Beyond Bioprocess Monitoring and Control

Design of a Novel and Robust Process Control Tool

  • Fig. 1: Schematic view of the developed fed-batch process with a PI feed-back controller to maintain the specific growth rate at a given set-point throughout the culture by regulating an exponential substrate feed (MFC, mass flow controller)
  • Fig. 2: Comparison of the specific growth rates of two different yeast strains cultured under different pH conditions with same PI feed-back control strategy
  • Prof. Ian W. Marison, School of Biotechnology – DCU

Maintaining a particular specific growth rate throughout a culture is essential for reproducible product quality in industrial bioprocesses and therefore, is a key sequence for the step-up from Quality by Analysis to Quality by Design. A novel and robust Process Control Tool designed to achieve close control of the specific growth rate in fed-batch cultures by regulating an exponential substrate feed is under development and shows high potential for general applicability.

Process Analytical Technology - Trend, Cult or Culture?

Whether a paper needs to be submitted or a business idea sold, nothing goes without the buzzword PAT. This three-letter acronym for Process Analytical Technology seems to be the current focus of many biopharmaceutical industries. The use and abuse of the concept has grown in recent years; the number of publications making allusion to PAT has increased from a few dozen in 2005 to several hundred in 2008, while workshops and meetings are flowering all over the world and several websites are entirely dedicated to PAT. Wrongfully considered as a simple tool for QbD or reduced to a synonym of bioprocess monitoring and control, PAT is more than a term for a trend in the field of industrial pharmaceutical and biotech production. Process Analytical Technology is about creating a business culture for quality and success. The "Guidance for Industry PAT" launched by the FDA [1] in September 2004, is based on four principles, Tools for Data Acquisitions, Analysis and Design, Process Analyzers, Process Control Tools, Continuous Improvement and Knowledge Management. PAT aims to enhance bioprocess understanding and implies a holistic approach where every risk is minimized by maximizing process understanding [2] thereby ensuring that quality is built into products by design. The initiative is a framework for the creation of a firm background for industrial success and innovation.

CPP and Control - the Cornelian Choice between Precarious Necessity and Costly Complexity

At the very beginning of the PAT era, the benefits to the industrial implementation of complex on-line process supervision were debatable.

Little interest was given to complex control strategies and sophisticated tools. On the other hand, nowadays, there is a tendency for over-monitoring, implying the tracking of several dozens of parameters to describe a simple process. But while more and more publications present complicated control strategies involving hybrid semi-parametric models and fuzzy artificial neuronal networks to address the issues related to nonlinearity inherent to the highly dynamic behavior of cells, the question about the real necessity of extensive monitoring and control should be raised. There is no doubt about an urgent need for fully-understood and well-designed processes in line with the PAT guideline. However, there should also be a quest for simplicity.
In the very first step of every process design, close attention should be paid to ensure that the greatest number of process variables are identified, analyzed and correlated. Correlations between critical process parameters (CPP) and product quality attributes also need to be drawn. However, it is crucial to reduce the complexity of the system as much as possible. Ultimately, a few CPPs and product quality attributes, as well as the appropriate devices used to follow their evolution are selected, while control strategies for the key parameters are designed and put in place. Again, only real process understanding allows for carefully thought-out decisions about the appropriate Process Control Tool. Sometimes it is initially necessary to employ overly complex systems in order to scale- back the complexity in retrospect and achieve the optimum level of simplicity.

LiB - Development of a PAT-based Plate-form for Control of Fed-batch Cultures

In the Laboratory of Integrated Bioprocessing (LiB), Prof. Ian Marison's group is currently developing a PAT-based platform for control of fed-batch cultures. Currently implemented as a platform for process design involving yeast cultures, the general concept ultimately aims to develop a simple and robust control strategy for mammalian fed-batch cultures for the production of pharmaceuticals and other high value-added products.

Product quality, for instance the activity of a recombinant protein, is often growth-related. Maintaining an appropriate specific growth rate may also be required to avoid the formation of overflow metabolites. Careful monitoring and control of the specific growth rate may therefore be crucial to ensure product quality as required by regulatory organizations such as EMEA. However, it is essential to keep in mind that the specific growth rate is not a process variable that can be measured directly; rather its estimation relies on the reliability of the other measurements. Whereas most models for growth rate estimation typically rely on auxiliary variables such as substrate consumption rate, the novel strategy proposed refers to real-time monitoring of the biomass.

As stated previously, high importance should be given to the selection of the process parameters to be monitored. In addition to two standard parameters, temperature and pH, that are not only monitored but highly controlled in most processes, different variables have been identified as critical and need to be followed on-line. In the present work, off-gas analysis, enabling the calculation of the oxygen up-take and the carbon dioxide evolution rates, is combined with the monitoring of all major medium components through the use of a Fourrier-transform mid-infrared spectrometer. Biomass is without a doubt a key process parameter but it remains challenging to follow its evolution. Dielectric spectroscopy, enabling the detection of viable cell volume inside the reactor through capacitance measurements is an effective monitoring tool, cumulating several interesting features, such as sensitivity, non-invasiveness and in situ sterilization. Elemental and mass balances based on the combined use of off-gas analysis, spectroscopic measurements and data emanating from the monitoring of pH evolution serve as basis for on-line data reconciliation to enhance the biomass measurements [3]. The specific growth rate is calculated on-line from the reconciled biomass measurements and used to estimate the error between the set-point and the actual value, thereby creating a feed-back term for a PI feed-back controller regulating the exponential substrate feed during the culture. The developed control strategy for fed-batch culture, allows a close control of the specific growth rate under different culture conditions showing potential as a platform of broad applicability.

And for the Near Future?

Continuous improvement of process analyzers, processes and strategies is a central point in the guidance for industrial PAT. The Process Control Platform presented in this article is currently being adapted to mammalian cell culture, a critical step in the proof of concept of general applicability. However, the quest for enhanced on-line monitoring allowing accurate and reliable real-time estimation of all critical process parameters will require additional research where novel control strategies based on groundbreaking robust, on-line tools such as calorimetry [4], and advances in analytical methods may bring new insights into Process Analytical Technologies.

[1] CDER: Guidance for Industry PAT - A Framework for Innovative Pharmaceutical Development, Manufacturing and Quality Assurance, U.S. Department of Health and Human Services Food and Drug Administration, 2004
[2] Wolde S. et al.: The chemometric analysis of point and dynamic data in pharmaceutical and biotech production (PAT) - some objectives and approaches; Chemometrics and Intelligent Laboratory Systems, Vol. 84, 2006
[3] Dabros M. et al.: Biotechnology Progress, 25 (2): 578-588 (2009)
[4] Marison I.W. and Sivaprakasam S.: Progress and challenges on design of high sensitivity biocalorimeter (bioRC1) for inline bioprocess monitoring, 14th European Congress on Biotechnology, 2009




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