Is the Drought Over for Pharming?
Science 25 April 2008:
Vol. 320. no. 5875, pp. 473 - 475
DOI: 10.1126/science.320.5875.473
News
Is the Drought Over for Pharming?
Jocelyn Kaiser
Despite technological, economic, and social issues, companies are plowing ahead, making drugs and other compounds in plants
In the bag. These cultured carrot cells are engineered to make a human drug.
CREDIT: PROTALIX
Many a child has been told "carrots are good for you." That advice could soon take on new meaning for people with Gaucher disease, an inherited metabolic disorder that leads to liver and bone problems. Patients must now be injected every 2 weeks with a manufactured enzyme that costs on average $200,000 a year, making it one of the most expensive drugs ever. If ongoing clinical trials go well, the 5000 Gaucher patients on the therapy could soon have a second option--a cheaper version of the enzyme that stays in the bloodstream longer and can be injected less often.
If the U.S. Food and Drug Administration (FDA) approves recombinant glucocerebrosidase, it will be good news not only for medicine but also for a community far removed from the clinic: plant scientists. Protalix Biotherapeutics in Karmiel, Israel, produces this new version of the protein in giant plastic bags, not in steel vats of mammalian cells like most biologics are. The bags are filled with transgenic carrot cells that are cultured and then processed to extract the drug. "If Protalix gets regulatory approval, that would [make it] the first plant-made pharmaceutical," says plant scientist Charles Arntzen of Arizona State University in Tempe. "For people who work in this field, it will be a very exciting step forward."
Arntzen is chasing an elusive dream: using whole plants as factories to make drugs. Nearly 20 years ago, when researchers first showed that a tobacco plant could be engineered to crank out an antibody, they envisioned harvesting cheap supplies of therapeutic proteins, antibodies, and vaccines from vast fields of crops. For this approach, researchers isolate the target gene and usually insert it into a bacterium called Agrobacterium that readily infects the plants and passes on the gene. The gene becomes part of the plant and is passed from one generation to the next, producing foreign protein much as if it were one of the plant's own genes.
However, technological hurdles and a lack of interest from drug companies have hamstrung "pharming," as have worries that pharma crops will escape from their experimental plots and taint the food supply. As a result, many companies have abandoned this research or gone under. And no plant-made drugs for humans have made it to the pharmacy.
But academic scientists and some companies have persisted, improving yields of plant-made drugs and developing innovative ways to keep pharming inside the lab, or the greenhouse. Several plant-made pharmaceuticals (PMPs) are now in patient trials (see chart). Moreover, the European Union, the Bill and Melinda Gates Foundation, and the U.S. Department of Defense are fertilizing the field with new funding. "We're actually not doing too bad," says Julian Ma, an immunologist at St. George's University of London in the U.K. "It's just that everyone is in a hurry."
Fields of dreams
The excitement over plant-made pharmaceuticals began with a 1989 paper in Nature showing that monoclonal antibodies could be produced in tobacco. The paper "really captured the imagination," says Ma. Monoclonal antibodies were being used to treat a growing number of diseases, from arthritis to cancer, but were expensive to make in mammalian cells. So-called plantibodies appeared to offer a cheaper production method--a kilogram might cost $100 rather than $3 million--and might be simpler to process because they would be free of animal pathogens.
Temporary transgenic. Fluorescing protein shows tobacco leaf's pharming potential.
CREDIT: BAYER AG
Other discoveries followed. In 1995, for instance, Arntzen's group reported in Science that potatoes engineered to make a cholera protein worked as a vaccine when the spuds were fed to mice. Such "edible vaccines" could offer developing countries cheap oral vaccines that didn't require refrigeration, Arntzen suggested (Science, 5 May 1995, p. 658).
A company called Large Scale Biology Corp. in Vacaville, California, came up with a shortcut. It didn't bother to create a new tobacco strain when it wanted to produce an antigen for a lymphoma vaccine. It simply sprayed tobacco plants with a tobacco mosaic virus carrying the appropriate gene. The leaves produced useful amounts of the vaccine protein within 14 days. The drug worked in mice, suggesting that vaccines tailored to lymphoma patients' tumors could be made in plants in just weeks. And because the plants carried the foreign gene only until they shed their leaves, they were potentially more acceptable than permanently modified crops.
Steps along the way. No plant-made human drug has made it through final clinical trials, but several "pharmed" proteins are close to or on the market as supplements, a vaccine reagent, and a medical device.
CREDIT: (GRAPHICS)N. KEVITIYAGALA/SCIENCE (SOURCE) INDIVIDUAL COMPANIES AND MOLECULARFARMING.COM
Scores of biotech companies sprang up to commercialize these discoveries, and some big agbiotech companies got involved as well. By the mid-1990s, more than 180 companies and organizations were working on pharming, according to the Biotechnology Industry Organization.
The companies soon ran into technological snags, however. Biotechnologists couldn't always get plants to express enough protein and had trouble purifying the protein product. Efforts to make edible vaccines stalled after researchers realized that the amount of antigen fluctuated widely from plant to plant. Arntzen thinks that oral vaccines made from dried plant material could work for developing countries, but a vaccine without a strictly controlled dose "would never be approved" in the United States, he says.
Another reality check: lukewarm interest from the big drug companies. They didn't much care that plant-made drugs would be cheaper to make because production is a small chunk of the cost of drug development; the big-ticket item is clinical trials. The companies were also leery of the regulatory hurdles, because both the drug and the new production process would have to clear FDA. "Most pharmaceutical companies aren't willing to take a chance on a drug produced in plants," says Roger Beachy, president of the Donald Danforth Plant Science Center in St. Louis, Missouri.
Also, like other genetically modified crops (see pp. 468, 472), pharma plants can be a public relations nightmare. In 2002, leftover corn plants engineered by ProdiGene Inc. to make a pig vaccine sprouted in a soybean field in Nebraska. For this and an Iowa mishap, the U.S. Department of Agriculture (USDA) fined the company $250,000 and made it pay $3 million to buy and destroy tainted soybeans. The incident stoked opposition from farmers and activists worried about "drugs in your cornflakes."
Other companies underestimated the public's concerns. A company called Ventria Bioscience that wanted to conduct field trials of rice containing two breast-milk proteins useful in combating diarrhea drew the ire of rice growers in California, then Missouri. It wound up in Kansas, where no other rice is grown.
USDA tightened its rules for field trials of pharma plants in 2003 to prevent mistakes like the ProdiGene episode. But skeptics were not assuaged. Bill Freese of the Center for Food Safety in Washington, D.C., says enforcement is "horrendous." As a result, "we don't think [drugs] should be in any food crops, indoors or outdoors," he adds. Many ecologists and some plant scientists are also leery of using food crops for pharma. "It's too dangerous," says Kenneth Palmer, former director of the vaccine program at Large Scale Biology.
Flower power. This transgenic safflower makes seeds containing insulin.
CREDITS: SEMBIOSYS
These concerns drove many companies away from using food crops such as corn for pharmaceuticals. A few big companies, such as Monsanto, dropped PMP research altogether. Stung by bad press and lack of interest from drug companies, many leading plant pharma companies have folded, including ProdiGene and Large Scale Biology. As Palmer puts it, "the field imploded."
Close to the clinic
Despite the setbacks, a handful of companies in the United States and Europe haven't given up. A few have plowed ahead with food crops, grown outdoors, for their pharma products; others have focused on other plants or on unconventional growing schemes.
Meristem Therapeutics in Clermont-Ferrand, France, plans to start final clinical trials for a corn-grown gastric lipase for cystic fibrosis patients by the end of the year. And the Canadian company SemBioSys Genetics Inc. uses transgenic safflower--"much less of a lightning rod than some other crops," says CEO Andrew Baum--to produce insulin, which should be in clinical trials this year. Companies such as Protalix and Biolex Therapeutics sidestep the growing of crops altogether: the former with its carrot-cell culture to make a Gaucher disease enzyme, and the latter by producing interferon using duckweed, tiny clonal plants grown as a layer in clear plastic bags. "We are careful not to be associated with whole-plant transgenic technology," says Protalix CEO David Aviezer.
New technologies are attracting attention. To boost expression, the German biotech Icon Genetics relies on bacteria to get transgeneladen viruses into tobacco plants. The company dips the plants into a solution of Agrobacterium that carries the DNA for a deconstructed tobacco mosaic virus, which in turn contains the gene for the desired drug. The bacterial bath, followed by a few seconds in a vacuum, gets far more of the virus into plant-leaf tissue than conventional spraying.
In a 2006 paper in the Proceedings of the National Academy of Sciences (PNAS), they reported that this method, combined with other techniques, increases the amount of antibody by up to 100-fold, reducing the size of the crop needed and making it feasible to grow plants commercially indoors. Compared with making a transgenic plant, which takes a year or two to develop, this "magnifection" can go from gene to grams of protein in a couple of weeks. "It's incredibly promising technology," says Ma, who, like other academic researchers, is trying out magnifection.
With help from the drug giant Bayer, which bought the company in 2006, Icon Genetics will open a clinical-grade manufacturing plant in June. It expects to begin trials with a cancer vaccine tailored to individual patients in 2009, says CEO Yuri Gleba.
Bayer's move is a healthy sign of regrowth for the pharming field, Ma and others say. And other new sources of support are helping too. Last month, Pharma-Planta, a €12 million, 5-year, European Union-funded project co-coordinated by Ma, described in PNAS an anti-HIV microbicide grown in corn or tobacco that could be ready for testing next year. The Defense Department and other U.S. government agencies have provided the Fraunhofer USA Center for Molecular Biotechnology in Newark, Delaware, nearly $14 million to use a technique like magnifection to make vaccines. It has tested anthrax and plague vaccines in nonhuman primates and a pandemic flu vaccine in ferrets. "[We] can do things much faster than any other technology," says Executive Director Vidadi Yusibov, slashing in half the 6 months it now takes to make flu vaccine the traditional way, in chicken eggs. The organization also has $8 million from the Gates Foundation for plant-based vaccines for malaria, sleeping sickness, and flu.
As visions of endless fields of pharma crops have faded, so have unrealistic expectations for pharming. Scientists say they now realize that they need to be smarter about the marketability of the drugs they develop in plants. They think the best bets--Protalix aside--may be high-volume biologics, such as microbicides, monoclonal antibodies, and vaccines, particularly for use in developing countries. Getting these first low-hanging fruits through clinical trials and FDA approval should allay concerns about safety and environmental risks. Says Palmer, now at the University of Louisville in Kentucky, "Once two or three products [win approval], the field should really take off."
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