Using Plants as Biofactories to Produce Therapeutic Peptides

David Craik, Ph.D.,

Professor of Biomolecular Structure, Institute for Molecular Bioscience,

University of Queensland

Plant-Based Peptide Production Could Yield Cheaper, More Stable Medicines

A class of ultra-stable plant peptides called cyclotides could solve the therapeutic peptide sector’s stability challenges and significantly reduce its manufacturing costs, according to Australian researchers from the University of Queensland.Cyclotides were identified in the 1960s in the plant Oldenlandia affinis, which is used in traditional medicine in West Africa. They are bioactive peptides characterized by their cyclic backbone and knotted arrangement of disulfide bonds, which give them a rigid 3-D structure.And this rigidity has application in medicine, said David Craik, Ph.D., professor from the Institute for Molecular Bioscience at the University of Queensland, who told delegates at TIDES Europe 2023 researchers are using cyclotides as scaffolds to stabilize candidate therapeutic peptides.“The reason we're interested in them is that they’re so stable that they perhaps can overcome some of the traditional limitations of peptide-based drugs,” he said.

Plant species producing these looped cyclotide molecules make a wide variety of them and in significant quantities, according to Craik. “Plants are natural factories for producing these molecules.“So the idea was if plants can produce this combinatorial library, then, as peptide chemists, we should be able to come along and insert a desired peptide epitope into one of these loops. So that’s really what we’ve been doing for the last 15 years or so, using cyclotides as super stable scaffolds in drug design.”There are more than 30 published examples where epitopes grafted into a cyclotide framework have shown activity against therapeutic targets in animal models, according to Craik, who added that one molecule is being studied in human trials as a treatment for multiple sclerosis (MS).

Manufacturing

Another potential benefit of cyclotides is the ease with which they can be manufactured at scale due to their underlying genetics. The genes encoding cyclotides consist of the core region flanked by an N terminal region and a C terminal region. They also have a signal that controls how the molecules fold – are cyclized – in the cell’s endoplasmic reticulum.This understanding is key to efficient mass production, according to Craik. “Having now understood this, we can now go ahead and use this sort of genetic architecture to make our own designer cyclotides.“We know plants produce these cyclotides in huge amounts, 1 gram per kilo of [plant] tissue. We come along as drug designers and modify those peptides to give them a therapeutic property. Then what we can do is make a modified gene and put that back into a crop plant to produce that molecule in high yield.”Citing soybean as a crop example, Craik said, “What we might do is express the therapeutic cyclotide in the leaves of the plant, and then we can harvest the leaves and refine the pharmaceutical product.“Given that these molecules are so stable, the refinement process is very simple. So that’s the main way in which we’re applying this technology. All we need is some sunlight and some water and some land, and we can produce high-value therapeutic peptides, essentially opening up a new bio farming industry.”Craik contrasted this with traditional biopharmaceutical manufacturing methods and argued that plant-based production offers a significant cost reduction.

“Why should we consider plants? Well of course for small-scale production solid phase peptide chemistry works well, and I’m not suggesting for a minute that we’ll ever replace that for lab-scale applications or even for, for small preclinical studies.“But once you start to get into hundreds of tons or hundreds of kilos to tons, we think we need to think of other ways,” he said.The industry standard for protein production is CHO cells. However, while CHO-based expression is effective, it is expensive and can be challenging to scale up. Expression in E. coli is a lower cost alternative, but the bacteria lack mechanisms for post-translational modification, making them unsuitable for cyclotide production.In contrast, Craik said plant cell cultures, plant cells or plants themselves are inexpensive to start up, sustainable and very easily scalable. “You just grow a thousand pots instead of a hundred pots if you want to scale up with no change in your infrastructure.”