Host cell proteins (HCPs) are process-related impurities derived from a host cell expression system that may be present in trace amounts in a final drug substance. During biologics development, it is important to demonstrate that a bioprocess is efficient in removing HCPs and that it provides consistent control of HCP levels. Several techniques are typically used for detection, quantitation, and risk evaluation of HCPs in biologics. The most common are enzyme-linked immunosorbent assays (ELISAs), Western blotting, sodium-dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and chromatographic separation methods (1). Genomics, proteomics, and bioinformatics also have been used recently in analysis and evaluation of host cell proteins (2).
PRODUCT FOCUS:MONOCLONAL ANTIBODIES
PROCESS FOCUS:PRODUCTION AND DOWNSTREAM PROCESSING
WHO SHOULD READ:MANUFACTURING, PROCESS DEVELOPMENT, QA/QC
KEYWORDS: ELISA, HOST-CELL PROTEINS, WESTERN BLOTTING, CROSS-REACTIVITY, RISK MANAGEMENT
From our recent understanding, thousands of potential HCP species could be present in a bioprocess (3,4,5). In a typical two-dimensional gel electrophoresis of HCPs from mammalian cells or Escherichia coli, hundreds of relatively abundant HCPs were detected. This is different from residual DNA, another process-related impurity, which is negatively charged in most bioprocesses and relatively easy to separate from the product stream. HCPs present a wide spectrum of variability, from different molecular sizes to charge heterogeneities. It is safe to say that any given biologic will always have a subset of HCPs with similar properties, which makes separating them from such products more challenging.
During the development of one monoclonal antibody (MAb), our group found that drug substance containing the antibody had a higher than normal level of HCPs. Initially, the process development team tested a variety of separation approaches trying to lower the HCP value, but with limited success. Further analytical studies led to a hypothesis that cross-reactivity of anti-HCP antibodies to this particular MAb might be the problem. The same preparation of polyclonal anti-HCP antibodies was used to both capture and report antibody for HCP quantitation in the HCP ELISA (Figure 1), so we hypothesized that even a small fraction of cross-reactivity could cause a measurable HCP signal (because of the assay’s nanogram sensitivity).
Systematic analysis indicated that ~30% of internally generated MAbs have such nonspecific cross-reactivity. We implemented a modified HCP ELISA to minimize this cross-reactivity and improve the accuracy of our HCP analysis. Other biologics, especially MAbs under development in the biotechnology industry, may have similar properties and should benefit from our findings. Improvement of this HCP assay will increase the accuracy of HCP quantitation to prevent the commitment of costly and unnecessary resources to improve our bioprocess when the actual cause for high HCP readings was antibody cross-reactivity.
Materials and Methods
All the MAbs we tested were from our own company. Precast gels for SDS-PAGE came from Bio-Rad Laboratories (catalog #345-0043, www.bio-rad.com). Antihuman IgG polyclonal antibodies came from Thermo Scientific (catalog #31143, www.thermo.com). And Streptavidin–IRDye-800 fluorescence conjugate came from Rockland Immunochemicals (catalog #S000-32, www.rockland-inc.com).
ELISA: We developed a process-specific Chinese hamster ovary (CHO) host cell protein ELISA and used it to support our group’s MAb development. A typical quantitation limit (QL) of 5 ng/mg (5 ppm) was demonstrated according to the ICH method validation guidelines (6). The ELISA is in a sandwich format, with HCP binding antibodies first coated onto a 96-well plate and incubated overnight, followed by an incubation with 1% BSA, then testing samples added and incubated for 90 minutes at room temperature. After multiple washes, the HCP reporting antibodies (biotinylated) were added and a colorimetric reaction determined the sample’s HCP level against a predetermined standard curve.
For cases in which a cross-reactivity occurred, we added an additional step of antihuman IgG antibody incubation before the HCP reporting antibody incubation. This step blocks nonspecific interactions among anti-HCP antibodies and MAbs under development.
Gel Electrophoresis and Western Blot: We used 10–20% gradient SDS-PAGE gels for separation of all MAbs, treating samples with SDS and reducing reagent. for total protein detection, the gel was fixed in a solution with 10% each of acetic acid and ethanol. Sypro Ruby protein stain was used for over-night fluorescence staining. After performing two washes in Milli-Q water (from Millipore, www.millipore.com) for five minutes each, we used a VersaDoc image system from Bio-Rad to acquire the gel images.
For Western blotting, the proteins separated in SDS-PAGE were transferred to a nitrocellulose membrane over night with NuPAGE transfer buffer (Invitrogen catalog #NP0006-1, www.invitrogen.com). After blocking with 1% BSA in PBS-T (0.1% Tween-20 in PBS), we used a biotin-labeled polyclonal antibody against CHO HCP to test for cross-reactivity between the internal MAbs and the anti-HCP antibody. A streptavidin-IR Dye800 was used to detect the interaction, and the image was acquired using an Odyssey system from LI-COR Biosciences (www.licor.com).
Cross-Reactivity Among Anti-HCP Antibodies and Therapeutic Monoclonal Antibodies: In typical biologics development efforts, a sandwich ELISA is used for quantitation of HCPs (Figure 1). The main reason to use this format is that HCPs present in the final drug substance must be reduced to ppm levels to minimize potential health risks, so an enrichment step (use of binding antibody) is necessary for their accurate measurement at such low levels. Because of the diversity of HCPs that may be present in the early bioprocess steps (3,5,7,8), for HCP quantitation we used a polyclonal antibody prepared against a null cell of the same production cell line for HCP analysis, as described in a recent review (2). Anti-HCP antibodies were raised as usual in either rabbits or goats, and the IgGs were purified using either a protein A or protein G column for use in the ELISA assay. As Figure 1 indicates, a portion of the anti-HCP IgGs was labeled with biotin and used as the reporting antibody for HCP detection. Using this format, a minimum of two antibody-generating epitopes is required for HCP detection.
During the development of one particular internal MAb candidate, HCP testing showed that the drug substance appeared to have higher levels of HCP than other MAbs under development even though they all use a very similar purification platform. Because typical HCP levels in biologics are reported at 1–100 ppm (9), we made an effort to further reduce the HCP level for this particular MAb. Initially, our efforts focused on process improvement. However, soon we realized that altering different purification conditions had very limited impact on HCP reduction (data not shown).
A hypothesis was proposed that this MAb might have cross-reactivity with anti-HCP antibodies, so a portion of the signal reported as HCPs actually resulted from binding to a subpopulation of the MAbs with anti-HCP antibodies (Figure 2). Because this particular MAb is a human IgG, we tested our hypothesis using polyclonal antihuman IgG antibodies in a sandwich HCP ELISA.
During the assay, after incubation of the drug substance with the capture antibody, we included a step in which the antihuman IgG antibody was added before the biotinylated anti-HCP antibodies. That additional step lowered the HCP signal (Figure 3), which confirmed that some of the HCP signal was indeed due to cross-reactivity. Further testing showed that the maximum blocking effect from antihuman IgG antibodies was reached at 100 µg/mL (data not shown), so all subsequent tests reported herein included the additional blocking step.
Development of a Modified Sandwich ELISA and Analysis of Additional MAbs: To demonstrate that decreasing HCP values were not the results of cross-reactivity among HCPs and antihuman IgG antibodies, we used HCP standards (in the absence of MAbs) to incubate with polyclonal antihuman IgG antibodies. Figure 4 indicates that incubation with antihuman IgG antibodies does not significantly affect HCP detection. When tested at different MAb concentrations, it can be seen that ~80% of the HCP value results from cross-reactivity (Figure 5).
When the MAb concentration increased to 2 mg/mL, HCP quantitation was no longer linear. Because the microplate coating of anti-HCP antibody is limited (normally
Table 1: Effect of antihuman IgG antibody blocking on CHO HCP ELISA quantitation; samples were tested with and without antibody incubation; all values were obtained with triplicate measurement.
To further evaluate the cross-reactivity phenomenon, we tested seven other MAbs with and without blocking antihuman IgG antibodies (Table 1). The majority of MAbs in this study do not exhibit cross-reactivity with anti-HCP antibodies. However, two of them (MAb-6 and MAb-8) in addition to original MAb-9 showed a more modest cross-reactivity with the anti-HCP antibodies.
We analyzed nine MAbs (eight used in the ELISA plus one more) by Western blot as an orthogonal technique to further evaluate the cross-reactivity phenomenon. Figure 6 showed that some had cross-reactivity with anti-HCP antibodies.Comparing the Western blot analysis with ELISA results (Table 1) indicated some correlation between the two assays, especially with heavy-chain cross-reactivity. However, Western blot binding is to a denatured sample, so the signal may be due to buried epitopes such as MAb-1, which showed little cross-reactivity in ELISA (Table 1) yet a visible binding by anti-HCP to the light chain (Figure 6).
HCP clearance is a major parameter in therapeutic biological compound development. A relatively low and consistent level of HCPs has to be achieved to demonstrate process robustness and minimize potential risks (10,11). We identified three internal MAb candidates that showed cross-reactivity with anti-HCP antibodies, thus artificially increasing their final drug substance HCP readings. Implementation of antihuman IgG antibody incubation in the HCP ELISA significantly reduced that cross-reactivity and brought the HCP readings more in line with typical levels for MAb products.
However, both MAb-8 and MAb-9 still had relatively high HCP values (19 ppm and 17 ppm, respectively). One possible reason for this high HCP value is that multiple binding sites are associated with the cross-reactivity, and even though the antihuman IgG antibody is polyclonal, it can block only a portion of those sites. That would leave others still available for nonspecific binding with anti-HCP antibodies.
Eight MAb candidates had high homology outside the three CDR regions we tested, and only three showed cross-reactivity. Currently it is unclear whether the CDR regions or specific cell lines in which those MAbs are being produced are responsible for the observed cross-reactivity. Alignment of the amino acid sequences from those MAbs did identify some possible sequences in the CDR region that might be associated with the cross-reactivity (data not shown). Western blot results supported the possibility that certain epitopes of the IgG heavy chain from some MAbs might have cross-reactivity with anti-HCP antibodies.
Further studies are needed to test whether those identified sequences in the CDR region are responsible for the observed cross-reactivity. In our collaboration with contract manufacturing organizations (CMOs) for MAb development, we have noticed that a small portion of those MAbs also reported relatively high HCP levels, with some even reaching a few hundred ppm (data not shown). So it will be interesting to see whether those particular MAbs also have cross-reactivity with anti-HCP antibodies generated by the CMOs.
It is reasonable to believe that among MAbs under development in the biopharmaceutical industry, a small proportion could possess the property of cross-reactivity with anti-HCP antibodies. So it is advisable to test this possibility with antihuman IgG antibodies for HCP ELISA quantitation to achieve the most accurate HCP analysis and prevent costly and time-consuming efforts in purification and process development.