Introduction
Growing global demand and public spotlight on the biopharmaceutical
industry is driving increased importance on production costs. This
spotlight also exacerbates the importance of viral contamination control
(Aranha, 2011). These external pressures position the industry to
consider alternatives to microbial fermentation and mammalian cell
culture production systems.
Plant cell suspension cultures have demonstrated promise as an
alternative. Plant cells are higher eukaryotes, able to produce a wide
array of complex protein products through a versatile set of expression
and processing techniques (Huang & McDonald, 2012; Nandi & McDonald,
2014). Plant cell cultures are relatively inexpensive to operate due to
their simple, often chemically-defined culture medium free from
animal-derived components (Häkkinen et al., 2018). They have been used
at the commercial manufacturing scale for production of multiple drug
products, including the secondary metabolite paclitaxel (Tabata, 2006)
and the recombinant human enzyme glucocerebrosidase produced by Protalix
Biotherapeutics (Ratner, 2010). Currently, Protalix is the only company
with an FDA approved recombinant biologic produced in plant cell
suspension culture (Tekoah et al., 2015), and they have several more
products in clinical development (Almon et al., 2017; Schiffmann et al.,
2019). Protalix’s process, which has paved the way for regulatory
approval of this technology, serves as an excellent guide for design of
future plant cell culture processes.
We have recently demonstrated the promise of plant cell culture
technology for production of a challenging recombinant human
therapeutic, the human enzyme butyrylcholinesterase (BChE). BChE is a
bioscavenger agent that protects against organophosphorus compounds that
are used in chemical warfare and also used as agricultural pesticides.
The previously reported cell culture system is able to produce BChE in a
metabolically-regulated transgenic rice culture (referred to as rice
recombinant BChE or rrBChE) over multiple cycles in a stirred tank
bioreactor (Corbin et al., 2016) and can operate semicontinuously for
>6 months with no decrease in the rrBChE production
(unpublished data). Using a combination of scalable, commonly used
operations including tangential flow filtration and column
chromatography, rrBChE can be purified to >95% with a 41%
overall process recovery at laboratory scale. Furthermore, rrBChE has
shown comparable structure, activity, and in vitroorganophosphate inhibition efficacy to native human BChE (hBChE) (Corbin
et al., 2018). These factors indicate that manufacturing-scale
implementation of this technology could lead to effective and affordable
production of this important drug.
Despite the promise of plant cell cultures for biopharmaceutical
production and their demonstrated efficacy and ease of use by Protalix,
manufacturing scale use of these cultures has been limited. Due to the
high cost of entry into the pharmaceutical manufacturing business, novel
processes are often viewed as too risky for development. To mitigate
risk associated with adoption of a new process, risk severity and
probability must both be considered.
Techno-economic analysis is one method to reduce economic uncertainty of
manufacturing costs and gauge risks. It can also be helpful to assess
process operation strategies and predict theoretical costs to identify
process and economic parameters with the highest impact on manufacturing
costs. This can be done using “back-of-the-envelope” calculations,
spreadsheets, computer modeling, and simulation tools such as SuperPro
Designer® (Petrides, Carmichael, Siletti, &
Koulouris, 2014).
Several traditional biopharmaceutical manufacturing processes have been
studied using SuperPro Designer® and other process
simulation tools, including tissue plasminogen activator (Rouf, Douglas,
Moo-Young, & Scharer, 2001; Rouf, Moo-Young, Scharer, & Douglas, 2000)
and monoclonal antibody (Xenopoulos, 2015) production in transgenic
mammalian cells. Other studies have focused on whole plant-based
biopharmaceutical processes, including lactoferrin (Nandi et al., 2005)
and lysozyme production in transgenic rice (Wilken & Nikolov, 2012),
and transient expression of monoclonal antibody (Mir‐Artigues et al.,
2019; Nandi et al., 2016), recombinant BChE (Tusé, Tu, & McDonald,
2014), antimicrobial proteins (McNulty et al., 2019), and Griffithsin
(Alam et al., 2018) in Nicotiana benthamiana plants. These
studies suggest that plant-based protein expression can produce high
quality recombinant proteins with a substantial cost savings, though the
magnitude of this savings depends on the specific molecule, as well as
the production and processing system.
However, to our knowledge, no such analyses have been performed for a
plant cell culture-based biomanufacturing process. In this work, we
present a techno-economic model, simulation, and analysis of a
large-scale version of the process our group has developed for
semicontinuous production of rrBChE in rice cell suspension culture. Our
design inputs draw from laboratory-scale process data we have generated
and demonstrate the potential cost savings that can be obtained by
implementing this process for production of a challenging human
biopharmaceutical. The base case facility is designed to produce 25kg of
purified rrBChE/year at >95% purity as bulk drug substance
with single-use bioreactors used in the seed train and stainless steel
bioreactors used for production. The rrBChE was assumed to be
cell-associated, extracted from the rice biomass, and purified using
tangential flow filtration and chromatographic operations. An additional
goal of this model development is to create a tool that can be easily
modified, adapted, and broadly applicable to similar processes. To the
best of our knowledge, this work represents the first techno-economic
analysis reported for production of recombinant protein in plant cell
culture and the first facility simulation model for semicontinuous
bioreactor operation over long time frames (~6 months).
We believe this analysis can be considered as a general model, and the
simulation tool can be used for widespread evaluation of
semicontinuously-operated cell culture platforms for production of
moderate-volume biopharmaceutical products.