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20 Pharmaceutical Technology ® Bio/Pharma Outsourcing Innovation eBook 2023 PharmTech.com Manufacturing Normal flow filtration. Nor ma l f low f i lt rat ion (NF F) operation is normally implemented in a DSP process at the point of harvest for clarification or at the very end of the process for bioburden reduction and terminal sterile filtration purposes. Such filters supply either a single pore size cut-off membrane, or a range of cut-offs with a large-to-small funneling function to improve filter capacity. NFF can be operated at constant pressure or constant FLUX [LMH; liter/m2/hr] and potential shear stress on the product must always be considered. Vendor, mem- brane chemistry, and pore sizes should be kept constant when scaling. Membrane area and conditioning vol- umes should be scaled linearly to process feed volume, whereby contaminants are regarded as comparable between different process runs and production scales. Stackable (i.e., parallelizable) filter elements should also be considered, as it can offer more f lexibility in usable membrane area, covering larger scales than self-contained filter capsules. Cross/tangential-flow filtration. In cross/tan- gent ia l-f low f i lt rat ion (CF F/ T F F), t he feed mate- rial is fed in a horizontal f low over the membrane; thereby, generally reducing the propensity for filter membrane fouling. The key optimization parameter is the FLUX (LMH) as a function of TMP (bar). Com- monly used formats are either hollow-fiber modules or membrane cassettes. Molecular weight cut-off (MWCO) is an important parameter and should be considered caref ully re- garding the molecular weight of contaminants to be depleted and product particle size itself. MWCO must be kept constant for all scales. CFF is either controlled using constant trans-mem- brane pressure or permeate f low FLUX. While filter load (volume per area) is kept steady during scale-up, the cross-f low rate is often scaled up by keeping the shear rate constant. This makes sense for shear-sen- sitive products (e.g., enveloped viruses). Chromatography. Most biolog ica l produc t ion processes comprise at least one chromatography step in a bind/wash/elute or f low-through modus to enrich product and deplete process-related resid- uals. The most used modalities are affinity, ion ex- change, size exclusion, hydrophobic interaction, and mixed-mode resins. When scaling, the critical, linear scaling param- eters are either f low velocity (cm/h) or product resi- dence time. To keep f low velocity constant between scales, column volume should be scaled-up linearly with the volume to be loaded by keeping the column height constant and increasing the column diam- eter. Volumes for all buffers should also be scaled linearly and f lux rates kept constant (column vol- umes/hour). Linear scale-up of column volumes is not always s t r a i g ht f or w a r d b e c au s e colu m n s t he m s e lve s , membra ne adsorber or monol it hs un its a re on ly available in discrete sizes. Options to mitigate this problem can be to run more than one product cycle or parallelization. Scaling up by constant column volumes/hour is used as an alternative approach and might become the industr y standard for certain modalities (e.g., af finit y, ion-exchange), helping to avoid r unning processes beyond column capacity. TABLE I. Key constant and scaled parameters that must be considered when scaling suspension cell cultures (5). Type of parameter (8) Parameter Description Unit Constant Scaling k L a (Volumetric O2 mass transfer coefficient) The efficiency that oxygen is delivered to the bioreactor at the set operating conditions 1/h P/V Specific power input W/m³ Vessel volumes per minute (VVM) gassing rate Volume of air sparged per unit volume of growth medium per minute Gas flow/reactor volume/minute Tip speed The velocity of the tip of the rotor m/sec-¹ Scale-independent Temperature - °F or °C %DO Percentage of dissolved oxygen % pH - - Cell seed density Number of viable cells in suspension volume Viable cells/mL Harvest time point Time at which harvest occurs Hours or days Scaled Scale-dependent Gas-flow rates - LPM Stirrer speed - RPM