Analytical and bioanalytical technologies for characterizing antibody–drug conjugates

Antibody drug conjugates as targeted cancer therapy: past development, present challenges and future opportunities

Antibody drug conjugates (ADCs) are promising cancer therapeutics with minimal toxicity as compared to small cytotoxic molecules alone and have shown the evidence to overcome resistance against tumor and prevent relapse of cancer. The ADC has a potential to change the paradigm of cancer chemotherapeutic treatment. At present, 13 ADCs have been approved by USFDA for the treatment of various types of solid tumor and haematological malignancies. This review covers the three structural components of an ADC-antibody, linker, and cytotoxic payload-along with their respective structure, chemistry, mechanism of action, and influence on the activity of ADCs. It covers comprehensive insight on structural role of linker towards efficacy, stability & toxicity of ADCs, different types of linkers & various conjugation techniques. A brief overview of various analytical techniques used for the qualitative and quantitative analysis of ADC is summarized. The current challenges of ADCs, such as heterogeneity, bystander effect, protein aggregation, inefficient internalization or poor penetration into tumor cells, narrow therapeutic index, emergence of resistance, etc., are outlined along with recent advances and future opportunities for the development of more promising next-generation ADCs.

Two-Dimensional Liquid Chromatography (2D-LC): Analysis of Size-Based Heterogeneities in Monoclonal Antibody–Based Biotherapeutic Products

Monoclonal antibodies (mAbs) dominate the pipelines in the biopharmaceutical industry today. Being complex products, this class of molecules has numerous critical quality attributes (CQAs). Their thorough characterization is a necessary and critical component of biopharmaceutical product development. One CQA is size-based heterogeneity. Aggregates are widely considered a CQA because of their likely impact on the immunogenicity of the product. There is no single analytical tool that can accurately characterize aggregates because of the significant diversity that they exhibit with respect to size, structure, and morphology. As a result, it is common practice to use multiple, orthogonal analytical tools for aggregate characterization. This article reviews efforts targeting the use of two-dimensional liquid chromatography (2D-LC) and mass spectrometry (MS) for aggregate characterization.

Overcoming challenges associated with the bioanalysis of cysteine-conjugated metabolites in the presence of antibody-drug conjugates

Aim: Investigations have shown that for the antibody-drug conjugate (ADC) belantamab mafodotin, concentrations of the cysteine-conjugated metabolite, Cys-mcMMAF, were overestimated in the presence of the ADC during sample processing when utilizing a historical SPE method. Results: A new assay was developed utilizing an acidic protein precipitation to remove the ADC early in the extraction process, thus eliminating the risk of overestimating Cys-mcMMAF in the presence of belantamab mafodotin. In vitro experiments demonstrated a linear relationship between the concentration of belantamab mafodotin and the release of Cys-mcMMAF. Extensive stability assessments were performed to cover storage of study samples. Conclusion: This work emphasized the critical importance of understanding the performance of a bioanalytical method for free toxic payload in the presence of the ADC.

Strategies for Setting Patient-Centric Commercial Specifications for Biotherapeutic Products

Commercial specifications for a new biotherapeutic product are a critical component of the product's overall control strategy that ensures safety and efficacy. This paper describes strategies for setting commercial specifications as proposed by a consortium of industry development scientists. The specifications for some attributes are guided by compendia and regulatory guidance. For other product quality attributes (PQAs), product knowledge and the understanding of attribute criticality built throughout product development should drive specification setting. The foundation of PQA knowledge is an understanding of potential patient impact through an assessment of potency, PK, immunogenicity and safety. In addition to PQA knowledge, the ability of the manufacturing process to consistently meet specifications, typically assessed through statistical analyses, is an important consideration in the specification-setting process. Setting acceptance criteria that are unnecessarily narrow can impact the ability to supply product or prohibit consideration of future convenient dosage forms. Patient-centric specifications enable appropriate control over higher risk PQAs to ensure product quality for the patient, and flexibility for lower risk PQAs for a sustainable supply chain. This paper captures common strategic approaches for setting specifications for standard biotherapeutic products such as monoclonal antibodies and includes considerations for ensuring specifications are patient centric.

Current LC-MS-based strategies for characterization and quantification of antibody-drug conjugates

The past few years have witnessed enormous progresses in the development of antibody-drug conjugates (ADCs). Consequently, comprehensive analysis of ADCs in biological systems is critical in supporting discovery, development and evaluation of these agents. Liquid chromatography-mass spectrometry (LC-MS) has emerged as a promising and versatile tool for ADC analysis across a wide range of scenarios, owing to its multiplexing ability, rapid method development, as well as the capability of analyzing a variety of targets ranging from small-molecule payloads to the intact protein with a high, molecular resolution. However, despite this tremendous potential, challenges persist due to the high complexity in both the ADC molecules and the related biological systems. This review summarizes the up-to-date LC-MS-based strategies in ADC analysis and discusses the challenges and opportunities in this rapidly-evolving field.

ADC Analysis by Hydrophobic Interaction Chromatography

Hydrophobic interaction chromatography (HIC) is a traditional technique used for the separation, purification, and characterization of proteins. As the number of antibody-drug conjugates (ADCs) continues to increase in clinical trials, HIC and other orthogonal methods utilizing changes in hydrophobicity are being used for ADC characterization and analysis. Unlike other techniques, HIC uniquely allows for protein analysis under mild nondenaturing conditions that preserve the native structure and activity of the molecules. Analysis of the ADC in its native form is advantageous. Herein, we describe a generic HIC protocol for the screening, analysis, and characterization of ADCs using an ammonium sulfate buffer and a high-pressure liquid chromatography system. Parameters affecting data quality and interpretation are addressed. In addition, several recommendations are included for method optimization and troubleshooting.

A method to directly analyze free-drug-related species in antibody-drug conjugates without sample preparation

Residual free-drug-related species that are present in antibody-drug conjugates (ADC) are a potential safety risk to patients and are therefore categorized as a critical quality attribute that must be strictly monitored and controlled. Among the many analytical methods developed for free-drug analysis, reversed-phase liquid chromatography (RP-LC) is the most common approach. Conventional RP-LC methods for free-drug analysis, however, involve labor-intensive sample preparation. Here we present a new RP-LC method to directly analyze free-drug-related species in an ADC sample without the need for sample preparation. In our work, free-drug-related species were very well separated from ADC peaks in the chromatography gradient. Typical performance issues observed in conventional RP-LC, such as column fouling, detection interference, and carryover, were not observed or were negligible with this new method. Three options were evaluated for free-drug quantitation: Strohl (2017) [1] use of an external free drug calibration curve for determination of absolute concentration; Perez et al. (2014) [2] calculation of relative percentage based on peak area ratio between free drug and ADC at a characteristic wavelength unique for drug payload; and (Beck et al., 2017) [3] calculation of relative percentage based on peak area ratio between free drug and corrected ADC peak area (at any wavelength). The method with calibration curve provides the highest sensitivity, the best accuracy and precision for determination of free drug present in the ADC. However, the second and third options were simpler because they eliminated the need for an external calibration curve, making them worth exploring. Due to its simplicity and compatibility with mass spectrometry, the new method is also a good choice for direct analysis of stability samples.

Novel trastuzumab-DM1 conjugate: Synthesis and bio-evaluation

Antibody-drug conjugates are now of considerable interest and are recommended for the treatment of cancers. Linkers are having a crucial role in potency and efficacy of these drugs. Herein, for the first time, we have used a water-soluble poly-ethylene glycol based linker (succinimidyl-[(N-maleimido propionamido)-diethyleneglycol] [SM(PEG)2]) for lysine amide coupling of DM1 drug to trastuzumab considering evaluation of the effect of using a hydrophilic linker on physicochemical and biological properties of the resulting conjugate in comparison to the conjugate containing succinimidyl 4-(N-maleimidomethyl) cyclohexane-1-carboxylate (SMCC) linker, which has a relative hydrophobic nature. The physicochemical properties of synthesized conjugates were investigated in terms of drug to antibody ratio, size variants and free drug quantities. In vitro biological activity of trastuzumab-DM1 conjugates was assessed on breast cancer cell lines expressing different levels of HER2 using binding affinity, antiproliferative, apoptosis, and antibody-dependent cell-mediated cytotoxicity (ADCC) assays. Synthesized conjugate containing hydrophilic linker, showed higher drug to antibody ratio, no aggregated form and higher cellular toxicity in comparison to SMCC bearing conjugate. Binding affinity and ADCC potential of conjugates was not affected upon the usage of hydrophilic linker. In conclusion, application of SM(PEG)2 for coupling of DM1 to trastuzumab enhance desirable characteristics of the resulting conjugate.

Application of triple quadrupole mass spectrometry for the characterization of antibody-drug conjugates

Antibody–drug conjugates (ADCs) are an inherently heterogeneous class of biotherapeutics, the development of which requires extensive characterization throughout. During the earliest phases of preclinical development, when synthetic routes towards the desired conjugate are being assessed, the main interest lies in the determination of the average drug-to-antibody ratio (DAR) of a given batch as well as information about different conjugation species. There has been a trend in mass spectrometry (MS)–based characterization of ADCs towards the use of high-resolving mass spectrometry for many of these analyses. Considering the high cost for such an instrument, the evaluation of cheaper and more accessible alternatives is highly motivated. We have therefore tested the applicability of a quadrupole mass analyzer for the aforementioned characterizations. Eight ADCs consisting of trastuzumab and varying stoichiometries of Mc-Val-Cit-PABC-monomethyl auristatin E conjugated to native cysteines were synthesized and served as test analytes. The average DAR value and molecular weights (Mw) of all detected chains from the quadrupole mass analyzer showed surprisingly high agreement with results obtained from a time-of-flight (TOF) mass analyzer and hydrophobic interaction chromatography (HIC)–derived values for all investigated ADC batches. Acquired Mw were within 80 ppm of TOF-derived values, and DAR was on average within 0.32 DAR units of HIC-derived values. Quadrupole mass spectrometers therefore represent a viable alternative for the characterization of ADC in early-stage development.

Chemical Structure and Concentration of Intratumor Catabolites Determine Efficacy of Antibody Drug Conjugates

Despite recent technological advances in quantifying antibody drug conjugate (ADC) species, such as total antibody, conjugated antibody, conjugated drug, and payload drug in circulation, the correlation of their exposures with the efficacy of ADC outcomes in vivo remains challenging. Here, the chemical structures and concentrations of intratumor catabolites were investigated to better understand the drivers of ADC in vivo efficacy. Anti-CD22 disulfide-linked pyrrolobenzodiazepine (PBD-dimer) conjugates containing methyl- and cyclobutyl-substituted disulfide linkers exhibited strong efficacy in a WSU-DLCL2 xenograft mouse model, whereas an ADC derived from a cyclopropyl linker was inactive. Total ADC antibody concentrations and drug-to-antibody ratios (DAR) in circulation were similar between the cyclobutyl-containing ADC and the cyclopropyl-containing ADC; however, the former afforded the release of the PBD-dimer payload in the tumor, but the latter only generated a nonimmolating thiol-containing catabolite that did not bind to DNA. These results suggest that intratumor catabolite analysis rather than systemic pharmacokinetic analysis may be used to better explain and predict ADC in vivo efficacy. These are good examples to demonstrate that the chemical nature and concentration of intratumor catabolites depend on the linker type used for drug conjugation, and the potency of the released drug moiety ultimately determines the ADC in vivo efficacy.

Antibody-drug conjugate characterization by chromatographic and electrophoretic techniques

Due to the inherent structure complexity and component heterogeneity of antibody drug conjugates (ADCs), separation technologies play a critical role in their characterization. In this review, we focus on chromatographic and electrophoretic approaches used to characterize ADCs with respect to drug-to-antibody ratio, drug distribution and conjugation sites, free small molecule drugs, charge variants, aggregates and fragments, etc. Chromatographic techniques including reversed-phase, ion exchange, size exclusion, hydrophobic interaction, two-dimensional liquid chromatography, and gas chromatography as well as capillary electrophoretic techniques including capillary electrophoresis sodium dodecyl sulfate, capillary zone electrophoresis and capillary isoelectric focusing are reviewed for their applications in the characterization of ADCs.

An enzymatic deconjugation method for the analysis of small molecule active drugs on antibody-drug conjugates

Antibody-drug conjugates (ADCs) are complex therapeutic agents that use the specific targeting properties of antibodies and the highly potent cytotoxicity of small molecule drugs to selectively eliminate tumor cells while limiting the toxicity to normal healthy tissues. Two critical quality attributes of ADCs are the purity and stability of the active small molecule drug linked to the ADC, but these are difficult to assess once the drug is conjugated to the antibody. In this study, we report a enzyme deconjugation approach to cleave small molecule drugs from ADCs, which allows the drugs to be subsequently characterized by reversed-phase high performance liquid chromatography. The model ADC we used in this study utilizes a valine-citrulline linker that is designed to be sensitive to endoproteases after internalization by tumor cells. We screened several proteases to determine the most effective enzyme. Among the 3 cysteine proteases evaluated, papain had the best efficiency in cleaving the small molecule drug from the model ADC. The deconjugation conditions were further optimized to achieve complete cleavage of the small molecule drug. This papain deconjugation approach demonstrated excellent specificity and precision. The purity and stability of the active drug on an ADC drug product was evaluated and the major degradation products of the active drug were identified. The papain deconjugation method was also applied to several other ADCs, with the results suggesting it could be applied generally to ADCs containing a valine-citrulline linker. Our results indicate that the papain deconjugation method is a powerful tool for characterizing the active small molecule drug conjugated to an ADC, and may be useful in ensuring the product quality, efficacy and the safety of ADCs.

Cutting-edge mass spectrometry methods for the multi-level structural characterization of antibody-drug conjugates

Antibody drug conjugates (ADCs) are highly cytotoxic drugs covalently attached via conditionally stable linkers to monoclonal antibodies (mAbs) and are among the most promising next-generation empowered biologics for cancer treatment. ADCs are more complex than naked mAbs, as the heterogeneity of the conjugates adds to the inherent microvariability of the biomolecules. The development and optimization of ADCs rely on improving their analytical and bioanalytical characterization by assessing several critical quality attributes, namely the distribution and position of the drug, the amount of naked antibody, the average drug to antibody ratio, and the residual drug-linker and related product proportions. Here brentuximab vedotin (Adcetris) and trastuzumab emtansine (Kadcyla), the first and gold-standard hinge-cysteine and lysine drug conjugates, respectively, were chosen to develop new mass spectrometry (MS) methods and to improve multiple-level structural assessment protocols.

Antibody-conjugated drug assay for protease-cleavable antibody-drug conjugates

BACKGROUND

Antibody-drug conjugates (ADCs) require multiple assays to characterize their PK. These assays can separately evaluate the ADC by quantifying the antibody or the conjugated drug and may give different answers due to assay measurement differences, heterogeneous nature of ADCs and potential biotransformations that occur in vivo.

RESULTS

We present a new version of the antibody-conjugated drug assay for valine-citrulline-linked monomethylauristatin E (vcMMAE) ADCs. A stable isotope-labeled internal standard, protein A affinity capture and solid-phase cleavage of MMAE using papain was used prior to LC-MS/MS analysis.

CONCLUSION

The assay was used to assess the difference in ex vivo drug-linker stability of native-cysteine versus engineered cysteine ADCs and to determine the number of drugs per antibody of a native-cysteine ADC in vivo.

Approaches to Interchain Cysteine-Linked ADC Characterization by Mass Spectrometry

Therapeutic antibody-drug conjugates (ADCs) harness the cell-killing potential of cytotoxic agents and the tumor targeting specificity of monoclonal antibodies to selectively kill tumor cells. Recent years have witnessed the development of several promising modalities that follow the same basic principles of ADC based therapies but which employ unique cytotoxic agents and conjugation strategies in order to realize therapeutic benefit. The complexity and heterogeneity of ADCs present a challenge to some of the conventional analytical methods that industry has relied upon for biologics characterization. This current review will highlight some of the more recent methodological approaches in mass spectrometry that have bridged the gap that is created when conventional analytical techniques provide an incomplete picture of ADC product quality. Specifically, we will discuss mass spectrometric approaches that preserve and/or capture information about the native structure of ADCs and provide unique insights into the higher order structure (HOS) of these therapeutic molecules.

Computational Construction of Antibody-Drug Conjugates Using Surface Lysines as the Antibody Conjugation Site and a Non-cleavable Linker

Antibody–drug conjugates (ADCs) constitute a category of anticancer targeted therapy that has gathered great interest during the last few years because of their potential to kill cancer cells while causing significantly fewer side effects than traditional chemotherapy. In this paper, a process of computational construction of ADCs is described, using the surface lysines of an antibody and a non-covalent linker molecule, as well as a cytotoxic substance, as files in Protein Data Bank format. Also, aspects related to the function, properties, and development of ADCs are discussed.

Preclinical Pharmacokinetic Considerations for the Development of Antibody Drug Conjugates

Antibody drug conjugates (ADCs) are an emerging new class of targeted therapeutics for cancer that use antibodies to deliver cytotoxic drugs to cancer cells. There are two FDA approved ADCs on the market and over 30 ADCs in the clinical pipeline against a number of different cancer types. The structure of an ADC is very complex with multiple components and considerable efforts are ongoing to determine the attributes necessary for clinical success. Understanding the pharmacokinetics of an ADC and how it impacts efficacy and toxicity is a critical part of optimizing ADC design and delivery i.e., dose and schedule. This review discusses the pharmacokinetic considerations for an ADC and tools and strategies that can be used to evaluate molecules at the preclinical stage.

Antibody-drug conjugate model fast characterization by LC-MS following IdeS proteolytic digestion

Here we report the design and production of an antibody-fluorophore conjugate (AFC) as a non-toxic model of an antibody-drug conjugate (ADC). This AFC is based on the conjugation of dansyl sulfonamide ethyl amine (DSEA )-linker maleimide on interchain cysteines of trastuzumab used as a reference antibody. The resulting AFC was first characterized by routine analytical methods (SEC, SDS-PAGE, CE-SDS, HIC and native MS), resulting in similar chromatograms,electropherograms and mass spectra to those reported for hinge Cys-linked ADCs. IdeS digestion of the AFC was then performed, followed by reduction and analysis by liquid chromatography coupled to mass spectrometry analysis. Dye loading and distribution on light chain and Fd fragments were calculated, as well as the average dye to antibody ratio (DAR) for both monomeric and multimeric species. In addition, by analyzing the Fc fragment in the same run, full glycoprofiling and demonstration of the absence of additional conjugation was easily achieved. As for naked antibodies and Fc-fusion proteins, IdeS proteolytic digestion may rapidly become a reference analytical method at all stages of ADC discovery, preclinical and clinical development. The method can be routinely used for comparability assays, formulation, process scale-up and transfer, and to define critical quality attributes in a quality-by-design approach.

Antibody therapy for pediatric leukemia

Despite increasing cure rates for pediatric leukemia, relapsed disease still carries a poor prognosis with significant morbidity and mortality. Novel targeted therapies are currently being investigated in an attempt to reduce adverse events and improve survival outcomes. Antibody therapies represent a form of targeted therapy that offers a new treatment paradigm. Monoclonal antibodies are active in pediatric acute lymphoblastic leukemia (ALL) and are currently in Phase III trials. Antibody-drug conjugates (ADCs) are the next generation of antibodies where a highly potent cytotoxic agent is bound to an antibody by a linker, resulting in selective targeting of leukemia cells. ADCs are currently being tested in clinical trials for pediatric acute myeloid leukemia and ALL. Bispecific T cell engager (BiTE) antibodies are a construct whereby each antibody contains two binding sites, with one designed to engage the patient's own immune system and the other to target malignant cells. BiTE antibodies show great promise as a novel and effective therapy for childhood leukemia. This review will outline recent developments in targeted agents for pediatric leukemia including monoclonal antibodies, ADCs, and BiTE antibodies.

Conformation and dynamics of interchain cysteine-linked antibody-drug conjugates as revealed by hydrogen/deuterium exchange mass spectrometry

Antibody-drug conjugates (ADCs) are protein therapeutics in which a target specific monoclonal antibody (mAb) is conjugated with drug molecules. The manufacturing of ADCs involves additional conjugation steps, which are carried out on the parent mAbs, and it is important to evaluate how the drug conjugation process impacts the conformation and dynamics of the mAb. Here, we present a comparative study of interchain cysteine linked IgG1 ADCs and the corresponding mAb by hydrogen/deuterium exchange mass spectrometry (HDX-MS). We found that ∼90% of the primary sequence of the ADC conjugated with either monomethyl auristatin E or F (vcMMAE/mcMMAF) displayed the same HDX kinetics as the mAb, indicating the ADCs and mAbs share very similar conformation and dynamics in solution. Minor increases in HDX kinetic rates were observed in two Fc regions in the ADCs relative to the mAb which indicated that both regions become more structurally dynamic and/or more solvent-accessible in the ADCs. The findings led to a subsequent inquiry into whether the local conformational changes were due to the presence of drugs on the interchain cysteine residues or the absence of intact interchain disulfides or both. To address this question, a side-by-side HDX comparison of ADCs, mAbs, reduced mAbs (containing 8 reduced interchain cysteine thiols), and partially reduced mAbs (conjugation process intermediate) was performed. Our results indicated that the slight increase in conformational dynamics detected at the two regions in the ADCs was due to the absence of intact interchain disulfide bonds and not the presence of vcMMAE or mcMMAF on the alkylated interchain cysteine residues. These results highlight the utility of HDX-MS for interrogating the higher-order structure of ADCs and other protein therapeutics.

Mass spectrometric characterization of transglutaminase based site-specific antibody-drug conjugates

Antibody drug conjugates (ADCs) are becoming an important new class of therapeutic agents for the treatment of cancer. ADCs are produced through the linkage of a cytotoxic small molecule (drug) to monoclonal antibodies that target tumor cells. Traditionally, most ADCs rely on chemical conjugation methods that yield heterogeneous mixtures of varying number of drugs attached at different positions. The potential benefits of site-specific drug conjugation in terms of stability, manufacturing, and improved therapeutic index has recently led to the development of several new site-specific conjugation technologies. However, detailed characterization of the degree of site specificity is currently lacking. In this study we utilize mass spectrometry to characterize the extent of site-specificity of an enzyme-based site-specific antibody-drug conjugation technology that we recently developed. We found that, in addition to conjugation of the engineered site, a small amount of aglycosylated antibody present in starting material led to conjugation at position Q295, resulting in approximately 1.3% of off-target conjugation. Based on our detection limits, we show that Q295N mutant eliminates the off-target conjugation yielding highly homogeneous conjugates that are better than 99.8% site-specific. Our study demonstrates the importance of detailed characterization of ADCs and describes methods that can be utilized to characterize not only our enzyme based conjugates, but also ADCs generated by other conjugation technologies.