RELEVANT PERFORMANCE PARAMETERS FOR A DRUG-SUBSTANCE UNIFORMITY STUDY
The DS specification parameters for biological products confirm the identity, purity, potency, quality and safety of the API.
The ultimate aim of a DS uniformity study is to ensure that individual DS containers are consistent with respect to all of
these critical quality attributes (CQAs) or specification parameters. For the purpose of demonstrating uniformity, it is not
necessary to test each of the DS specification parameters, since a subset of the parameters may be used as a surrogate for
the others. Typically, the quantitative specification parameters can be used to demonstrate uniformity. Representative assays
may include protein concentration by absorbance (e.g., UV280), high performance liquid chromatography (HPLC) or bioassay.
Of these, the UV280 is the simplest and fastest measurement with an acceptable degree of accuracy and precision. Protein concentration
also allows evaluation of possible mechanisms for introduction of non-uniformity to the DS batch (e.g., dilution due to residual
flush liquid, protein adsorption to the filter material, or inherent uniformity challenges with the upstream pool). Other
parameters, such as pH, osmolality, conductivity and/or purity (e.g., aggregate concentration) may also be used as measures
for DS uniformity. It is important to consider the potential failure modes for uniformity when selecting performance parameters
such that they are sensitive enough to pick up a lack of uniformity. For example, if dilution with residual water is a potential
source for non-uniformity, pH may not be the best parameter to use because water for injection (WFI) may not significantly
impact the buffering capacity of the drug substance formulation buffer. Additionally, stabilizing agents such as polysorbate
may be a critical component of the DS and it may be necessary to demonstrate that the concentration of such agents is within
required limits. Other factors, such as shear could impact DS product quality (particle size, aggregation, potency), and a
risk assessment should be considered to identify and mitigate these potential outcomes. Validation provides evidence of sufficient
uniformity such that any sample location across the fill is representative of the entire DS lot with respect to the CQAs.
Validation allows batch release and stability testing to be performed on a representative sample collected from a single location
within the DS fill operation.
Figure 1: Flow diagram of drug substance filtration. (FIGURE IS COURTESY OF THE AUTHOR)
Another key question is when to take the samples to demonstrate uniformity and how to establish appropriate validation acceptance
criteria. Typically, samples may be taken at the beginning, middle, and end of bulk filtration, and if they meet prospective
acceptance criteria, the entire batch can be considered to be homogenous. The sample collection strategy is an important consideration:
Does one take a point sample (i.e. directly from the filter bell), or a pool sample from the actual container? The latter
is more relevant but needs to be balanced against the potential risk of contamination during sampling, and mixing considerations
prior to taking the sample. Typically, the beginning sample is taken as a pool sample from the first container, as this reflects
how the contents of the DS will be forward processed. If the DS lot-release and stability samples are taken as point samples
at the middle of bulk filtration, the use of point samples during a uniformity study may be favorable for consistency and
to minimize potential for contamination.
There are several ways to establish acceptance criteria to demonstrate DS uniformity, which may be used in isolation or in
combination, as appropriate. Prior to selecting a preferred strategy, it is important to understand how the DS will be used
during the subsequent DP formulation and testing, and to ensure that the requirements for these steps are addressed in the
DS approach. Several methods for DS testing, and their pros and cons, are briefly discussed below.
Calculation based on DP specification limits
This method is based on a back calculation of the requirements for DP processing and DP specification criteria. The DS batch
may be divided and used over several DP batches. In order to maintain this flexibility, it is necessary to ensure that any
DS batch or part thereof used for DP processing will meet the DP specification requirements. It is necessary to take into
account the smallest DP stock keeping unit (SKU), including product concentration and manufacturing volume, which will be
used. The advantage of this method is that it is based on processing needs. However, it may be complicated by the existence
of multiple SKUs and the desired safety margin between a passing uniformity result for DS compared to the associated specification
range for DP.