- TORONTO (CP) -- Hold the
pickles, hold the lettuce, but if you want immune protection against SARS,
don't hold the tomato.
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- American researchers have managed to grow a SARS vaccine
in genetically engineered tomatoes, reporting that mice fed the tomatoes
developed antibodies to the coronavirus that killed nearly 800 people in
2003.
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- Their findings, published Tuesday in the medical journal
Proceedings of the National Academy of Sciences, drew interest from others
in the scientific community. But experts were quick to caution that vaccine-laden
produce isn't heading to your dinner plate anytime soon.
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- "There's still a long ways to go between knowing
whether this is a promising lead and conducting further research or whether
other forms of technology would be more likely to be useful," said
Dr. Bob Brunham, director of the British Columbia Centre for Disease Control.
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- Dr. Brunham noted the scientists, from Thomas Jefferson
University's Biotechnology Foundation Laboratories, reported only that
the mice fed the vaccine-filled tomatoes developed antibodies.
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- They did not report on whether they "challenged"
the mice with the virus to see if those antibodies were the type needed
to actually protect against the disease. The simple presence of antibodies
can't be assumed to be protective.
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- "They didn't show ... that their antibodies would
neutralize a virus in tissue culture," said Dr. Brunham, who co-chairs
the SARS accelerated vaccine initiative or SAVI, a project established
in Canada following Toronto's deadly experience with the since-disappeared
disease.
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- The regulatory hurdles such a vaccine would face would
be sizeable, given that no vaccine produced in plants has ever been licensed
for human use, said Frank Plummer, scientific director of Canada's National
Microbiology Laboratory, where researchers have also been working on a
SARS vaccine.
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- "I think there are some practical hurdles for using
that kind of thing," Mr. Plummer said. "If they can be overcome,
it might be a very cheap way of producing proteins for vaccines. But it
hasn't made it there yet."
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- The notion of growing vaccine in plants " rather
than in animal cells or eggs, as is currently the industry norm "
dates back to 1992. Biologist Charles Arntzen proposed the idea of genetically
modifying bananas to serve as cheap, edible vaccines to protect against
a range of infectious diseases.
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- Others have taken up the notion, including renowned virologist
Hilary Koprowski, lead author of this work. Mr. Koprowski produced the
first oral polio vaccine (though another, designed by Dr. Albert Sabin,
is the one the world uses) and developed the rabies vaccine.
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- In this work, Mr. Koprowski and his team first genetically
engineered a low-nicotine tobacco plant to produce what's called the spike
protein of the SARS coronavirus. The spike protein is the key protein on
the SARS virus that triggers an immune response.
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- Tobacco is the easiest plant to genetically modify, co-author
Maxim Golovkin explained in an interview from Philadelphia. Once the team
was confident they could get tobacco to produce or "express"
the SARS protein, they tried the same technique in other vegetables.
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- "When the best works, you know what to apply to
those plants which are recalcitrant and much harder to produce transgenic
[models], like tomatoes," Mr. Golovkin said.
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- While Mr. Arntzen initially thought edible plants could
be produced that would immunize simply through consumption, that is no
longer considered a viable option.
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- There would be no way to guarantee that each fruit or
vegetable contained enough antigen to provoke immunity; nor would there
be any way of ensuring the person ate the whole dose. The picture of an
18-month-old spitting out half his peas comes readily to mind.
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- "Forget about this," Mr. Golovkin said flatly
when asked if a melange of vegetables might one day immunize kids rather
than a series of jabs. Government regulators would never allow a vaccine
to be licensed without guarantees the dose was standardized, he said.
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- "How would you normalize the amount of any vaccine
or any pharmaceutical drug delivered from eating something?" he noted.
"None of the countries of the world would allow you to deliver a drug
when it's not normalized."
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- Instead, Mr. Koprowski's team is freeze-drying produce
containing viral antigen, removing the water and grinding what's left into
a powder that can be formed into capsules or tablets.
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- They envisage vaccines that can be taken like a dietary
supplement, cheaply produced and stored for long periods of time without
suffering degradation.
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- "It's as easy as putting the seed in the soil and
growing the plant up to four or five weeks when it's most potent,"
Mr. Golovkin said.
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