QUALITY BY DESIGN AND SCALE – UP
Unit operations are the individual basic steps in a process that when linked together define the process train and result in the final product. In practice, a unit operation is defined as an individual step that is carried out on one piece of equipment. In biopharmaceutical API unit operations may include fermentation or bioreactor processes, cell separation through centrifugation or microfiltration, virus removal or inactivation, cell lysis and inclusion body precipitation, product refolding, and purification steps. Besides, unit operations for drug product manufacturing procedures would be similar to those seen in the manufacture of a small molecule of comparable dosage form, namely mixing, fluid transfer, sterile filtration, dose filling, lyophilization, and so on. Of course, unit operations will be dependent on the manufacturing process for the specific dosage form, but careful pre-formulation and characterization studies will enable relatively straightforward process design and ease subsequent scale-up activities. Modeling of unit operations for both small and large molecules is a recognized gap in our ability to achieve Quality by design (QbD). The application of accepted engineering methods to the problem is the subject of active research.
Bioburden Considerations
Bioburden is the amount of microbial flora that can be detected on an item, on a surface, or in a solution. As mentioned previously, microbial contamination and bioburden are mostly important for biotechnology-derived parenteral products since these products are typically capable of supporting microbial growth. Special care should be taken to ensure not only that the final packaged product does not contain microbial contamination but also that manufacturing equipment is also free from contamination. Monitoring bioburden and determining potential levels of microbial contamination on equipment surfaces are particularly important with respect to the material being evaluated.
In general, bioburden counts in parenteral solutions are obtained by conducting the total aerobic counts and total yeast and mold counts as specified in the USP microbial limits test (61) or an equivalent test. In addition, membrane filtration of larger than specified volumes may also be used to detect any microbial contamination when sample results are expected to contain a negligible number of microbial flora or in the presence of potential confounding factors, such as antimicrobial preservatives. It is important to note that the presence of a high bioburden count can present an endotoxin contamination problem, as whole microbial cells and spores can be removed by sterilizing grade filtration (0.2 μm), while endotoxins are not. These issues also underscore the importance of cleaning methods and their respective validation as well as assessing relevant product contamination on manufacturing equipment.
Scale-Up and Process Changes
The FDA defines process validation as “ establishing documented evidence that provides a high degree of assurance that a specific process will consistently produce a product meeting its predetermined quality attributes ”. While validation studies are typically performed at full scale, in most cases scale-down or laboratory-scale
models were used to initially develop the manufacturing process. Consequently, scale-down process pre-characterization and characterization studies are considered crucial to successful process validation for both API and drug product manufacturing schemes. Although they do require qualification work and a significant
commitment of time and resources, characterization studies provide significant insight into the critical process and control parameters for each unit operation as well as improved success rates for process validation due to a better, more complete understanding of the process. In engineering terms, characterization studies identify the critical parameters useful for dimensional analysis that enable successful process scale-up.
While the above explanation attempts to simplify the scale-up process, it is not meant to trivialize it. In fact, scale-up is probably the most difficult manufacturing challenge for traditional small molecules, let alone biopharmaceuticals. Issues such as homogeneous mixing, bulk product holding and transfer, and sterile filtration
could all be potentially compounded due to the increased scale and introduced stress. However, a QbD approach to rational drug design should enable simplified process scale-up and validation. This is only true if experimental design approaches have been utilized to identify the design space for the processes involved in the
production of the molecule. This is also where the greatest benefit of developing empirical phase diagrams early in development could materialize. Essentially, the QbD approach identifies the quality attributes of the product based on scientific rationale as opposed to attempting to fit the proverbial square peg into a round hole through a trial-and-error approach. This rational design approach goes further to identify the limiting factors of each unit operation and provides the means of attempting to correlate how each unit operation affects the final product quality attributes.
In order to initiate a successful QbD program, the first step is to identify those process parameters that are essential to product quality and develop well-validated analytical methodologies to monitor those parameters. In short, the process involves the identification of the potential design space for the production of the molecule and confirmation of that design space through rational, deliberate experimentation. Ideally, process monitoring should be done in real-time to minimize production time and if possible online; however, this may not always be the case or even necessary depending on ing upon the relative duration of the process to the test. Recognizing potential quality metrics earlier in the development process could also potentially facilitate
greater flexibility during product development and subsequent process characterization. Certainly, manufacturing site-specific differences could also potentially introduce variability into processes. It is for this reason that site-specific personnel training, process/technology transfer and validation, and stability assessments are required to ensure product quality.
By definition, a process designed under the auspices of QbD should enable a degree of process knowledge that allows for controlled process changes without affecting the final product or requiring regulatory approval. For immediate – and controlled-release solid dosage products, SUPAC guidelines provide direction on the studies to conduct to determine the impact of a process change. Although there is some regulatory guidance available for biological products (e.g., “ Changes to an Approved Application for Specified Biotechnology and Specified Synthetic Biological Products ” or “ FDA Guidance Concerning Demonstration of Comparability of Human Biological Products, Including Therapeutic Biotechnology-Derived Products ” ), process changes need to be evaluated on a case – by – case basis. The comparative analysis of process changes should also be evaluated with respect to defined product specifications. PAT will be invaluable in determining the potential impact of process changes. While stability is often the main metric for small-molecule drug products, bioactivity and immunogenicity will need to be added metrics for biopharmaceuticals. Therefore, any process change should be approached subjectively and care should be taken to validate the relative impact on the safety and efficacy of the product.
CONCLUSION
Although the goals are the same, developing biotechnology molecules present challenges that are unique compared to the development of conventional small molecules. The innate complexity of the molecular and macromolecular structures requires three dimensionally viable stability assays and understanding. The complexity of possible physiological responses and interactions requires an enhanced understanding of the formulation and processing stresses to identify the minor but critical changes that result in product unacceptability. A key to addressing these challenges is the development of analytical techniques with the sensitivity and reliability to detect and monitor such changes and to provide data to another gap-closing activity — modeling unit operations. Also, the need to develop meaningfully kinetic models is obvious to everyone involved in the development of both large and small molecules. Linking this type of information to the major efforts in the discovery arena is a necessary step to bringing the products of the future to market.
The use of biotechnology products is increasing exponentially and many opportunities exist to improve their development. The first step may be defining rational biotechnology-derived drug “ developability ” standards that can be assessed during preclinical/early development testing. Such a tiered approach is based upon
the potential risk, the confidence in methodology, and the benefits t have of course been proven strategies for small molecules, and a preliminary version applicable to biotechnology drug products.
Reference:
Production and Processes -PHARMACEUTICAL MANUFACTURING HANDBOOK (SHAYNE COX GAD, PH.D., D.A.B.T.)
