Biopharmaceutical proteins are one of the central pillars of modern healthcare systems. The simplest polypeptides can be produced in bacteria or yeast, but more complex proteins and glycoproteins are produced in mammalian cells – the current gold standard among biomanufacturing platforms. But plants, particularly tobacco due to its biomass yield, are receiving increased attention for the niche product market. Potentially, plants could present greater economy, scalability and sustainability for mainstream biopharma manufacturing. But in the face of strong incumbent technology and intense competition, can these relative newcomers stand their ground?
Biopharmaceuticals include vaccines and prophylactic antibodies for disease prevention, labeled antibodies and ligands for disease diagnosis/monitoring, and diverse therapeutic proteins ranging from replacement enzymes and hormones, to antibodies that target and destroy cancer cells. Some of these proteins, such as insulin, serve large markets, but the vast majority, including most antibodies, are indicated for diseases with a relatively low incidence. In these cases, the entire annual global demand is typically in the kilograms to hundreds of kilograms range. A few antibodies have achieved blockbuster status and the demand for these exceptional products is several tonnes per year, but if we want to use antibodies for the prevention, diagnosis or treatment of more widespread illnesses, including Alzheimer’s disease, malaria or HIV/AIDS, then demand may increase to 100 tonnes per year or more for each product. Meeting such demand will be difficult using microbes or mammalian cells because the scalability of a production suite is limited by the working volume of today’s largest bioreactors, typically 20,000 L for conventional stainless-steel fermenters and 2000 L for single-use alternatives. Assuming optimal yields, constant campaigns and consistent perfect performance, such facilities would produce 7.5 and 0.75 tonnes per year, respectively (1).
The obvious solution to match supply and demand is to commission parallel systems with multiple bioreactors operating simultaneously, or to build additional production facilities. The problem with this approach, however, is the cost of the infrastructure. Monoclonal antibodies are among the most expensive drugs currently available on the market, in part due to the high cost of manufacturing. Costs are borne by patients and healthcare providers because the target population for these drugs is relatively small, and the treatment course relatively short. Antibodies developed to prevent the transmission of prevalent diseases, such as HIV/AIDS would need to be administered as a long term regular microbicide to tens of millions of at-risk people, most of whom live in countries with sparse healthcare resources. The costs need to come down. And to achieve this, the industry needs a new manufacturing paradigm. It’s well worth considering plants.
The green advantage
The use of plants for the manufacture of biopharmaceuticals was first demonstrated in 1989 but, for a long time, the industry viewed this as a fringe movement because of the untested nature of the technology, low product yields, and absence of a regulatory framework. In comparison, mammalian cells are reliable, yields regularly exceed 5 g L-1, and the regulatory framework is rock solid. Over the last few years, however, plants have started to flourish, (both metaphorically and literally) as a new production platform. Expression levels of ~3 g kg-1 biomass have been achieved (2). Together with the infrastructure available from industrial agriculture, we could “farm” antibodies and other biopharmaceutical proteins at unprecedented scales; the cost of growing more plants is tiny compared with the costs of building more fermentation suites (3).
Individual plants can be viewed as living, single-use bioreactors that can be grown from seeds, using virtually free and unlimited resources (sunlight and water). Plants have no compatibility issues with other equipment, and the costs for cleaning and disposal are minimal (composting, compared with thermal deactivation of fermenter waste). Plants also do not support the propagation of human pathogens and have a built-in barrier to prevent contamination with mammalian viruses (3). Genetically modified plants are primed with the means to produce the biopharmaceutical product, analogous to a transformed mammalian cell line. As an alternative, unmodified plants can be used for transient expression by infiltrating the leaves with appropriate vectors. The advantage of transient expression is that production can be ramped up quickly – indeed, much more quickly than any cell-based process (4).
Safety and scalability aside, plants also offer several advantages over mammalian cells in terms of product specifications. For example, plants can produce glycoproteins with diverse glycan structures, allowing for the development of products with designer quality and functionality profiles, such as extended serum half-life, preferable interactions with immune system cells, and modified antibody effector functions (5). Plants can also produce proteins that are highly toxic towards mammalian cells, such as antibody–drug conjugates, which are currently produced in a complex process involving mammalian cells for the antibody component, and microbes for the toxin, followed by in vitro conjugation and further purification. All of this could be achieved in a single plant, with only one round of downstream processing and purification. One major difference between mammalian cells and plants is that the former secrete products into the medium, whereas proteins expressed in plants, such as tobacco, must be released by shredding and grinding the leaves. Although this requires extra clarification steps to remove the resulting particulates, the process flows are similar for plants and mammalian cells and there is now little difference in costs – an important consideration given that downstream processing can account for up to 80 percent of all production costs (3).
I believe that the advantages of plants in terms of safety, scalability, sustainability and product diversity can be valuable economic and environmental assets when competing with mammalian cell cultures, even though the latter are well-established and trusted by industry. The best way to overcome industry inertia and embrace disruptive technologies is to focus on the cost-effective production of relevant biopharmaceutical proteins – niche products that benefit specifically from the advantages of plant-based systems. Plants already stand their ground in these niche markets, but it is likely they will begin to encroach on the markets for more mainstream biopharmaceutical proteins as the technology takes hold.
Johannes F. Buyel is head of the Integrated Production Platforms department at the Fraunhofer Institute for Molecular Biology and Applied Ecology IME, and senior scientist at RWTH Aachen University, Germany.
Johannes F. Buyel is head of the Integrated Production Platforms department at the Fraunhofer Institute for Molecular Biology and Applied Ecology IME, and senior scientist at RWTH Aachen University, Germany.
