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Anurag S. Rathore is a professor in the Department of Chemical Engineering at the Indian Institute of Technology Delhi, Tel. +91.9650770650, email@example.com.
Constraints associated with equipment can make scale-down a challenging exercise.
Creation and qualification of scale-down models is essential for performing several critical activities that support process validation and commercial manufacturing. This article is the second of two parts covering scaling down biopharmaceutical operations, Part I was published in April 2006 and detailed fermentation. In this segment, we present some guidelines and examples for scale-down of common downstream unit operations used in biotech processes — chromatography and filtration.
In theory, chromatography is one of the more straightforward unit operations to scale down. Factors at the large scale such as column-header design and packing can be difficult to control and may result in reduced accuracy of the scale-down model. This section presents a review of general guidelines for scaling down.
Use representative feed streams, preferably from full-scale manufacturing. First perform a complete analysis of product concentration, quality attributes and other feed stream characteristics (such as pH and conductivity) to ensure a good match. If possible, use feed streams from different lots at small scale to learn about the effect of feed stream quality on the performance. In addition, acquire some knowledge of the storage and stability of the feed stream material. Storage-related degradation of the inventory could affect the outcome of the small-scale runs.
Use a representative chromatography resin. It would be ideal to have resin from the lot being used at full scale. If you can't obtain GMP-released resin, then make a match based on the vendor's certificate of analysis.
Figure 1 Chromatographic profiles of small- and large-scale columns. Raw material is a proprietary aqueous solution that has undergone one step of preliminary purification. The small column (green data) is 1.6 cm diameter by 12 cm. The large column (black data) is 63 cm diameter by 12 cm.
Buffers and other solutions should be made up and released by manufacturing. If it is not possible to obtain these, mix solutions with GMP-released raw materials, or material of an equivalent grade from an approved vendor. Follow exactly the solution recipes from manufacturing. The pH, temperature and conductivity probes that check buffer properties must be comparable with those used at the large scale. Measure pH and conductivity at the same temperature spelled out in the recipe. The pH and conductivity standards should also be comparable (preferably identical) to those used in manufacturing.
Hardware. Our experience has been that an automated small-scale chromatography system, such as the äKTA Explorer, excels at small-scale studies. It offers precise, accurate and reproducible control of flow rates and gradients. However, you still have to check and calibrate the accuracy and precision of in-line detectors (pH, conductivity, temperature) by using off-line probes.
The type of column is generally not a major issue; however, whenever possible use the same bed support material. We prefer to not use column diameters of less than 1.0 cm since the ratio of system dead volume to column volume can result in pool concentrations significantly lower than those seen at large scale. The packing procedures and solutions should be similar to those used at large scale.
Controlling temperature at small scale to mimic that of cold rooms or cold-boxes can be a challenge. The use of water baths with column jacketing can provide precise temperature control, but the tubing leading up to the column can frequently act as a mini heat exchanger, making it difficult to control the actual inlet temperature. Using heat exchangers on the inlet tubing and column jacketing may be necessary for accurate, consistent and precise temperature control.
Table 1 Comparison of scale-down model with a large-scale cation-exchange column. Input material roughly 98% antibody, 3 ppm DNA, 3000 ppm CHOP and