Harvesting Drugs From Medicinal
Annual wormwood, Artemisia annua L., yields the important antimalarial
Ancient medical texts, some dating back to the early Greeks, talk about
medicinal plants. Now modern scienceincluding some done by USDA's
Agricultural Research Serviceis
taking this ancient art to new levels.
Plant physiologist Stephen O. Duke heads ARS' Natural Products Utilization
Research Unit at Oxford, Mississippi. Part of his research involves discovering
how plants make their beneficial compounds and how to better extract them.
One example of his work is understanding the plant known as annual wormwood,
Artemisia annua. This gray-green aromatic plant and its relatives in the
genus Artemisia have been used to make absinthe and flavored wines since
earliest times. Now this plant family could bring a new gift: Its natural
pest-fighting defense may protect humans from malaria.
It's no secret that malaria-fighting drugs have done a lot for
civilizationthe Panama canal is one testimony of their success. But what
happens when the organisms that cause the disease develop resistance to current
Right now, scientists are preparing to solve this problem before it ever
occurs by having alternative treatments ready. One of these understudy cures
could be artemisinin, a natural compound produced by Artemisia plants.
Chemist Agnes Rimando and plant physiologist Stephen Duke use steam
distillation to determine the essential oil content of wormwood
Medical researchers, especially in the military, want to know more about
wormwood's malaria-fighting properties. That's because when duty calls U.S.
troops to a tropical area, the disease is always a potential problem.
The question Duke wants to answer is exactly how the plant produces this
potentially life-saving compound. Knowing the physiology would play a role in
increasing the supply of this beneficial compound.
"It was already known that wormwood has little balloonlike glands on
its leaf surface," Duke says. "We found that as the plant matures,
these balloons fill with artemisinin. Pest protection is nature's goal. As the
plant matures, the glands swell bigger and finally burst, covering the plant
with self-made pesticide."
It is the natural pesticide aspect of annual wormwood that brings it even
more within Duke's expertise. His research group's primary mission is to find
natural products to control agriculture pestsespecially in America's
There's money to be made in fighting pests, but it's mainly in protecting
the nation's agronomic superstarscorn, wheat, and soybeans. That can mean
that important, but less prominent, crops such as avocados and orchids are ripe
for new protective treatments.
The scientists in Duke's research unit search the plant and microbial
kingdoms for pest-fighters that work in harmony with the environment.
Artemisinin, for example, may also yield its protective powers to otherwise
"We found that without the genetic coding for the gland, Artemisia
won't produce artemisinin," says Duke. "For the plant, it's
essential to have the genetic instructions for both the toxin and the storage
Microscopic view of a hypericin-containing gland of St. John's-wort,
Knowing how a plant's DNA programs it for pest protection may lead
researchers to ways to provide the same pest resistance to currently vulnerable
Products for People Too
Besides the crop protection aspects of their research, Duke and his team of
scientists are also looking at the pharmaceutical value of these plants. In
fact, Artemisia is just one of the plants they're exploring.
Another plant that's intrigued Duke is St. John's-wort, which belongs to the
genus Hypericum. Greek texts dating back to 2000 B.C. have noted uses
and harvesting techniques for this plant. Currently, its claim to fame is as an
alternative treatment for depression.
"St. John's-wort is the preferred treatment for mild depression in
Europe," says Duke. "Physicians there choose it four to five times
more often than synthetic drugs because they believe it has fewer side effects.
Europeans get their supply from Albania, but it grows wild in the United
Much to the chagrin of ranchers who know St. John's-wort as a pest, horses
and cattle that eat the plant develop a sensitivity to light, resulting in a
rash, Duke says. Understanding the biochemistry involved might help reduce the
But now, however, Duke's main concern is extracting a red pigment from the
plant to benefit people.
St. John's-wort tablets are standardized by the amount of the red pigment,
called hypericin, which some researchers suspect is the active ingredient.
Hypericin is being studied as both an anti-viral and anti-cancer drug.
St. John's-wort has yet to receive FDA approval for use as a treatment for
depression in the United States. But several companies are doing phase I and II
clinical trials, a step toward gaining approval. Currently, Americans can buy
St. John's-wort as a diet supplement.
Getting the Good Stuff Out
Finding an economical extraction method for hypericin would be helpful in
developing St. John's-wort both for medicinal purposes and as an anti-viral
agent. Since plant extraction and physiology are aspects of Duke's work, he
began with research on the plant.
A highly effective chloroform dip procedure enables plant biotechnologist
Camilo Canel to extract over 90 percent of artemisinin from the glandular hairs
of wormwood leaves.
It was already known that hypericin was concentrated in small black and red
dots found on the flowers and leaves of St. John's-wort and that it was
effective in pest control. But hypericin, if given in a high enough
concentration, is toxic to all living things, including St. John's-wort. The
plant protects itself by sealing the hypericin dots off with a thin cell layer.
Normally, hypericin is extracted by chopping the plant up and extracting it
with ethanol. But Duke may have a better way.
"The plant has other enzymes that can destroy hypericin when the cells
containing the toxin are breached," he says. "Crushing the plant
releases these hypericin-destroying proteins, defeating your purpose. We are
looking at chemical extraction methods that may work betterlike soaking
the leaves in a solution that gently removes the hypericin without having to
While breeders and pharmaceutical companies are waiting to see what this
plant can do, they can also stay tuned to Duke's research team. Already these
scientists are hot on the trail of new helpful plants and better extraction
techniques. Last fall, Duke had just finished completing his staffing, and the
scientists were busy ordering equipment to start new research projects.
"All I can say is we are excited about the new research projects we've
begun, and we're always looking for new ideas," says Duke. "We hope
to excel in this newand yet ancientfield of natural pesticides and
Lee, Agricultural Research Service Information Staff, 6303 Ivy Lane,
Greenbelt, Maryland 20770, phone (301) 344-2781.
Stephen O. Duke is in the USDA-ARS
Natural Products Utilization
Research Unit, National Center for the Development of Natural Products,
Room 1011, University of Mississippi, Oxford, MS 38677; phone (601) 232-1036,
fax (601) 232-7062.
"Harvesting Drugs From Medicinal Plants" was published in
the April 1998 issue of Agricultural Research magazine. Click here
to see this issue's table of contents.