Currently, the primary methods for detecting HCP (Host Cell Protein) include enzyme-linked immunosorbent assay (ELISA) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The former method is characterised by its simplicity, rapidity and high throughput, and is recommended by national pharmacopoeias as the method for detecting residual HCP in biological products.

This paper primarily elaborates on the development of an HCP immunoassay detection method based on USP<1132>, covering aspects such as the analytical method development timeline, preparation and characterisation of key reagents within the analytical method, and the development and validation of the analytical method.

1

Development Cycle

Currently, the method widely employed for HCP detection is the sandwich enzyme-linked immunosorbent assay (ELISA). In drug development, both generic and proprietary HCP ELISA methods are typically established. Generic HCP ELISA kits may be utilised during the early stages. By the phase III clinical stage, pharmaceutical companies will adopt proprietary kits.

Figure 1 Schematic Diagram of HCP ELISA Assay

The development plan for HCP ELISA assays should be determined according to the project's stage. During early product development, commercial HCP kits may be selected, with this analytical method typically viable through Phase II clinical trials (Figure 2A). For Phase III and beyond, platform process HCP methods or upstream process-specific HCP methods should be prioritised. Should commercial kits be intended for use through Phase III clinical trials and post-marketing, comprehensive validation must demonstrate that key kit components (e.g., standards, antibodies) are suitable for HCP detection within the specific manufacturing process. Given that kits originate from external suppliers, biopharmaceutical manufacturers have limited control over these components. Consequently, when critical reagents (e.g., antibodies) are substituted, method consistency must be demonstrated both before and after the change.

A platform process HCP method refers to a detection method developed using company-specific host cell lines, HCP reference standards produced according to the upstream platform process, and corresponding antibodies. When upstream conditions are similar across projects, the same method may be employed for HCP detection. Figure 2B illustrates that if a platform-based HCP method exists during product development, it may be employed throughout all development stages. Should cell culture processes deviate significantly from the platform process, potentially introducing markedly different HCP populations, transition to a dedicated HCP method is required prior to Phase III clinical trials/process validation.

    Figure 2 USP<1132> Analytical Methods Selection Strategy in Different Stage of Product Development

A：HCP Analytical Methods in the Pre-development Stage Without a Platform   

B：HCP Analytical Methods in the Post-development Stage With a Platform

2

HCP Antigen Preparation

2.1 Mock Transfect Cells Preparation

The first step is to establish an mock cell line that does not express the product gene. This is achieved by using either untransfected (i.e., parental) cells homologous to those used for the separate production of the product, or by transfecting a production cell line with a vector lacking the gene encoding the product. The primary advantage of the latter approach is the expression of selective markers (e.g., dihydrofolate reductase or glutamine synthetase). A further benefit is that empty cells grown under conditions simulating transfection are closer to the cell culture conditions used in the manufacturing process. Production cell lines transfected with empty vectors generate HCPs under cell culture conditions approximating the manufacturing process.

As variations in cell viability, metabolism, and density may alter the HCP profile of antigens, cell culture conditions can be slightly modified to generate different HCP combinations (e.g., allowing extended fermentation times, artificially altering culture conditions, or reducing cell survival rates) to obtain a broader HCP spectrum.

HCP antigens may be generated from representative small-scale, pilot-scale or production-scale batches. Pilot or production scale best simulates the actual process. However, a single large-scale production run may fail to reflect normal variations within the cell culture process; conversely, combining several small-scale productions can better reflect the variability arising from different aspects of the cell culture process.

2.2 HCP Harvest

HCP antigens may be prepared from cell lysates, HCCF or mixtures of both, as illustrated in Figure 3. Should the antigen be prepared from HCCF, cell harvesting may be delayed to permit greater cell lysis and HCP release, thereby broadening the HCP spectrum. Following completion of the upstream fermentation process, minimal processing of the resulting HCP antigen is employed to minimise HCP loss. Typically, HCCF steps identical or similar to the actual downstream production process are selected for cell debris removal. The HCCF is then treated via ultrafiltration/diafiltration, buffered with replacement buffers (e.g., PBS or HEPE), and concentrated. Loss of HCPs during filtration should be minimised, for instance by employing membranes with a molecular weight cut-off of 10 kDa or smaller.

Figure 3 USP<1132> Tipycal Preparation Process of Mammals HCP Antigen

2.3 HCP Quality Analysis

It is recommended to perform several analyses prior to immunisation:

1) Protein content: Measure protein concentration to establish a standard concentration for future use and determine the total amount of antigen prepared. The most commonly used methods for total protein assay are the bis-4-chloroaniline (BCA) assay, Bradford assay, and amino acid analysis (AAA);

2) Absence of products (as demonstrated by Western blotting, immunoassay and/or MS analysis);

3) Characterisation via 1-D or 2-D polyacrylamide gel electrophoresis (PAGE) to identify as many HCP proteins as possible (Figure 4).

Figure 4 SDS-PAGE Illusatration of Mammal Cell HCP

HCP antigens must serve not only as immunogens but also as reference standards. Consequently, the following must be ensured:

1) Production volumes must be sufficiently large to provide multi-year stockpiles (typically 10-20 years);

2) Antigen preparation processes must be meticulously documented to facilitate traceability during subsequent reproductions;

3) HCP antigen composition must be sufficiently comprehensive to withstand manufacturing process variations throughout the product lifecycle;

4) HCP reference standards must undergo stability monitoring to prevent degradation.

3

HCP Antibody Preparation

3.1 Animal Immunization and Serum Purification

Given that the prepared anti-HCP antibodies may have an extended shelf life (e.g., corresponding to the anticipated product or platform lifecycle), the quantity of antibodies produced should be maximised. As CHO cells are mammalian, utilising species more distantly related to mammals on the phylogenetic tree (such as chickens) facilitates the generation of antibodies against conserved mammalian proteins, thereby enhancing antibody coverage. Prior to immunisation, animals should be screened for pre-existing antibodies against the product drug, and any reactive animals should be excluded.

Multiple booster immunisations may be administered per animal to generate high-titre, high-affinity antibodies. Low molecular weight HCP antigens often exhibit poor immunogenicity; these may be collected separately and subjected to distinct immunisation strategies to enhance immunogenicity. Prior to pooling antisera, individual sera should undergo titre or Western blot analysis to screen out and remove low-titre antibodies, those reacting only to partial HCPs, and non-specific binders.

When purifying antibodies from pooled sera, different purification methods yield varying results (as shown in Figure 5). The final antibody library obtained should also demonstrate HCP coverage and specificity.

Figure 5 USP<1132> Pros and Cons of Different Purification Methods

3.2 HCP Anitibody Coverage Research

There are two methods for assessing coverage: two-dimensional electrophoresis combined with Western blotting (2-D Western blot) or immunoaffinity capture. USP<1132> compares these two approaches for antibody coverage assessment (Figure 6). In practice, current antibody coverage testing has derived numerous additional methodologies based on these two fundamental principles.

Figure 6  USP<1132>Comparasion of 2-D Western Blot and Immunoaffinity binding 2-D SDS-PAGE

3.2.1 2-D+Western blot

Two identical gels were used for two-dimensional electrophoresis of the same sample, with one gel subjected to Western blot staining and the other to silver/fluorescent staining. Software was then employed to match and analyse protein spots across both gels, calculating coverage levels (Figure 7). This method is relatively straightforward to perform but suffers from poor reproducibility and difficulties in data comparison and analysis. Furthermore, the HCP in this method exists under denaturing conditions, differing from the native state of HCP in ELISA assays (Figure 8).

Figure 7 2-D Western blot Coverage Analysis Flowchart

Figure 8 USP<1132> Coverage Comparison Schematic Diagram

Left：2-D SDS-PAGE of CHO HCP Dyed with Fluorescence Dye

Right：Western Blot Analysis of the Same Sample Shown in the Figure 8 (Left)

3.2.2 2-D Differential in Blot Electrophoresis（DIGE）

Differential Fluorescence Blotting Electrophoresis enables the analysis of a single sample using a single gel preparation, ultimately displaying two fluorescent channels on the same blot membrane: one for total HCP proteins and another for HCPs specifically labelled by antibody recognition. DIGE/DIBE technology eliminates inter-gel variations and gel deformation. Coupled with dedicated HCP analysis software, it enables automatic spot matching with 2D/3D real-time tracking of alignment results, directly calculating coverage metrics. This approach circumvents objective or subjective discrepancies inherent in using dual gels.

Figure 9 DIBE with Different Fluorescence Dye

3.2.3 Immmunoaffinity 2-D SDS-PAGE

Immunoaffinity two-dimensional electrophoresis is a technique based on immunochromatography columns and two-dimensional electrophoresis. First, HCP antibodies are conjugated to the chromatography packing material. Subsequently, HCP samples are loaded onto the column, where the HCP antibodies capture recognised HCP molecules, while unrecognised HCP molecules elute. Finally, the enriched HCP is eluted. Both pre-capture and post-capture HCP fractions undergo two-dimensional electrophoresis and silver staining analysis to calculate antibody coverage (Figure 10).

Figure 10 Immunoaffinity 2-D SDS-PAGE Flowchart

3.2.4 Immunoaffinity Liquid Chromatography-Mass Spectropy/Mass Spectropy (LC-MS/MS) Analysis

Building upon immunoaffinity, LC-MS/MS may also be employed for coverage analysis and can quantitatively assess HCPs. The LC-MS/MS methodology offers several advantages:

1) It enables the identification of potential high-risk HCPs during the early stages of product development;

2) It allows for the determination of relative levels of individual HCPs. However, the extremely low levels of individual HCPs in the final product pose significant challenges to the sensitivity of this method.

3.2.5 2D/MS Analysis Based on Immuno-Magnetic Beads

Magnetic beads typically exhibit superparamagnetic properties, enabling rapid separation of bound and unbound proteins under magnetic fields. This simplifies procedures and reduces reaction times. Functional groups externally modified onto the bead surface bind to active proteins, serving as carriers for antigen-antibody reactions. These form complexes with strong magnetic responsiveness, enabling directional movement under magnetic force. This separates the complexes from other substances in the liquid, achieving the separation, concentration, and purification of specific proteins. For HCP antibody coverage detection, HCP antibodies are conjugated to magnetic beads. The HCP sample is then incubated with these beads. During this process, the HCP antibodies capture and bind recognisable HCPs, while unrecognised HCPs, not captured, are removed during the washing step. Finally, recognisable HCPs are collected under elution conditions such as low pH. Pre-capture HCP samples and captured eluate samples are subjected to two-dimensional electrophoresis and silver staining analysis respectively, with antibody coverage calculated.

Figure 11 Immuno-Magnetic Beads and Protein Binding Process