Summary
Host cell protein (HCP) assays are important tools for monitoring product purity and demonstrating consistency of a manufacturing process. HCPs are typically measured using enzyme-linked immunosorbent assays (ELISAs) with polyclonal antibodies (6, 7). ELISAs come as off-the-shelf kits or can be process-specific assays developed for particular products. An important criterion for these assays is that the antibodies they use must be able to recognize a broad range of HCPs that might be expressed and purified during a manufacturing process. But that is often difficult. Usually several rounds of animal immunizations are necessary to secure broad antigen recognition.
An alternative and attractive way for monitoring HCPs is one- or two-dimensional SDS-PAGE combined with sensitive protein silver staining followed by protein identification using mass spectrometry and/or Western blotting. The MS method can also detect nonimmunoreactive proteins and provides the identity of any contaminating HCPs, although the method is not as sensitive as an ELISA. Another advantage of the alternative method is the speed of analytical development. It often takes 12 months or more to develop a sufficient ELISA, whereas 1D SDS-PAGE, Western blots, and mass spectrometry can be applied immediately to give results within a week.

Figure 5: Drug substance (DS) from different process steps (samples 1-3) was loaded on a 1D SDS-PAGE gel and stained with sensitive silver staining. The gel was overloaded with DS for detection of very small amounts of contaminating proteins. Protein bands were cut out and analyzed by MALDI MS to identify contaminating HCPs. Eight specific HCP’s were identified and marked HCP1-HCP8.

Reference:
Mortz, E. et al. “Proteomics technology applied to upstream and downstream process development of a protein vaccine”, Bioprocess International 6, p36-43, 2008.