Critical Quality Attributes
What do critical quality attributes (CQA) mean to bioprocessing? Anne Kowal, Ph.D., associate director at Millennium Pharmaceuticals considered this question in her presentation. According to Kowal, “A CQA is a physical, chemical, microbiological, or biological property or characteristic that should be within an appropriate limit, range, or distribution to ensure the desired product quality.”
The significance for effective product development is the generation of an integrated and well-defined control strategy, leading to an optimal end result. As Dr. Kowal explained, it is very difficult to determine which attributes are truly critical. What might be observed in a general assay is the knowledge that a parameter is different, but the investigator may never know what the impact on the product quality will be.
“Many inputs complicate the analysis of biotherapeutics. Glycosylation impacts the molecule’s performance and other secondary modifications, such as deamination in the complementarity determining region of antibodies can severely degrade efficacy.”
Other quality attributes may be extrinsic, such as microbial contaminants and the choice of cell culture components. All these factors introduce heterogeneity into biotherapeutic molecules that makes teasing out differences between “normal” and more critical variations a challenging task. While the final goal is to link these attributes to clinical outcomes, this may not be possible in the real world since it would be impractical to test every variation in the molecule in human trials.
Dr. Kowal related two case studies on a monoclonal antibody that the company is developing, describing typical challenges faced in the process. Because of time constraints and deadlines, only existing platform methods were available for assessing the material. Moreover, just very small samples of the product were available for analysis, particularly to support planned process changes, and there was no quantitative charge profile assay for comparability or stability.
Dr. Kowal judged that the deaminated antibody fit the requirements for a known potential CQA. Typical high pI of the antibody made assessment of existing IEF stability data difficult, and the sialylated product was a known potential CQA. Therefore, the development of a quantitative charge profile was given high priority. The Kowal group developed a user-friendly cation-exchange method, which demonstrated that there was a significant amount of general process variability.
“We identified a number of antibody variants present in both the batches,” Dr. Kowal explained, “including dimers, aggregates, fragments, and various minor changes, in short, nothing particularly unexpected.”
Using two robust potency assays, a binding assay and a cell-based bioassay, Dr. Kowal and her colleagues were able to correlate the critical quality attributes of dimer formation and truncation of the molecules with reduced activity of the antibody. “We have found that a strong bioassay is the key to a good evaluation of product variants. Multiple assays may help to determine nuances associated with efficacy when in vivo evaluation isn’t possible. Understanding safety questions can be complex. In the final analysis one may have to rely on general knowledge and whatever can be gleaned from clinical experience (or available nonclinical models).”