BIOPHARMACEUTICAL MANUFACTURING - Risk Management Applications in Pharmaceutical and Biopharmaceutical Manufacturing - page 340

Scale-Up of Downstream Operations
MEMBRANE/FILTRATION OPERATIONS Membranes and filters are an integral part
of most protein purification schemes, and are used for a variety of operations,
such as control of bioburden and particulate levels, medium exchange during
cell growth, cell harvest, product concentration, diafiltration, formulation, and
for removal of viruses and other agents. The important process parameters that
should be kept in mind when selecting membranes/filters for scale-up opera-
tions are cross-sectional area, trans-membrane pressure, volume processed per
unit area, filtration area, shear rate, operating time, temperature, protein con-
centration, solution viscosity, and, where applicable, number of uses [20]. A
conservative approach to scale-up involves increasing flow rates and filter areas
while keeping other variables constant. Additional details on this topic can be
found in PDA Technical Report No. 15, Validation of Tangential Flow Filtration
in Biopharmaceutical Application [21].
CENTRIFUGATION Centrifugation is used to separate or concentrate materials
suspended in liquid medium, based on sedimentation rates in an increased gravi-
tational field. That is, two particles of different size, density, and shape will settle
in a tube/bowl at different rates in response to gravity. For biotechnology opera-
tions, batch centrifugation is often used at the laboratory scale, while continuous
centrifugation is preferred at the production scale. Scale-up operations typically
use centrifugation for separating whole cells from the supernatant. It is also used
after precipitation steps to separate solid from liquid phases.
For cell culture operations that involve secreted proteins, filtration may be
preferable because of the relatively mild operating conditions. Another advantage
of filters is the relatively simple cleaning validation as compared to the cleaning
validation required for centrifuges. However, as process volume increases, the
economics of using filters decreases and makes them unsuitable for very large-
scale operations [20].
CHROMATOGRAPHY The majority of processes currently used to manufacture
biotechnology products employ column chromatography as the main method
for product recovery and purification. It is capable of combining relatively
high throughputs with high selectivity to either capture the product or purify
it from accompanying impurities. Key process parameters for scaling up and
using chromatographic columns include protein and product loading, linear
velocity, buffer volume, bed height, temperature, cleaning capacity, and gel
lifetime. Other parameters include buffer properties (e.g., pH, ionic strength)
and measures of packed bed quality (such as number of theoretical plates or
asymmetry in a pulse test).
The most commonly used types of chromatographic columns in commercial
production are ion exchange columns (impurities are removed by manipulating
the pH and conductivity of equilibration wash and elution buffers), hydrophobic
interaction columns (contaminants and proteins are selectively removed on the
basis of their hydrophobic interactions), and affinity chromatography (provides