C-terminal lysine variants in fully human monoclonal antibodies: Investigation of test methods and possible causes

Isolation and characterization of rabies monoclonal antibody charge variants

Current postexposure prophylaxis of rabies includes vaccines, human rabies immunoglobulin (RIG), equine RIG, and recombinant monoclonal antibodies (mAb). In the manufacturing of rabies recombinant mAb, charge variants are the most common source of heterogeneity. Charge variants of rabies mAb were isolated by salt gradient cation exchange chromatography (CEX) to separate acidic and basic and main charge variants. Separated variants were further extensively characterized using orthogonal analytical techniques, which include secondary and tertiary structure determination by far and near ultraviolet circular dichroism spectroscopy. Charge and size heterogeneity were evaluated using CEX, isoelectric focusing (IEF), capillary-IEF, size exclusion chromatography, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and western blotting. Antigen binding affinity was assessed by enzyme linked immuno-sorbent assay and rapid florescence foci inhibition test. Results from structural and physicochemical characterizations concluded that charge variants are formed due to posttranslational modification demonstrating that the charge heterogeneity, these charge variants did neither show any considerable physicochemical change nor affect its biological function. This study shows that charge variants are effective components of mAb and there is no need of deliberate removal, until biological functions of rabies mAb will get affected.

Multi-Attribute Method (MAM) Analytical Workflow for Biotherapeutic Protein Characterization from Process Development to QC

The multi-attribute method (MAM) has emerged significantly in recent years to support biotherapeutic protein characterization from process development to the QC environment. MAM is a liquid chromatography mass spectrometry (LC-MS) based peptide mapping approach, which combines the benefits from liquid chromatography coupled to high resolution accurate mass mass spectrometry (LC-HRAM MS), enabling direct assessment of protein sequence and product quality attributes with site specificity. These product quality attributes may impact efficacy, safety, stability, and process robustness. MAM is intended to replace conventional analytical approaches as it offers a more streamlined strategy for parallel monitoring of multiple attributes in a single analysis with high sensitivity and confidence, and ultimately supports more robust Quality by Design (QbD) approaches and faster decision cycles for biotherapeutic development. MAM consists of three main stages. The first stage is sample digestion, which typically entails proteolytic digestion of the protein. The second stage is reversed-phase chromatographic separation of the generated peptides and detection by HRAM MS in two phases. During MAM Phase I (discovery phase), data-dependent acquisition (DDA) MS/MS is performed to enable confident identification of peaks and development of a peptide workbook. During MAM Phase II (monitoring phase), full MS acquisition is only carried out for the monitoring of predefined product quality attributes (PQAs). The third stage is data processing, which entails analysis and reporting for each of the two phases including evaluation of sequence coverage, assessment of PQAs and peptide workbook creation during phase I, and targeted monitoring of predefined product attributes and new peak detection (NPD) during phase II. The latter is a comparative analysis that uses a base peak alignment algorithm to determine any non-monitored differences between the LC-MS chromatograms of a test sample and a reference standard. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: In-solution sample digestion Alternate Protocol: Automated sample digestion Basic Protocol 2: Reversed-phase chromatographic separation and detection by HRAM-MS (RPLC-HRAM MS) Basic Protocol 3: Data processing and reporting.

Multi-attribute method (MAM) to assess analytical comparability of adalimumab biosimilars

Adalimumab drug product (Humira ®), the first fully human monoclonal antibody (mAb) approved by FDA in 2002, led the top ten list of best-selling mAbs in 2018 and has been the most profitable drug in the world. With the expiration of patent protection in Europe in 2018 and in United States by 2023, the landscape is changing as up to 10 adalimumab biosimilars are expected to enter the market in the US. Biosimilars offer the potential to lower costs on health care systems and increase patient accessibility. The analytical similarity of seven different adalimumab biosimilars was accomplished in the present study using the multi-attribute method (MAM), a LC-MS based peptide mapping technique that allows for primary sequence assessment and evaluation of multiple quality attributes including deamidation, oxidation, succinimide formation, N- and C- terminal composition and detailed N-glycosylation analysis. In the first step, characterization of the most relevant post-translational modifications of a reference product was attained during the discovery phase of MAM. During the second step, as part of the MAM targeted monitoring phase, adalimumab batch-to batch variability was evaluated to define statistical intervals for the establishment of similarity ranges. The third step describes biosimilarity evaluation of predefined quality attributes and new peak detection for the assessment of any new or modified peak compared to the reference product. This study highlights a new perspective of the MAM approach and its underlying power for biotherapeutic comparability exercises in addition to analytical characterization. MAM offers a streamlined comparability assessment workflow based on high-confidence quality attribute analysis using high-resolution accurate mass mass spectrometry (HRAM MS) and the capability to detect any new or modified peak compared to the reference product.

Characterising the structural and cellular role of immunoglobulin C-terminal lysine in secretory pathways

Improved understanding of expression of recombinant immunoglobulin (IgG)-based therapies can decrease manufacturing process costs and bring down costs to patients. Deletion of C-terminal Lysine (C-Lys) from IgG molecules has been shown to greatly impact yield. This study set out to characterise structural components of IgG C-terminal variants which modulate protein expression by examination of the consequences of mutations at the C-terminal of IgG on expression and by the use of fluorescent C-terminal fragment fusion proteins. Cell-based and cell-free experiments were also implemented to characterise how the C-terminal differentially engages with cellular pathways to modulate expression. IgG variants engineered by removal of the C-terminal Lys were expressed at significantly lower rates than control variants by CHO (and HEK) cells. Engineered constructs of mCherry fused with short regions of the C-terminal regions of IgG mimicked the ordering of expressability observed for IgG variants. These fluorescent C-terminal fragment fusions offered the potential to profile how sequences (and point mutations) modified expression. Via combinations of cell and cell-free systems, screening across a range of variants of IgG and mCherry reporter constructs has shown that interactions between specific C-terminal amino acid sequences and the ribosome can regulate the rate and extent of expression. This study highlights the importance of amino acid sequence regulatory events determining the efficiency of production of desirable recombinant proteins, showing that wildtype C-terminal lysine is a necessary capping molecule for IgG1 expression. From a wider perspective, these data are especially significant towards the design of novel entities. The approach has also provided information about novel short C-terminal tags which may be used to provide selective synthesis of specific subunits in the production of multisubunit products. Alternative strategies for removing C-terminal amino acid heterogeneity whilst maintaining efficient rates of expression have been provided.

Observation of Heavy-Chain C-Terminal Des-GK Truncation in Recombinant and Human Endogenous IgG4

Therapeutic IgG mAbs have shown presence of three variations of their heavy chain C-termini, including the unprocessed C-terminal lysine, the processed C-terminal lysine, and C-terminal amidation. These variants are also present in endogenous human IgGs, although the level of unprocessed C-terminal lysine is very low. Here we report a new heavy-chain C-terminal variant, i.e., the des-GK truncation, which exists in both recombinant and endogenous human IgG4. The des-GK truncation was found in negligible amount in IgG1, IgG2 and IgG3 subclasses. Observation of a significant level of heavy-chain C-terminal des-GK truncation in endogenous human IgG4 suggests that low level of this variant present in therapeutic IgG4 is unlikely to be a safety concern.

CRISPR Technologies in Chinese Hamster Ovary Cell Line Engineering

The Chinese hamster ovary (CHO) cell line is a well-established platform for the production of biopharmaceuticals due to its ability to express complex therapeutic proteins with human-like glycopatterns in high amounts. The advent of CRISPR technology has opened up new avenues for the engineering of CHO cell lines for improved protein production and enhanced product quality. This review summarizes recent advances in the application of CRISPR technology for CHO cell line engineering with a particular focus on glycosylation modulation, productivity enhancement, tackling adventitious agents, elimination of problematic host cell proteins, development of antibiotic-free selection systems, site-specific transgene integration, and CRISPR-mediated gene activation and repression. The review highlights the potential of CRISPR technology in CHO cell line genome editing and epigenetic engineering for the more efficient and cost-effective development of biopharmaceuticals while ensuring the safety and quality of the final product.

Interlaboratory Evaluation of a User-Friendly Benchtop Mass Spectrometer for Multiple-Attribute Monitoring Studies of a Monoclonal Antibody

In the quest to market increasingly safer and more potent biotherapeutic proteins, the concept of the multi-attribute method (MAM) has emerged from biopharmaceutical companies to boost the quality-by-design process development. MAM strategies rely on state-of-the-art analytical workflows based on liquid chromatography coupled to mass spectrometry (LC-MS) to identify and quantify a selected series of critical quality attributes (CQA) in a single assay. Here, we aimed at evaluating the repeatability and robustness of a benchtop LC-MS platform along with bioinformatics data treatment pipelines for peptide mapping-based MAM studies using standardized LC-MS methods, with the objective to benchmark MAM methods across laboratories, taking nivolumab as a case study. Our results evidence strong interlaboratory consistency across LC-MS platforms for all CQAs (i.e., deamidation, oxidation, lysine clipping and glycosylation). In addition, our work uniquely highlights the crucial role of bioinformatics postprocessing in MAM studies, especially for low-abundant species quantification. Altogether, we believe that MAM has fostered the development of routine, robust, easy-to-use LC-MS platforms for high-throughput determination of major CQAs in a regulated environment.

Characterization of therapeutic mAb charge heterogeneity by iCIEF coupled to mass spectrometry (iCIEF-MS)

Imaged capillary isoelectric focusing (iCIEF) has emerged as an important technique for therapeutic monoclonal antibody (mAb) charge heterogeneity analysis in the biopharmaceutical context, providing imaged detection and quantitation by UV without a mobilization step. Besides quantitation, the characterization of separated charge variants ideally directly by online electrospray ionization-mass spectrometry (ESI-MS) is crucial to ensure product quality, safety, and efficacy. Straightforward direct iCIEF-MS coupling combining high separation efficiency and quantitative results of iCIEF with the characterization power of MS enables deep characterization of mAb charge variants. A short technical setup and optimized methodical parameters (30 nl/min mobilization rate, 2%-4% ampholyte concentration, 0.5-2 mg/ml sample concentration) allow successful mAb charge variant peak assignment from iCIEF to MS. Despite a loss of separation resolution during the transfer, separated intact mAb charge variants, including deamidation as well as major and minor glycoforms even from low abundant charge variants, could be characterized by online ESI-MS with high precision. The presented setup provides a large potential for mAb charge heterogeneity characterization in biopharmaceutical applications.

Using New Peak Detection to Solve Sequence Variants Analysis Challenges in Bioprocess Development

Recombinant therapeutic proteins have become the major class of drugs to treat various human diseases in recent years. Low levels of protein sequence variants (SVs) have been reported to be present in recombinant therapeutic proteins. The consequences of potential unwanted immune response from SVs of recombinant therapeutic proteins have increasingly drawn attention from regulatory authorities and the biopharmaceutical industry. It is highly desirable to detect low-level SVs during clone selection and early process development as part of the control strategy. Peptide mapping with LC-MS/MS analysis has been applied as a powerful tool to characterize post-translation modifications of therapeutic proteins. Despite the recent advancements in mass spectrometry hardware and software, it is still quite challenging and time-consuming to detect and identify low-level SVs. In this study, we present an optimized approach using new peak detection to detect and identify low level SVs with high confidence and high speed. The new approach makes sequence variants analysis by LC-MS/MS broadly applicable and practical in bioprocess development of therapeutic proteins.

Two-Dimensional Capillary Zone Electrophoresis-Mass Spectrometry: Intact mAb Charge Variant Separation Followed by Peptide Level Analysis Using In-Capillary Digestion

Characterization of charge heterogeneity is an essential pillar for pharmaceutical development and quality control of therapeutic monoclonal antibodies (mAbs). The highly selective and commonly applied capillary zone electrophoresis (CZE) method containing high amounts of ε-aminocaproic acid (EACA) provides a detailed and robust charge heterogeneity profile of intact mAb variants. Nevertheless, the exact location of protein modifications within these charge profiles remains ambiguous. Electrospray ionization mass spectrometry (ESI-MS) is a promising tool for this purpose; however, EACA is incompatible with electrospray. In this context, we present a two-dimensional CZE-CZE-MS system to combine efficient charge variant separation of intact mAbs with subsequent peptide analysis after in-capillary digestion of selected charge variants. The first dimension is based on a generic CZE(EACA) method in a fused silica capillary. In the second dimension, a neutral-coated capillary is used for in-capillary reduction and digestion with Tris(2-carboxyethyl)phosphine (TCEP) and pepsin, followed by CZE separation and MS/MS-characterization of the resulting peptides. The setup is demonstrated using stressed and nonstressed mAbs where peaks of basic, main, and acidic variants were transferred in a heart-cut fashion, digested, and characterized on the peptide level. Sequence coverages of more than 90% were obtained for heavy chain (HC) and light chain (LC) for four different mAbs, including low-abundance variants (<2% of the main peak). Frequently observed modifications (deamidation, oxidation, etc.) could be detected and localized. This study demonstrates a proof-of-concept for identification and localization of protein modifications from CZE charge heterogeneity profiles and, in this way, is expected to support the development and quality control testing of protein pharmaceuticals.

Risk-Based Control Strategies of Recombinant Monoclonal Antibody Charge Variants

Since the first approval of the anti-CD3 recombinant monoclonal antibody (mAb), muromonab-CD3, a mouse antibody for the prevention of transplant rejection, by the US Food and Drug Administration (FDA) in 1986, mAb therapeutics have become increasingly important to medical care. A wealth of information about mAbs regarding their structure, stability, post-translation modifications, and the relationship between modification and function has been reported. Yet, substantial resources are still required throughout development and commercialization to have appropriate control strategies to maintain consistent product quality, safety, and efficacy. A typical feature of mAbs is charge heterogeneity, which stems from a variety of modifications, including modifications that are common to many mAbs or unique to a specific molecule or process. Charge heterogeneity is highly sensitive to process changes and thus a good indicator of a robust process. It is a high-risk quality attribute that could potentially fail the specification and comparability required for batch disposition. Failure to meet product specifications or comparability can substantially affect clinical development timelines. To mitigate these risks, the general rule is to maintain a comparable charge profile when process changes are inevitably introduced during development and even after commercialization. Otherwise, new peaks or varied levels of acidic and basic species must be justified based on scientific knowledge and clinical experience for a specific molecule. Here, we summarize the current understanding of mAb charge variants and outline risk-based control strategies to support process development and ultimately commercialization.

Multiattribute Monitoring of Charge-Based Heterogeneity of Recombinant Monoclonal Antibodies Using 2D HIC-WCX-MS

Charged heterogeneity of monoclonal antibody (mAb) products is regarded as a critical quality attribute (CQA) depending on its impact on the safety and efficacy profile of the product. Hence, manufacturers are expected to perform a comprehensive characterization of the charge heterogeneity to ensure that the manufactured product meets its specifications. Further, monitoring is also expected during the product lifecycle to demonstrate consistency in product quality. However, conventional analytical methods for characterization of hydrophobic and charge variants are nonvolatile salt-based and require manual fraction collection and desalting steps before analysis through mass spectrometry can be performed. In the present study, a workflow of a two-dimensional liquid chromatography method using mass spectrometry (MS)-compatible buffers coupled with native mass spectrometry was performed to characterize hydrophobic variants in the first dimension and charge variants in the second dimension without any need for manual fractionation. This novel two-dimensional (2D) hydrophobic interaction chromatography (HIC)-weak cation-exchange chromatography (WCX)-MS workflow identified 10 variants in mAb A, out of which 2 variants are exclusive to the 2D orthogonal method. Similarly, for mAb B, a total of 11 variants are identified, including 5 variants exclusive to the 2D orthogonal workflow. When compared to stand-alone, HIC resolved only 4 variants for both mAbs and WCX resolved 7 variants for mAb A and 6 variants for mAb B. In addition, the proposed method allows direct characterization of hydrophobic/charge variant peaks through native mass spectrometry in a single-run workflow.

Characterization of light chain c-terminal extension sequence variant in one bispecific antibody

Protein modifications such as post-translational modifications (PTMs) and sequence variants (SVs) occur frequently during protein biosynthesis and have received great attention by biopharma industry and regulatory agencies. In this study, an aberrant peak near light chain (LC) was observed in the non-reduced capillary electrophoresis sodium dodecyl sulfate (nrCE-SDS) electrophoretogram during cell line development of one bispecific antibody (BsAb) product, and the detected mass was about 944 Da higher than LC. The corresponding peak was then enriched by denaturing size-exclusion chromatography (SEC-HPLC) and further characterized by nrCE-SDS and peptide mapping analyses. De novo mass spectra/mass spectra (MS/MS) analysis revealed that the aberrant peak was LC related sequence variant, with the truncated C-terminal sequence “SFNR” (“GEC”deleted) linked with downstream SV40 promotor sequence “EAEAASASELFQ”. The unusual sequence was further confirmed by comparing with the direct synthetic peptide “SFNREAEAASASELFQ”. It was demonstrated by mRNA sequencing of the cell pool that the sequence variant was caused by aberrant splicing at the transcription step. The prepared product containing this extension variant maintained well-folded structure and good functional properties though the LC/Heavy chain (HC) inter-chain disulfide was not formed. Several control strategies to mitigate the risk of this LC related sequence variant were also proposed.

Host cell protein identification in monoclonal antibody high molecular weight species

High molecular weight (HMW) species are product-related variants that may impact therapeutic product safety and efficacy. Therefore, HMW species and aggregates are considered critical quality attributes and their levels should be closely monitored and controlled during drug development, commercial manufacturing, and shelf-life storage period for therapeutic monoclonal antibody drug products. Various biophysical and analytical methods have been developed to characterize the HMW species to understand their mechanisms of formation and assess potential product risk. However, host cell protein (HCP) analysis has seldom been conducted to characterize the impurities in aggregates. In this work, HCP analysis of enriched HMW species and drug substance (DS) from five different monoclonal antibodies (mAbs) was performed. More HCPs are identified in the enriched HMW than in the DS, thus demonstrating a potential interaction between HCPs and HMW. Certain HCPs, including commonly detected HCPs and problematic HCPs, were enriched in HMW fractions. Especially, the most abundant HCP from mAb1, CC motif chemokine, was 46 times more abundant in enriched HMW than DS. The enriched HMW was further fractionated into enriched dimers and enriched very HMW (vHMW) fractions. The CC motif chemokine was found to interact mainly with mAb1 dimer species rather than vHMW fraction. Removing the HMW species from mAb1 significantly decreased the CC motif chemokine level in the final mAb1 DS. Our findings demonstrate that some HCPs are more preferentially bound to HMW species and this finding may provide a new opportunity for removing HCPs in downstream purification steps.

Observation of Heavy-Chain C-Terminal Amidation in Human Endogenous IgG

Therapeutic IgG mAbs expressed from Chinese hamster ovary (CHO) cells are known to contain three C-terminal variants in their heavy chains, namely, the unprocessed C-terminal lysine, the processed C-terminal lysine, and C-terminal amidation. Although the presence of C-terminal amidation in CHO-expressed IgGs is well studied, the biological impact of the variant on the safety and efficacy of biotherapeutics has not been well understood. To further our biological understanding of C-terminal amidation, we analyzed a series of IgG samples, including both endogenous human IgGs as well as recombinant IgGs of different subclasses expressed from both CHO and murine cell lines, for their heavy-chain C-terminal variants by LC-MS/MS based peptide mapping. The results demonstrate that heavy-chain C-terminal amidation is a common variant occurring in IgG of all four subclasses (IgG1, IgG2, IgG3 and IgG4). The variant is generally present in recombinant IgG mAbs expressed from CHO cell lines but not in IgG mAbs expressed from murine cell lines, whereas the IgGs expressed from murine cell lines contain a much larger amount of unprocessed C-terminal lysine. Additionally, a significant amount of heavy-chain C-terminal amidation is observed in endogenous human IgGs, indicating that small amount of the variant present in therapeutic IgGs does not pose a safety concern.

Antibody charge variant modulation by in vitro enzymatic treatment in different CHO cell cultures

Charge variants represent a critical quality attribute that must be controlled during the development and manufacturing of monoclonal antibodies (mAb). Previously, we reported the development of a cost-effective enzymatic treatment capable of removing the C-terminal lysine from a mAb produced by a Chinese hamster ovary (CHO) GS cell line. This treatment resulted in a significant decrease in basic charge variants and a corresponding improvement in the main peak, enabling a longer cell culture production duration for titer improvement. Here, we describe this enzymatic treatment protocol in detail and demonstrate its applicability to two additional mAbs produced by distinct industrial cell lines. The simple addition of carboxypeptidase B (CpB) at a ratio of 1:10,000 (w/w) to whole cell cultures significantly improved the main peaks for both mAbs without affecting other critical quality attributes, including size exclusion chromatography impurities and N-glycans. Our results demonstrate that this in vitro CpB treatment protocol can be used as a platform strategy to improve main peak for mAbs that exhibit high levels of basic variants attributable to C-terminal lysines. An in vitro enzymatic treatment in general may be another good addition to existing in vivo CHO cell culture strategies for titer improvement and control of critical quality attributes. This article is protected by copyright. All rights reserved.

Structural identification and absolute quantification of monoclonal antibodies in suspected counterfeits using capillary electrophoresis and liquid chromatography-tandem mass spectrometry

Monoclonal antibodies (mAbs) represent a major category of biopharmaceutical products which due to their success as therapeutics have recently experienced the emergence of mAbs originating from different types of trafficking. We report the development of an analytical strategy which enables the structural identification of mAbs in addition to comprehensive characterization and quantification in samples in potentially counterfeit samples. The strategy is based on the concomitant use of capillary zone electrophoresis analysis (CZE-UV), size exclusion chromatography coupled to multi-angle light scattering (SEC-MALS) and liquid chromatography hyphenated to tandem mass spectrometry (LC-MS/MS). This analytical strategy was applied to the investigation of different samples having unknown origins seized by the authorities, and potentially incorporating an IgG 4 or an IgG 1. The results achieved from the different techniques demonstrated to provide orthogonal and complementary information regarding the nature and the structure of the different mAbs. Therefore, they allowed to conclude unequivocally on the identification of the mAbs in the potentially counterfeit samples. Finally, a LC-MS/MS quantification method was developed which specificity was to incorporate a different mAbs labeled with stable isotopes as internal standard. The LC-MS/MS quantification method was validated and thus demonstrated the possibility to use common peptides with the considered IgG in order to achieve limit of quantification as low as 41.4 nM. The quantification method was used to estimate the concentration in the investigated samples using a single type of internal standard and experimental conditions, even in the case of mAbs with no stable isotope labeled homologues available.

Generation of a Novel High-Affinity Antibody Binding to PCSK9 Catalytic Domain with Slow Dissociation Rate by CDR-Grafting, Alanine Scanning and Saturated Site-Directed Mutagenesis for Favorably Treating Hypercholesterolemia

Inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9) has become an attractive therapeutic strategy for lowering low-density lipoprotein cholesterol (LDL-C). In this study, a novel high affinity humanized IgG1 mAb (named h5E12-L230G) targeting the catalytic domain of human PCSK9 (hPCSK9) was generated by using CDR-grafting, alanine-scanning mutagenesis, and saturated site-directed mutagenesis. The heavy-chain constant region of h5E12-L230G was modified to eliminate the cytotoxic effector functions and mitigate the heterogeneity. The biolayer interferometry (BLI) binding assay and molecular docking study revealed that h5E12-L230G binds to the catalytic domain of hPCSK9 with nanomolar affinity (K_D = 1.72 nM) and an extremely slow dissociation rate (k_off, 4.84 × 10⁻⁵ s⁻¹), which interprets its quite low binding energy (−54.97 kcal/mol) with hPCSK9. Additionally, h5E12-L230G elevated the levels of LDLR and enhanced the LDL-C uptake in HepG2 cells, as well as reducing the serum LDL-C and total cholesterol (TC) levels in hyperlipidemic mouse model with high potency comparable to the positive control alirocumab. Our data indicate that h5E12-L230G is a high-affinity anti-PCSK9 antibody candidate with an extremely slow dissociation rate for favorably treating hypercholesterolemia and relevant cardiovascular diseases.

Identification, characterization and control of a sequence variant in monoclonal antibody drug product: a case study

Sequence variants (SV) in protein bio therapeutics can be categorized as unwanted impurities and may raise serious concerns in efficacy and safety of the product. Early detection of specific sequence modifications, that can result in altered physicochemical and or biological properties, is therefore desirable in product manufacturing. Because of their low abundance, and finite resolving power of conventional analytical techniques, they are often overlooked in early drug development. Here, we present a case study where trace amount of a sequence variant is identified in a monoclonal antibody (mAb) based therapeutic protein by LC-MS/MS and the structural and functional features of the SV containing mAb is assessed using appropriate analytical techniques. Further, a very sensitive selected reaction monitoring (SRM) technique is developed to quantify the SV which revealed both prominent and inconspicuous nature of the variant in process chromatography. We present the extensive characterization of a sequence variant in protein biopharmaceutical and first report on control of sequence variants to < 0.05% in final drug product by utilizing SRM based mass spectrometry method during the purification steps.

"High-risk" host cell proteins (HCPs): A multi-company collaborative view

Host cell proteins (HCPs) are process-related impurities that may copurify with biopharmaceutical drug products. Within this class of impurities there are some that are more problematic. These problematic HCPs can be considered high-risk and can include those that are immunogenic, biologically active, or enzymatically active with the potential to degrade either product molecules or excipients used in formulation. Some have been shown to be difficult to remove by purification. Why should the biopharmaceutical industry worry about these high-risk HCPs? What approach could be taken to understand the origin of its copurification and address these high-risk HCPs? To answer these questions, the BioPhorum Development Group HCP Workstream initiated a collaboration among its 26-company team with the goal of industry alignment around high-risk HCPs. The information gathered through literature searches, company experiences, and surveys were used to compile a list of frequently seen problematic/high-risk HCPs. These high-risk HCPs were further classified based on their potential impact into different risk categories. A step-by-step recommendation is provided for establishing a comprehensive control strategy based on risk assessments for monitoring and/or eliminating the known impurity from the process that would be beneficial to the biopharmaceutical industry.

C-terminal lysine clipping of IgG1: impact on binding to human FcγRIIIa and neonatal Fc receptors

Monoclonal antibodies (mAbs) display numerous structural attributes, some of them may impact their safety and/or efficacy profiles. C-terminal lysine clipping is a common phenomenon occurring during the bioproduction of mAbs and leads to variable amounts of final process-related charge variants. If Fc-glycosylation has been by far the most documented critical quality attribute (CQA), the potential impacts of mAb C-terminal lysine content is far less reported, particularly on the ability of these basic variants to bind human Fc receptors. To address this question, three charge variant species having zero (K0), one (K1) and two (K2) C-terminal lysine(s) were isolated with high purity from an in-house human IgG1 by preparative strong-cation exchange (SCX) chromatography. A comprehensive biophysical characterization of these three fractions was undertaken, demonstrating their high similarity in terms of structural homogeneity, with a particular attention paid on their respective N-glycosylation profiles. The binding affinity of the fractions to human FcγRIIIa-Val¹⁷⁶ was assessed both by affinity chromatography and surface plasmon resonance (SPR), and to human neonatal Fc receptor (FcRn) by affinity chromatography. Results demonstrate that the three charge variants did not show any significant binding difference for the two tested human Fc receptors, translating certainly to comparable biological properties. As a consequence, C-terminal lysine clipping of the present therapeutic IgG1 should not impact both FcRn-dependent pharmacokinetic profiles and FcγRIIIa-driven cytotoxic activities. The methods used in this study can be widely applied to other IgG1 to define criticality of the C-terminal lysine clipping as a CQA.

Using continuous chromatography methodology to achieve high-productivity and high-purity enrichment of charge variants for analytical characterization

Charge variants of biological products, such as monoclonal antibodies (mAbs), often play an important role in stability and biological activity. Characterization of these charge variants is challenging, however, primarily due to the lack of both efficient and effective isolation methods. In this work, we present a novel use of an established, high productivity continuous chromatography method, known as multi-column counter-current solvent gradient purification (MCSGP), to create an enriched product that can be better utilized for analytical characterization. We demonstrate the principle of this separation method and compare it to traditional batch HPLC (high performance liquid chromatography) or FPLC (fast protein liquid chromatography) methods, using the isolation of charge variants of different mAbs as a case study. In a majority of cases, we are able to show that the MCSGP method is able to provide enhanced purity and quantity of samples when compared to traditional fractionation methods, using the same separation conditions. In one such case, a sample prepared by MCSGP methodology achieved 95% purity in 10 hours of processing time, while those prepared by FPLC and HPLC achieved purities of 78% and 87% in 48 and 300 hours of processing time, respectively. We further evaluate charge variant enrichment strategies using both salt and pH gradients on cation exchange chromatography (CEX) and anion exchange chromatography (AEX) resins, to provide more effective separation and less sample processing following enrichment. As a result, we find that we are able to utilize different gradients to change the enrichment capabilities of certain charged species. Lastly, we summarize the identified mAb charge variants used in this work, and highlight benefits to analytical characterization of charge variants enriched with the continuous chromatography method. The method adds a new option for charge variant enrichment and facilitates analytical characterization of charge variants.

Development of a novel, fully human, anti-PCSK9 antibody with potent hypolipidemic activity by utilizing phage display-based strategy

Background

Proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates serum LDL cholesterol (LDL-C) levels by facilitating the degradation of the LDL receptor (LDLR) and is an attractive therapeutic target for hypercholesterolemia intervention. Herein, we generated a novel fully human antibody with favourable druggability by utilizing phage display-based strategy.

Methods

A potent single-chain variable fragment (scFv) named AP2M21 was obtained by screening a fully human scFv phage display library with hPCSK9, and performing two in vitro affinity maturation processes including CDR-targeted tailored mutagenesis and cross-cloning. Thereafter, it was transformed to a full-length Fc-silenced anti-PCSK9 antibody FAP2M21 by fusing to a modified human IgG1 Fc fragment with L234A/L235A/N297G mutations and C-terminal lysine deletion, thus eliminating its immune effector functions and mitigating mAb heterogeneity.

Findings

Our data showed that the generated full-length anti-PCSK9 antibody FAP2M21 binds to hPCSK9 with a K_D as low as 1.42 nM, and a dramatically slow dissociation rate (k_off, 4.68 × 10⁻⁶ s⁻¹), which could be attributed to its lower binding energy (-47.51 kcal/mol) than its parent counterpart FAP2 (-30.39 kcal/mol). We verified that FAP2M21 potently inhibited PCSK9-induced reduction of LDL-C uptake in HepG2 cells, with an EC₅₀ of 43.56 nM. Further, in hPCSK9 overexpressed C57BL/6 mice, a single tail i.v. injection of FAP2M21 at 1, 3 and 10 mg/kg, dose-dependently up-regulated hepatic LDLR levels, and concomitantly reduced serum LDL-C by 3.3% (P = 0.658, unpaired Student's t-test), 30.2% (P = 0.002, Mann-Whitney U-test) and 37.2% (P = 0.002, Mann-Whitney U-test), respectively.

Interpretation

FAP2M21 with potent inhibitory effect on PCSK9 may serve as a promising therapeutic agent for treating hypercholesterolemia and associated cardiovascular diseases.

Productivity improvement and charge variant modulation for intensified cell culture processes by adding a carboxypeptidase B (CpB) treatment step

The goal of cell culture process intensification is to improve productivity while maintaining acceptable quality attributes. In this report, four processes, namely a conventional manufacturing Process A, and processes intensified by enriched N-1 seed (Process B), by perfusion N-1 seed (Process C), and by perfusion production (Process D) were developed for the production of a monoclonal antibody. The three intensified processes substantially improved productivity, however, the product either failed to meet the specification for charge variant species (main peak) for Process D or the production process required early harvest to meet the specification for charge variant species, Day 10 or Day 6 for Processes B and C, respectively. The lower main peak for the intensified processes was due to higher basic species resulting from higher C-terminal lysine. To resolve this product quality issue, we developed an enzyme treatment method by introducing carboxypeptidase B (CpB) to clip the C-terminal lysine, leading to significantly increased main peak and an acceptable and more homogenous product quality for all the intensified processes. Additionally, Processes B and C with CpB treatment extended bioreactor durations to Day 14 increasing titer by 38% and 108%, respectively. This simple yet effective enzyme treatment strategy could be applicable to other processes that have similar product quality issues.

Integration of liquid chromatography mass spectrometry with a heavy peptide response curve accurately measures unprocessed C-terminal lysine during peptide mapping analysis of therapeutic antibodies in a single run

Therapeutic monoclonal and bispecific antibodies are susceptible to modification after protein biosynthesis. These post-translational modifications (PTMs) not only contribute to mass and charge heterogeneity, but they can also negatively impact the molecule's activity, half-life, and immunogenicity. Therefore, identification and quantification of PTMs are critical to ensure the safety and efficacy of an antibody therapeutic as well as demonstrate product consistency and process control. Unprocessed C-terminal lysine on the heavy chain (HC) is a prevalent modification that contributes to this charge heterogeneity in antibodies. Peptide mapping through liquid chromatography tandem mass spectrometry (LC-MS²) enjoys higher selectivity and sensitivity for measuring this PTM relative to global PTM methods, but differences in the ionization efficiencies of the unprocessed C-terminal K peptide and the truncated C-terminal K peptide result in its overestimation. Consequently, large discrepancies in this PTM's measured abundance may exist between different characterization assays used in regulatory filings, which can be further compounded by large variability when multiple mass spectrometers are used to quantify C-terminal K during a therapeutic's lifespan. In this study, we propose a simple new method to quantify unprocessed C-terminal K in antibodies in a single LC-MS² run that incorporates heavy isotopic standards for both the unprocessed and truncated C-terminal K peptide to build a response curve and correct for the disparity in ionization efficiency between these two different peptide sequences. The approach was evaluated across two different Orbitrap-based mass spectrometers using multiple monoclonal and bispecific therapeutic antibodies, resulting in accurate (<10% error, as determined with peptide standards) and precise C-terminal K quantification during peptide mapping analysis.

LC-MS based case-by-case analysis of the impact of acidic and basic charge variants of bevacizumab on stability and biological activity

The present study investigates the impact of charge variants on bevacizumab's structure, stability, and biological activity. Five basic and one acidic charge variants were separated using semi-preparative cation exchange chromatography using linear pH gradient elution with purity > 85%. Based on the commercial biosimilar product's composition, two basic variants, one acidic and the main bevacizumab product, were chosen for further investigation. Intact mass analysis and tryptic peptide mapping established the basic variants' identity as those originating from an incomplete clipping of either one or both C-terminal lysine residues in the heavy chain of bevacizumab. Based on peptide mapping data, the acidic variant formation was attributed to deamidation of asparagine residue (N84), oxidation of M258, and preservation of C-terminal lysine residue, located on the heavy chain of bevacizumab. None of the observed charge heterogeneities in bevacizumab were due to differences in glycosylation among the variants. The basic (lysine) variants exhibited similar structural, functional, and stability profiles as the bevacizumab main product. But it was also noted that both the variants did not improve bevacizumab's therapeutic utility when pooled in different proportions with the main product. The acidic variant was found to have an equivalent secondary structure with subtle differences in the tertiary structure. The conformational difference also translated into a ~ 62% decrease in biological activity. Based on these data, it can be concluded that different charge variants behave differently with respect to their structure and bioactivity. Hence, biopharmaceutical manufacturers need to incorporate this understanding into their process and product development guidelines to maintain consistency in product quality.

Micro-Heterogeneity of Antibody Molecules

Therapeutic monoclonal antibodies (mAbs) are mostly of the IgG class and constitute highly efficacious biopharmaceuticals for a wide range of clinical indications. Full-length IgG mAbs are large proteins that are subject to multiple posttranslational modifications (PTMs) during biosynthesis, purification, or storage, resulting in micro-heterogeneity. The production of recombinant mAbs in nonhuman cell lines may result in loss of structural fidelity and the generation of variants having altered stability, biological activities, and/or immunogenic potential. Additionally, even fully human therapeutic mAbs are of unique specificity, by design, and, consequently, of unique structure; therefore, structural elements may be recognized as non-self by individuals within an outbred human population to provoke an anti-therapeutic/anti-drug antibody (ATA/ADA) response. Consequently, regulatory authorities require that the structure of a potential mAb drug product is comprehensively characterized employing state-of-the-art orthogonal analytical technologies; the PTM profile may define a set of critical quality attributes (CQAs) for the drug product that must be maintained, employing quality by design parameters, throughout the lifetime of the drug. Glycosylation of IgG-Fc, at Asn297 on each heavy chain, is an established CQA since its presence and fine structure can have a profound impact on efficacy and safety. The glycoform profile of serum-derived IgG is highly heterogeneous while mAbs produced in mammalian cells in vitro is less heterogeneous and can be "orchestrated" depending on the cell line employed and the culture conditions adopted. Thus, the gross structure and PTM profile of a given mAb, established for the drug substance gaining regulatory approval, have to be maintained for the lifespan of the drug. This review outlines our current understanding of common PTMs detected in mAbs and endogenous IgG and the relationship between a variant's structural attribute and its impact on clinical performance.

Protein NMR of biologicals: analytical support for development and marketed products

Application of NMR spectroscopy to derive in-depth characterization of structure and dynamical properties of biomolecules is well established nowadays in many laboratories. Most of these methods rest on the availability of protein labeled with stable isotopes like ¹³C and ¹⁵N. In this report examples are presented on the application of NMR spectroscopic methods to characterize biopharmaceutical proteins in cases no isotope labeled material are available. This is typically found in protein samples used in the development of formulations and production processes. Another important focus of this report is the application of NMR methodology in the field of counterfeit drugs of biologicals and biosimilars. Especially here, NMR does offer relevant structural and quantitative data due to the high versatility of the NMR equipment. An excurse regarding the high medical relevance for a detailed spectroscopic analysis of counterfeits will be presented.

An 18O-Labeling Assisted LC-MS Method for Accurate Quantitation of Unprocessed C-Terminal Lysine in Therapeutic Monoclonal Antibodies

Unprocessed C-terminal lysine (C-term Lys) is one of the most common causes for the formation of basic variants in therapeutic monoclonal antibodies (mAbs). Although the C-term Lys variants are routinely quantified by a LC-MS-based peptide mapping method using the relative MS responses from both C-terminal peptides (with and without Lys), this approach often leads to overestimation of Lys-containing peptide due to the intrinsic difference in ionization efficiency. Herein, we report an ¹⁸O-labeling assisted LC-MS method, which takes advantage of the carboxypeptidase B-catalyzed Lys removal and ¹⁸O-labeling to achieve improved accuracy of C-term Lys quantitation. The fidelity of this method was first demonstrated using synthetic peptide mixture standards that mimic a wide range of C-term Lys levels. Finally, the newly developed method was applied in a case study where C-term Lys variants in mAb samples manufactured from different processes were accurately quantified and compared. This new method provides a valuable solution for studies where accurate C-term Lys levels are needed to assist decision-making and root-cause investigation.

Development and validation of a mass spectrometric method to determine the identity of rituximab based on its microheterogeneity profile

Analytical methods have been considered the "eyes" for development, characterization and batch releasing of biotherapeutics over the past 40 years. One of the most powerful analytical platform for biotherapeutic analysis is mass spectrometry coupled to liquid chromatography (LC-MS). Due to its wide flexibility and instrumental configurations, LC-MS can determine different physicochemical attributes of proteins, e.g. molecular mass, primary sequence, and posttranslational modifications. Intact molecular mass analysis of therapeutic proteins is essential to confirm their identity. Analytical methods must be validated to support drug quality information during its approval process. Although there are international guidelines that provide general information on validation of analytical methods, practical examples about the design, selection of validation attributes and acceptance criteria of identity LC-MS methods are scarce. Here, according to the recommendations of Q2R1 ICH guideline, we showcase the validation of an LC-MS-TOF method to identity rituximab by determining its intact and deglycosylated molecular mass profiles. The proposed method specifically identified the m/z profile and deconvoluted mass profile of rituximab from deglycosylated rituximab and from excipient blank (specificity) with a maximum error of 76.63 ppm (accuracy) and a maximum Relative Standard Deviation (RSD) of 0.00315% (precision). Besides, the system suitability test, which was based on the expected mass value of the mass calibrator, confirmed the reliability of the analytical results. In summary, validation showed that the proposed method is suitable for identifying rituximab based on its glycosylated (intact) and deglycosylated mass profile.

Quantitation and modeling of post-translational modifications in a therapeutic monoclonal antibody from single- and multiple-dose monkey pharmacokinetic studies using mass spectrometry

Post-translational modifications (PTMs) of therapeutic monoclonal antibodies (mAbs) are important product quality attributes (PQAs) that can potentially impact drug stability, safety, and efficacy. The PTMs of a mAb may change remarkably in the bloodstream after drug administration compared to in vitro conditions. Thus, monitoring in vivo PTM changes of mAbs helps evaluate the criticality of PQAs during the product risk assessment. In addition, quantitation of the subject exposures to PTM variants helps assess the impact of PTMs on the safety and efficacy of therapeutic mAbs. Here, we developed an immunocapture-liquid chromatography/mass spectrometry (LC/MS) method to quantify in vivo PTM changes a therapeutic mAb overtime in single- and multiple-dose monkey pharmacokinetic (PK) studies. We also built mathematical models to predict the in vivo serum concentrations of PQAs, the subject exposures to PQAs, and the relative abundance of PQAs in single- and multiple-dose regimens. The model predictions are in good agreement with the experimental results. The immunocapture-LC/MS method and mathematical models enable bioanalytical chemists to quantitatively assess the criticality of PQAs during drug development.

Unambiguous Sequence Characterization of a Monoclonal Antibody in a Single Analysis Using a Nonspecific Immobilized Enzyme Reactor

Accurate sequence characterization is essential for the development of therapeutic antibodies by the pharmaceutical industry. Presented here is a methodology to obtain comprehensive sequence analysis of a monoclonal antibody. An enzyme reactor of immobilized Aspergillopepsin I, a highly stable nonspecific protease, was used to cleave reduced antibody subunits into a peptide profile ranging from 1 to 20 kDa. Utilizing the Thermo Orbitrap Fusion's unique instrument architecture combined with state-of-the-art instrument control software allowed for dynamic instrument methods that optimally characterize eluting peptides based on their size and charge density. Using a data-dependent instrument method, both collisional dissociation and electron transfer dissociation were used to fragment the appropriate charge state of analyte peptides. The instrument layout also allowed for scans to be taken in parallel using both the ion trap and Orbitrap concurrently, thus allowing larger peptides to be analyzed in high resolution using the Orbitrap while simultaneously analyzing tryptic-like peptides using the ion trap. We harnessed these capabilities to develop a custom method to optimally fragment the eluting peptides based on their mass and charge density. Using this approach, we obtained 100% sequence coverage of the total antibody in a single chromatographic analysis, enabling unambiguous sequence assignment of all residues.

Macro- and Micro-Heterogeneity of Natural and Recombinant IgG Antibodies

Recombinant monoclonal antibodies (mAbs) intended for therapeutic usage are required to be thoroughly characterized, which has promoted an extensive effort towards the understanding of the structures and heterogeneity of this major class of molecules. Batch consistency and comparability are highly relevant to the successful pharmaceutical development of mAbs and related products. Small structural modifications that contribute to molecule variants (or proteoforms) differing in size, charge or hydrophobicity have been identified. These modifications may impact (or not) the stability, pharmacokinetics, and efficacy of mAbs. The presence of the same type of modifications as found in endogenous immunoglobulin G (IgG) can substantially lower the safety risks of mAbs. The knowledge of modifications is also critical to the ranking of critical quality attributes (CQAs) of the drug and define the Quality Target Product Profile (QTPP). This review provides a summary of the current understanding of post-translational and physico-chemical modifications identified in recombinant mAbs and endogenous IgGs at physiological conditions.

Structure, heterogeneity and developability assessment of therapeutic antibodies

Increasing attention has been paid to developability assessment with the understanding that thorough evaluation of monoclonal antibody lead candidates at an early stage can avoid delays during late-stage development. The concept of developability is based on the knowledge gained from the successful development of approximately 80 marketed antibody and Fc-fusion protein drug products and from the lessons learned from many failed development programs over the last three decades. Here, we reviewed antibody quality attributes that are critical to development and traditional and state-of-the-art analytical methods to monitor those attributes. Based on our collective experiences, a practical workflow is proposed as a best practice for developability assessment including in silico evaluation, extended characterization and forced degradation using appropriate analytical methods that allow characterization with limited material consumption and fast turnaround time.

Combination of temperature shift and hydrolysate addition regulates anti-IgE monoclonal antibody charge heterogeneity in Chinese hamster ovary cell fed-batch culture

Charge heterogeneity has been broadly studied as a critical quality attribute during monoclonal antibody (mAb) production that may subsequently affect product stability and biopotency. However, the charge variation distribution is poorly controlled, so methods of more effective control need to be explored. In this study, the combined effects of temperature shift (37-34, 37-32, or 37-30 °C) and hydrolysate addition (0.100 g/L) to culture feed on the charge heterogeneity of anti-IgE mAb were investigated. The results showed that the distribution of charge variation was significantly regulated by the combination of hydrolysate addition with a highly sub-physiological temperature (34 °C). In addition, under this condition, the main peak content significantly increased, and the acidic peak content significantly decreased. Furthermore, we explored Lys variant content, which is the major basic variant content, as well as its relationship with temperature shift and hydrolysate addition. Lys variant levels were positively related to the Lys and Arg concentrations in the medium and negatively related to carboxypeptidase B and carboxypeptidase H transcript levels. The combination of temperature shift and hydrolysate addition can thus effectively improve anti-IgE mAb charge heterogeneity and significantly increase main variant levels and decrease acidic variant levels.

LC/MS-based Intact IgG and Released Glycan Analysis for Bioprocessing Applications

Robust plate based antibody glycan analysis platforms are urgently needed for biopharmaceutical development and manufacturing as well as for clinical biomarker research. A 96-well plate based workflow has been developed to analyze both intact IgG antibodies and released N-glycans using an Orbitrap Fusion Mass Spectrometer and an LC/MS method on the Waters UNIFI platform. Here, such a workflow including protein A purification, PNGaseF digestion, 2-AB labeling, and SPE clean-up is described. The measured IgG glycan profile is consistent with that obtained from non-plate based method and commercial kit and has the advantage of less hands-on time. Also the application of the workflow in cell culture monitoring and clonal selection work is demonstrated. Apart from checking the major glycan structure changes among clones, post translational modifications (PTMs) such as C-terminal lysine residue clipping and N-terminal pyroglutamic acid formation can also be deduced from the workflow.

Immunoglobulin Fc-Fused Peptide without C-Terminal Arg or Lys Residue Augments Neuropilin-1-Dependent Tumor Vascular Permeability

Neuropilin-1 (NRP1), which functions as a coreceptor for vascular endothelial growth factor (VEGF) and is implicated in vascular permeability and tumorigenesis, has been targeted by peptides that specifically bind to the VEGF-binding region on NRP1. Like natural VEGF ligands, all known peptides with NRP1-binding activity bind only through a carboxy (C)-terminal R/K-x-x-R/K sequence motif (x stands for any amino acids); this strict requirement is called the C-end rule (CendR). Here, we report immunoglobulin Fc-fused NRP1-specific peptides deviating from CendR. We screened a yeast surface-displayed Fc-fused non-CendR peptide library against NRP1 and isolated Fc-V12, wherein V12 peptide comprising 12 amino acids has a PPRV sequence at its C-terminal end. Although Fc-V12 lacked the CendR motif, it showed selective binding to the VEGF-binding region of NRP1 and triggered cellular internalization of NRP1, which resulted in enhanced extravasation into tumor tissues and tumor tissue penetration of the Fc-fused peptide along with the coinjected chemical drug in tumor-bearing mice. Through a saturation mutagenesis study, we identified that the Val residue at the C-terminus of Fc-V12 is crucial for NRP1 binding. We further improved NRP1 affinity of Fc-V12 (K_D = ∼761 nM) through directed evolution of the upstream sequence of PPRV to obtain Fc-V12-33 (K_D = ∼17.4 nM), which exhibited enhanced NRP1-mediated vascular permeability as compared with Fc-V12. Our results provide functional Fc-fused non-CendR peptides, which bind to the VEGF-binding region of NRP1 and enhance vascular permeability, expanding the sequence space of NRP1-targeting peptides.

Site-Specific Conjugation to Native and Engineered Lysines in Human Immunoglobulins by Microbial Transglutaminase

The use of microbial transglutaminase (MTG) to produce site-specific antibody-drug conjugates (ADCs) has thus far focused on the transamidation of engineered acyl donor glutamine residues in an antibody based on the hypothesis that the lower specificity of MTG for acyl acceptor lysines may result in the transamidation of multiple native lysine residues, thereby yielding heterogeneous products. We investigated the utilization of native IgG lysines as acyl acceptor sites for glutamine-based acyl donor substrates. Of the approximately 80 lysines in multiple recombinant IgG monoclonal antibodies (mAbs), none were transamidated. Because recombinant mAbs lack the C-terminal Lys447 due to cleavage by carboxypeptidase B in the production cell host, we explored whether blocking the cleavage of Lys447 by the addition of a C-terminal amino acid could result in transamidation of Lys447 by a variety of acyl donor substrates. MTG efficiently transamidated Lys447 in the presence of any nonacidic, nonproline amino acid residue at position 448. Lysine scanning mutagenesis throughout the antibody further revealed several transamidation sites in both the heavy- and light-chain constant regions. Additionally, scanning mutagenesis of the hinge region in a Fab' fragment revealed sites of transamidation that were not reactive in the context of the full-length mAb. Here, we demonstrate the utility of single lysine substitutions and the C-terminal Lys447 for engineering efficient acyl acceptor sites suitable for site-specific conjugation to a range of glutamine-based acyl donor substrates.