Researchers have isolated potential drugs from this plant,
the wild tomatillo.
Some of the most successful drugs we know about have come
from plants, including the cancer drug Taxol, the malarial treatment
artemisinin, and even aspirin. The latter two were identified because of their
use in herbal remedies that targeted known symptoms.
Many plants produce a complex mixture of chemical compounds,
and the lure of finding new drugs has resulted in what has been termed
"bioprospecting," or searching plants for compounds that have activity
in various drug assays. Choosing plants that are already used in traditional
herbal remedies would seem to be the right place to start, but it's not clear
which (or what percentage) of these remedies are based on anything more than a
placebo effect. Now, a study in PNAS suggests that a combined
knowledge of evolution and local lore might hold the key to finding the plants
with the best chance of producing a useful drug.
The challenge of working with plants that are used as herbal
remedies is that there are so many: most indigenous cultures seem to have had
them, and most involve plant species that don't have a global range. In
addition, some of the more popular ones seem to have been shared between
cultures, and it's not clear whether that is a product of their effectiveness.
How do we make sense of all this mess? The authors of the
new study make two assumptions to help narrow things down. The first is that,
if a remedy is successful, it will be discovered by more than one culture. The
second is that a given remedy wouldn't be limited to a single species, but is
likely to be made by that species' closest relatives.
What that means is that cultures are in geographically
remote locations will have a similar set of remedies made from plants. But,
because they are so far removed from each other, they generally won't have
access to precisely the same species, but will be able to identify closely
related ones. So, if you can identify closely related species that are used for
similar purposes by different cultures, chances are this group contains a
product with interesting properties.
To test this, the authors obtained information about
medicinal plants used by cultures from three very distinct locations: Nepal,
New Zealand, and South Africa. They then compared these plants to a list of
species that we have an evolutionary history for, primarily from DNA sequence
comparisons. Each area had thousands of plants, and a fraction of these were
used for medicinal purposes (under five percent for everywhere but Nepal, which
used 14 percent of the available plants).
The authors identified all the medicinal plants on the
evolutionary tree, and then looked for cases where a species in a given genus
that were used in all three cultures. These were dubbed "hot nodes,"
or groups of species that were frequently put to medicinal use in a given area.
They found a hot node in one area was 38 percent more likely to predict the
presence of medicinal plant used for the same condition in other areas than
you'd expect by random chance.
This wasn't always statistically significant, so the authors
examined the evolutionary tree as a whole. They found that hot nodes include
133 percent more medicinal plants than a random sample of species would.
Although the results weren't always significant, the authors
consider the frequent identification of cross-cultural medicines as providing
an important validation. "That we recover a phylogenetic signal in our
cross-cultural comparisons is strongly indicative of independent discovery of
efficacy," the write, "and provides unique large-scale evidence that
plant bioactivity underlies traditional medicine." In other words,
although some of these remedies may be placebos, the tendency of remedies to
line up with evolutionary history across distant cultures suggests that there
are real effects here.
Will this make developing drugs any easier? Here, the
authors admit the message is a bit mixed. As it turns out, a comprehensive
study to identify medicinal plants in a given country is a lot of work, so it
isn't a simple solution. But it might pay off in the end. Not only can they
help focus bioprospectors on the right plant, but also on the sorts of ailments
it might work for, how it's administered, and so on. All of which could speed
up the drug testing process considerably.
Texto: John Timmer
Data: 11.09.2012
Data: 11.09.2012
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