there discovered the properties of their
local plants independently.
To Saslis-Lagoudakis, these trends suggest that plants don’t
make their way into a healer’s repertoire through superstition or chance.
Instead, it’s their medical properties – their bioactivity – that makes them
useful. And since drug manufacturers search for those same properties, the
evolutionary relationships between traditionally used plants could help to
guide their search.
But Michael Heinrich from University College London cautions
that there could be other explanations for the results. Saslis-Lagoudakis
thinks that the close relationships between traditionally used plants reflect
their chemistry. Heinrich wonders if it reflects their “weediness”. Weeds are
more likely to be found and used, and families that are rich in weeds – such as
daisies and mints – are a common part of traditional repertoires. “If you have
to search for something to treat your diarrhoea, would you walk up to the Welsh
mountains and try to get a rare endemic species or just use what grows in your
backyards?” says Heinrich.
Still, it seems that bioprospectors are already on the path
of using traditional knowledge, even if they’re not aware of it. When
Saslis-Lagoudakis listed all the plants that have yielded chemicals either
already in use, or going through trials, he found that they’re more likely to
belong to groups being used in traditional medicine, and to the “hot” branches
of his family tree.
Within these branches, question marks hang over more than 80
percent of species. They haven’t been checked by bioprospecting companies, and
many aren’t being used by traditional healers. We have no idea what chemicals
they contain, and Saslis-Lagoudakis writes that they “have high potential to
deliver new medicines”.
He thinks that even in the era of cheap powerful molecular
biology, traditional knowledge can make bioprospecting programmes more
effective in three ways. They can tell us which conditions plants are used to
treat, which could help to focus our tests. They can reveal which parts are
used, and which organs can be ignored. And they can show how the plants
are processed before being used, which “indicates how best to prepare a
plant sample for testing.”
“We hope our new methods in how traditional knowledge
can be used to search for new drugs will direct bioprospectors back towards
traditional medicine, and encourage more ethnobotanical fieldwork,” he says.
Heinrich agrees with the need for more fieldwork, especially
in “the many understudied regions of the world, such as Southern Africa, New
Zealand, and South and Central America”. But he cautions that bioprospecting
companies also take other considerations into account, like how different new
compounds will be to existing ones, and how more effective they will be to
existing gold standards. It’s not just about the chemistry; it’s about the
applications too.
Reference: Saslis-Lagoudakis, Savolainen, Williamson,
Forest, Wagstaff, Baral, Watson, Pendry and Hawkins. 2012. Phylogenies reveal
predictive power of traditional medicine in bioprospecting. PNAS http://dx.doi.org/10.1073/pnas.1202242109
Photos by Stan Shebs (left)
and Neelix (right)
Data: 11.09.2012
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