Thousands at risk from Ebola need hope, and that hope might be growing right now in a Kentucky greenhouse. Plants such as tobacco, corn, and potatoes can build complex proteins. Researchers have now taken control of these plants’ internal machinery to produce some of the most important therapeutic proteins, antibodies, and vaccines ever made. These are called plant-made pharmaceuticals, or PMPs.
The process of turning a gene into a protein into a crop is being streamlined, optimized, and regulated. Clinical trials to test the first “grown” proteins are ongoing. PMPs are coming of age, and the experimental Ebola drug ZMapp has made them famous.
The crisis of Ebola in West Africa is expanding daily. According to the CDC, more than 1,400 people have died from suspected Ebola infection in the current outbreak. Millions more are at risk. Ebola has no cure and no vaccine.
Two American aid workers, Dr. Kent Brantly and missionary Nancy Writebol, just came out of isolation at Atlanta’s Emory University Hospital. They beat the odds and recovered from infection with Ebola. Brantly and Writebol both received ZMapp.
Although doctors can’t prove that ZMapp was the cause of these patients’ recovery, the results are encouraging. The remaining few doses of the untested drug were shipped to the afflicted areas in West Africa.
On August 16, officials in Monrovia, Liberia, confirmed that three infected doctors were given ZMapp. On Monday, officials reported that one of the doctors, Dr. Abraham Borbor, had died from the infection. Borbor was deputy chief medical doctor at Liberia’s largest hospital.
Even if ZMapp ultimately proves successful, more of the treatment won’t be available for weeks or months. Why? The plants need time to grow.
ZMapp “grows” inside of genetically modified Nicotiana benthamiana plants (a fragile cousin of tobacco). Mapp Biopharmaceutical and LeafBio developed this treatment cocktail only months ago. It combines antibodies that previously showed success in increasing Ebola survival rates in monkeys. The plants manufacture these antibodies using their own internal machinery.
Moving from Antisera to Antibodies
ZMapp’s origins began with a 100-year-old treatment for infections with no cure: antiserum. Originally, antiserum was made from the serum of people (or animals) that survived the same infection. Serum is the “clear” part of blood. It has no red blood cells or clotting proteins. Serum contains many other proteins, including antibodies.
A survivor’s serum contains antibodies against the virus or bacteria that caused infection. Antiserum treatment involves taking the survivor’s serum and injecting it into someone recently exposed to the same disease. The serum antibodies help quickly activate the immune system of the newly infected person.
Before the discovery of antibiotics like penicillin and vaccines to prevent infection, antisera were used to treat infections. These included pneumonia, diphtheria, cholera, and more. Now, antisera combat toxins from snakebites and fight rabies and tetanus infections.
Infected rabbits, rats, and even horses were used to generate antisera. This was costly and inefficient. Modern understanding of the genetics and structure of antibodies helped refine the antiserum concept. Instead of using many different antibodies (polyclonal antibodies, or pAb) researchers now choose only the most effective ones. Then they recreate the genetic code for one especially helpful antibody (called a monoclonal antibody, or mAb).
The genes for that mAb are typically inserted into bacteria or cultures of animal cells. These cell cultures then produce large quantities of mAbs, which are purified and used for treatment. No serum or animal products are involved. This eliminates the risks of working with deadly infectious agents in animals.
In a press release regarding the success of the precursor to ZMapp, called MB-003, Mapp Biopharmaceutical president Dr. Larry Zeitlin said, “We were pleased to see how well the humanized mAbs of MB-003 performed. We also were pleasantly surprised by the superiority of the plant-derived mAbs compared to the same mAbs produced in traditional mammalian cell culture."
This method creates highly effective medicines. These include Herceptin (trastuzumab), a mAb chemotherapy for breast cancer. Humira (adalimumab) is a mAb treatment against autoimmune disease. The tag-team pairing of actoxumab and bezlotoxumab is in development to treat Clostridium difficile infection. There are more than 350 mAb treatments in development and more than 30 currently on the market.
Maintaining huge vats of cell cultures to manufacture mAbs costs a lot of money. Introducing the mAb genes into plants creates a cheaper, high-volume production method. Using plants also reduces the risk of contamination in the final product.
“With the growing global demand for vaccines and other biologics, development of technologies for making safer and better products remains to be critical,” said Dr. Vidadi Yusibov, executive director of the Fraunhofer Center for Molecular Biotechnology, in a recent press release.
‘Infecting’ Plants Turns Them into Protein Factories
Relatives of tobacco are most commonly used for this novel process. Researchers altered tobacco-like plants grown in Owensboro, Kentucky, to produce three antibodies against Ebola and create ZMapp.
N. benthamiana is grown indoors in factory-like farms so it doesn’t put commercial tobacco or other crops at risk. The leaves, which soak up liquids quickly, are easily infected with viruses or bacteria.
N. benthamiana is a perfect plant for the process of agroinfiltration. The desired antibody gene is added as a DNA fragment to an engineered bacterium called Agrobacterium tumefaciens. This was once the source of crown gall disease in plants. Now it has been harnessed to saturate a plant with useful DNA. Plants are dipped whole into a solution of A. Tumefaciens and the leaves absorb the bacteria. Then, the plants’ hijacked manufacturing centers begin producing the desired protein in bulk.
A working indoor “pharm” is a well-oiled machine. Robots grow plants, treat mature plants with a bath to convert them into miniature factories, then harvest and process them.
The leaf of the tobacco plant is the main storage site for proteins. Therefore, processing and purification must begin immediately upon harvest. Extracting the desired protein is still expensive and time consuming. Tobacco and its cousins contain toxic alkaloids that have to be removed before the protein can be administered as a drug.
It’s going to be a while before more of the ZMapp mAb cocktail can be grown, refined, and purified for use. The process also needs to be scaled to address the thousands of doses that will be needed. ZMapp still needs extensive human testing to determine if it’s safe and effective.
Plant-Made Drugs Also Tested for HIV, MRSA, West Nile
Beyond ZMapp for treating Ebola, other plant-made mAbs are moving toward clinical trials. Mapp Biopharmaceutical continues work on plant-produced mAb therapies for Marburg virus (a cousin of Ebola) and for respiratory syncytial virus. A microbicidal cocktail of mAbs against HIV and herpes is also close to advancing to phase 1 clinical trials.
Other researchers are developing plant-made mAbs for West Nile virus. Rabies and hepatitis mAbs are being produced in tobacco. CaroRx is a plant-made mAb used to treat the bacteria that cause tooth decay and is currently in phase 2 clinical trials.
The possibilities are very exciting for extending antibody protection to other diseases like tuberculosis, MRSA, and even HIV. Using plants to produce these protective antibodies can result in lower costs and a faster response to an outbreak of disease.
PMPs have applications beyond therapeutic antibodies. Quick production of huge amounts of targeted proteins is one big benefit of producing vaccines with plants. When an epidemic like swine flu hits, vaccine stocks can be depleted quickly. Plant production of vaccines can fill in the gaps.
At the Fraunhofer Center for Molecular Biotechnology in Delaware, racks of N. benthamiana plants can be programmed to produce a wide variety of vaccines. The next time an epidemic hits, PMP production could create 2.5 million units of vaccine in just one week.
Plant-produced vaccines have been developed for cholera, toxic Escherichia coli, hepatitis B, Norwalk virus, and HPV. More are on the way. There is even ongoing research into edible vaccines. This would allow patients to simply consume a food to create immunity.
Other therapeutic proteins, such as clotting factors and other blood products, are being produced in plants as well. Dr. Henry Daniell, director of translational research at the School of Dental Medicine at the University of Pennsylvania, is collaborating with drugmaker Bayer to produce anticoagulants in lettuce leaves.
“In addition to ZMapp, there are lots of recent developments in this field,” Daniell told Healthline. “[The journal] Nature is featuring one of our publications on hemophilia produced in lettuce chloroplasts. This is developed with a $100 million agreement with a major pharmaceutical company. So, this field is moving forward rapidly.”
The freeze-dried lettuce cells are resistant to stomach acids. They can be taken orally and still be absorbed in the intestine, unlike most delicate protein products. With the increasing need for more therapeutic proteins, a fast and affordable manufacturing technique would be a huge benefit. And major pharmaceutical companies are sitting up and taking notice.
The current outbreak of Ebola is the largest and most serious ever seen. Even if efforts to stem the tide of new infections are successful, another outbreak will occur. Plant-grown antibodies and vaccines will allow for rapid production of drugs in response to the epidemics of the future.