Scaling Down of Biopharmaceutical Unit Operations — Part 1: Fermentation - The fermentation process can be challenging to scale down and several factors must be evaluated for each step. - BioPha


Scaling Down of Biopharmaceutical Unit Operations — Part 1: Fermentation
The fermentation process can be challenging to scale down and several factors must be evaluated for each step.

BioPharm International

Use GMP-released raw materials that are identical to those used for the full-scale process. Lot-to-lot variations between raw materials, including master or working cell-bank vials, as well as all media components, antifoam additions, and acid and base stock solutions, can greatly impact process performance. If the same lot of raw materials is not available, have the vendor supply a representative lot based on the certificate of analysis. Similarly, prepare buffers and all bulk media solutions according to the GMP-approved manufacturing procedures for the full-scale process. Match current manufacturing procedures for the order of addition of media ingredients, mode of sterilization and addition (filter, heat or steam), mixing times and temperatures, media hold times, and the preparation of mixing tanks.

After assembling a system with all the similar geometries, it is time to tune the controls. All volume-independent operational control-parameter setpoints should be identical to the center point of the operating ranges of the large-scale fermentation process. The volume-independent parameters include:

  • process temperature
  • pH
  • inoculation percentages (v/v) for each step
  • schedule of feed-media additions.

If oxygen transfer rates are equivalent, then the dissolved-oxygen control setpoint and vessel backpressure should also be held constant.

Except for agitation, use a linear adjustment for all the volume-dependent operational control-parameter setpoints. The scale factor should be equivalent to the ratio of overall process volumes. For example, if the process volumes are 300 L and 15 L, the scale factor (as a divisor) is 20.

The volume-dependent parameters include:

  • Pre and post-sterilization volumes of growth media. The volumes of initial growth media at the beginning and end of sterilization should be equal to the initial and final volumes in the manufacturing process divided by the scale factor. Excessive water gain or loss during sterilization can result in slightly offset volumes, which may result in product and cell mass dilution and possibly an increase in overall time (if the process target is based on optical density). It is crucial to know the extent of these volume differences in order to assess the accuracy of the scale-down model.
  • Feed media delivery rates. Adjust all feed rates based on the scale factor. The method of delivery (based on unit weight or unit volume per hour) and the control (flowmeter vs. air pressure) should be the same as the large-scale process; otherwise it will be difficult to equate the two processes.
  • Total airflow. Scale down the total airflow linearly to ensure similar oxygen transfer and carbon dioxide stripping between the different scales.
  • Oxygen flow rate. In most cases, the maximum oxygen flowrate, whether using pure oxygen or oxygen-enriched air, should be scaled-down linearly. However, if there are profound differences in the sparger or vessel geometry between scales, then the total flowrate of oxygen, relative to air, may need to be changed to ensure equivalency in oxygen transfer rates. The oxygen flowrate should be increased only if the agitation has been increased to its maximum tolerance for the process, or if the culture is extremely shear sensitive.

Set agitation to provide either representative oxygen transfer rate (k L a), tip speed (v T ), Reynolds Number (N RE ), or power-input per unit volume (P/V), according to the equations listed in Table 2. Assuming equivalent vessel geometries and sparger design, the best bet for agitation is to provide a representative oxygen transfer rate (k L a) between scales. If k L a data are not available at the different scales, then set agitation to provide an equivalent power input per unit volume (P/V). This should result in similar mixing profiles across scales, and thus similar oxygen transfer and dispersion. Scale down by constant tip speed or constant Reynolds Number has also been reported.

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