The first goal of cleaning validation — demonstrating that the membranes are returned to a state where the process will perform
reproducibly each run — can be met by tracking process performance as well as membrane data from run to run. Typical measures
of process performance are process flux, process time, product yield, and product purity. While these measurements tie most
directly to process success and are the most relevant, they require that product feedstock be committed to the cleaned membranes
without knowing whether the membranes have yet reached their reuse limit. Therefore, in addition to process data, it is desirable
to also track clean membrane data — most typically membrane integrity and clean membrane permeability. Membrane integrity
should be monitored before and after each use, and the vendor's specification for membrane failure should be used as a criterion
for replacing the membranes. Clean membrane permeability — water flux divided by transmembrane pressure (TMP) — should be
measured and trended from run to run. Although a drop in clean membrane permeability may not be as directly related to process
performance as the actual process flux, it is a simple measurement that can be an early indication of degradation of membrane
performance. The reduction of clean membrane permeability to a certain percentage of the initial value may be used as a membrane
change-out criteria. However, it is important to note that measured permeability values are often related to the system on
which the measurement is taken, especially for high permeability membranes (ultrafiltration > 30 kDa and all microfiltration
membranes) where measured TMPs are very low and often within the measurement error. Differences in the placement of pressure
transmitters in relation to the membranes and differences in pressure loss in piping also add to variations in permeability
measurements from system to system. For these reasons, a change-out criterion based on a permeability measurement should only
be set when the data is collected on the same system on which it will be applied.
The second goal of cleaning validation is ensuring that contaminants are adequately removed to prevent run-to-run carryover.
Carryover studies can either be performed between batches using placebo product, or by testing rinse solutions or extraction
solutions. The placebo product, rinse solution, or extraction solution is analyzed for residual product protein, other host
cell proteins, DNA, excipients, and residual cleaning and storage reagents. Care should be taken not to set the specification
for any contaminant level as the limit of detection (LOD) of the assay, as LODs often decrease as assays improve with new
technology. Carryover studies must be performed out to the established limit of membrane reuses. Membrane re-use validation
and carryover validation are often done concurrently at full-scale during production in order to minimize cost and effort.
However, care must be taken to verify that product quality is not compromised while running a process for which the validation
is still open. This can be done by putting in place appropriate sampling, analysis, and quarantining of final product until
it is shown to be acceptable. In addition to run-to-run carryover, users of TFF membranes must also demonstrate that the preservative
(shipping) solution is completely removed from the devices after a specified flushing protocol prior to the first use with
product. The assay showing clearance of the preservative or storage solution should be easy to use, and validation of an assay
which can be run on the manufacturing floor will save processing time versus having to submit samples for QC testing and waiting
for results before beginning a process run.
Finally, the third goal of cleaning validation is to demonstrate that bioburden and endotoxin levels are kept under control
using the specified cleaning protocol. This is of particular importance if the TFF unit operation is near the end of the purification
process. Sterility requirements and maximum endotoxin levels should be specified — often not just for the final processed
product pool but for incoming buffers and product as well. Bioburden and endotoxin elimination from the membranes and system
using the specified cleaning reagents (concentration, temperature, and exposure time) must be documented. In addition, storage
solutions should be evaluated for bacteriostatic capability over the intended storage time.
Special considerations surround the cleaning validation of a microfiltration system due to exposure to cells. Whether bacterial,
yeast, or mammalian, the cleaning of the system must be adequate to show inactivation and removal of all cells and cell debris
between runs. The biggest validation issue surrounding cell harvest is the containment of recombinant organisms and equipment
decontamination. If a claim is made that the filtrate from the TFF system will be free of recombinant organisms, then protocols
should specify testing of the filtrate for such.
Prior to validating a TFF process, it is important to ensure that the process has been adequately defined during process development
and accurately scaled up onto a well-designed system. If these criteria have been met, the task of executing the overall validation
strategy — which should include process characterization, process validation, and cleaning validation segments — becomes much
more straightforward with a high probability of success.
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4. USP. USP XXIII; Section 88: Biological reactivity tests, in vivo. Rockville (MD): USP.