Plant-based expression system produces higher yields than mammalian alternatives, says CEO

By Flora Southey

- Last updated on GMT

(Image: Getty/digicomphoto)
(Image: Getty/digicomphoto)

Related tags Mammalian cell culture Biosimilars

Leaf Expression Systems’ technology enables the manufacture of proteins, vaccines and complex biomolecules without using animal-based products, says recently appointed chief executive.

The UK-based contract biopharmaceutical development firm uses Nicotiana benthamiana​, an Australian cousin of the tobacco plant, for the plant-based expression of proteins, metabolites and complex natural products.

According to CEO Simon Saxby, the firm’s ‘Hypertrans’ expression technology offers an alternative to more costly processes, such as mammalian cell culture systems, and can result in higher protein yields.

Biopharma-Reporter (BPR) ​spoke with Simon Saxby (SS)​ to find out how Leaf Expression Systems’ technology works, how it compares to alternative systems, and in what ways biopharmaceutical companies can benefit from plant-based technology.

BPR: Where did the idea come from originally?

SS: ​The concept of using plants as bioreactors for protein production had been around for several decades, but the Hypertrans technology used by Leaf Expression Systems was developed in 2007 by Professor George Lomonossoff and Dr Frank Sainsbury while they were researchers at the John Innes Centre, Norwich Research Park. Professor George Lomonossoff won the Biotechnology and Biological Sciences Research Council’s Innovator of the Year award in 2012 for his work on Hypertrans, and funding was secured for Leaf Expression Systems to be established as a spin out company in 2015, with the grand opening taking place in January 2017.

BPR: Why tobacco plants?

SS:​ We use Nicotiana benthamiana​. This Australian plant is a close relative of the tobacco plant (Nicotiana tabacum​). It is a research model plant used by numerous scientists all over the world for experiments, which is why many standard protocols and methods are well established for Nicotiana benthamiana​. It is particularly easy to infiltrate and transform with Agrobacterium​ – a genus of gram-negative bacteria. In addition, it is easy to grow and it reliably produces a large amount of biomass in a short period of time.  

BPR: How exactly are vaccines manufactured out of tobacco plants?

SS:​ The vaccine components (genes) are cloned into a transformation plasmid. This plasmid is transferred to Agrobacterium​, which can naturally transfer DNA to plants. The Agrobacterium​ is infiltrated using a vacuum into the plant leaves where it transfers the vaccine genes to the plant. The vaccine genes are then expressed by the plant to produce the active vaccine proteins, which is then  extracted and purified from the plant tissue to yield the final vaccine product.

Nicotiana benthamiana is a close relative of the tobacco plant (pictured): Getty/zbg2
Nicotiana benthamiana is a close relative of the tobacco plant (pictured): Getty/zbg2

BPR: And complex molecules?

SS: ​The process for molecules is similar to that for vaccines; however, in this case, the entire pathway from precursor to product is introduced into the plants.

BPR: What kind of currently available drugs could be candidates?

SS: ​Any protein-based drugs – such as antibodies, enzymes, and vaccines – could potentially be produced in plants. For example, we have recently been working on the plant-based expression of several therapeutic antibodies – including an HIV antibody and an Avastin (bevacizumab) biosimilar. Another example is Medicago’s plant-expressed flu vaccine, currently in Phase III clinical trials.

BPR: How fast is the process and what kind of yields are possible?

SS:​ The actual expression process (from treatment of plant with Agrobacterium​ to harvest) usually takes less than a week. We have observed yields of a few grams of protein per kilogram of plant material for some proteins. Developments in expression technology and upstream processes are being made and these yields are expected to rise in the coming years.

BPR: How does your plant-based expression system differ from other plant-based tech available to industry?

SS:​ Traditional plant-based technologies are based around the generation of stable transgenic plants. This process can take anywhere from several months to years to establish plant lines, due to the number of generations that are required. In addition, the expression levels observed in stable transgenic lines are often several fold lower than what is achieved from transient expression. The transient plant-based expression system used at Leaf Expression Systems is rapid, allowing protein production in around a week after infiltration. Also, the Hypertrans ​expression system results in higher protein yields than most other expression systems.

BPR: What advantages does plant-based expression have over more common methods, such as mammalian Chinese hamster ovary (CHO) for monoclonal antibody (mAb) manufacture?

SS:​ Our expression system enables us to produce meaningful quantities of proteins, vaccines or biomolecules within 12 weeks of receiving the sequences from our clients. The plants grow rapidly, infiltration is a simple process, and within a week or two of the plants being infiltrated, we are able to harvest and purify the products. In common with other plant-based systems we also offer the advantage of not using any animal-based products in the manufacturing process – either upstream or downstream, removing a considerable regulatory concern over product purity and potential adventitious infection of patients.

Leaf grows its plants in controlled environment rooms
Leaf grows its plants in controlled environment rooms

BPR: Is it fair to say that plant-based protein expression is under-represented in current commercial biologics and vaccines?

SS: ​Very much so. There are a number of reasons:

  • Despite plant-based technologies being around for many years, there has been significant underfunding for R&D of plant-based systems as industry has focused on improving the safety profiles and yields from mammalian and bacterial processes.
  • Because all of the first biologic drugs were produced in either mammalian or bacterial systems, and because pharma and biotech developed these systems to meet the understandably strict regulatory requirements, switching to plant-based systems posed too much of an economic and financial risk.
  • To date, only Elelyso (Protalix Biotheraputics/Pfizer) for the treatment of Gaucher disease has received market authorisation. Elelyso is made from transgenic carrot cell suspensions. When additional plant produced therapeutics receive regulatory approval, e.g. Medicago’s ‘flu vaccine’ which is currently in Phase III trials, I expect the landscape for plant-based drug manufacture will change significantly.

"Any protein-based drugs – such as antibodies, enzymes, and vaccines – could potentially be produced in plants"​ - Simon Saxby, Leaf Expression Systems

BPR: What has been the initial reaction from potential partners?

SS:​ The reaction has been varied, from surprise that you can produce drugs in plants to acceptance and understanding of the platform. Obviously, there is some reluctance to adopt plant production due to a lack of understanding of the process and regulatory concerns.

BPR: What immediate action do you need/want to take at the company to drive business going forward?

SS:​ The immediate focus will be on raising Leaf Expression Systems’ profile, and our offering in the marketplace by using data to demonstrate the value proposition that working with us will bring to our clients. We will be looking to enter into both risk sharing and straight commercial relationships to produce proteins, vaccines and complex biomolecules that are either too expensive or too difficult to produce in other expression systems, such as mammalian cell culture systems. We will also be working with clients and partners to establish Hypertrans and Leaf as the technology and partner of choice for production of their new molecules – especially when there is the need for a rapid response to disease outbreaks such as in previous instances of Zika or Ebola, allowing big pharma to focus on longer term solutions to these outbreaks.

Simon Saxby, who joined Leaf Expression Systems as CEO this year, has more than 30 years’ experience in the global life science industry. Prior to joining Leaf, he was CEO of Stratophase Ltd, and has also worked as project director for the design and build of the UK Government’s Cell and Gene Therapy Catapult manufacturing centre in Stevenage.

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