Biophysical Properties and Heating-Induced Aggregation of Lysine-Conjugated Antibody-Drug Conjugates

Formulation Development of a COVID-19 Recombinant Spike Protein-Based Vaccine

The purpose of this study was to develop a formulation for a recombinant prefusion spike protein vaccine against SARS-CoV-2. It was found that the spike protein was susceptible to aggregation due to mechanical stress. Therefore, formulation studies were initiated focused on screening pharmaceutical excipients capable of preventing this. The screening of a panel of potential stabilizing conditions found that Tween 20 could inhibit mechanically induced aggregation. A concentration-dependent study indicated that a higher concentration of Tween 20 (0.2% v/v) was required to prevent conformational changes in the trimer. The conformational changes induced by mechanical stress were characterized by size exclusion chromatography (SEC) and hydrogen-deuterium exchange mass spectrometry (HDX-MS), indicating the formation of an extended trimeric conformation that was also unable to bind to antibodies directed to the S2 domain. Long-term stability modeling, using advanced kinetic analysis, indicated that the formulation containing 0.2% (v/v) Tween 20 at a neutral pH was predicted to be stable for at least two years at 2 °C to 8 °C. Additional stabilizer screening conducted by thermal shift assay indicated that sucrose and glycerol were able to significantly increase the spike protein melting temperature (Tm) and improve the overall thermostability of the spike protein in a short-term stability study. Thus, while 0.2% (v/v) Tween 20 was sufficient to prevent aggregation and to maintain spike protein stability under refrigeration, the addition of sucrose further improved vaccine thermostability. Altogether, our study provides a systematic approach to the formulation of protein-based COVID-19 vaccine and highlights the impact of mechanical stress on the conformation of the spike protein and the significance of surfactants and stabilizers in maintaining the structural and functional integrity of the spike protein.

Agitation-Induced Aggregation of Lysine- And Interchain Cysteine-Linked Antibody-Drug Conjugates

Drug conjugation to an antibody can affect its stability, which depends on factors such as the conjugation technique used, drug-linker properties, and stress encountered. This study focused on the effects of agitation stress on the physical stability of two lysine (ADC-K) and two interchain cysteine (ADC-C) conjugates of an IgG1 monoclonal antibody (mAb) linked to either ∼4 MMAE or DM1 payloads. During agitation, all antibody-drug conjugates (ADCs) exhibited higher aggregation than the mAb, which was dependent on the conjugation technique (aggregation of ADC-Ks > ADC-Cs) and drug-linker (aggregation of ADCs with MMAE > ADCs with DM1). The aggregation propensities correlated well with higher self-interaction, hydrophobicity, and surface activity of ADCs relative to the mAb. The intermediate reduced mAb (mAb-SH) showed even higher aggregation than the final product ADC-Cs. However, blocking mAb-SH's free thiols with N-ethylmaleimide (NEM) strongly reduced its aggregation, suggesting that free thiols should be minimized in cysteine ADCs. Further, this study demonstrates that a low-volume surface tension method can be used for estimating agitation-induced aggregation of ADCs in early development phases. Identifying liabilities to agitation stress and their relationship to biophysical properties may help optimize ADC stability.

Enhancing thermal stability in the CH2 domain to suppress aggregation through the introduction of simultaneous disulfide bonds in Pichia pastoris

Protein aggregations decrease production yields and impair the efficacy of therapeutics. The CH2 domain is a crucial part of the constant region of human IgG. But, it is also the least stable domain in IgG, which can result in antibody instability and aggregation problems. We created a novel mutant of the CH2 domain (T250C/L314C, mut10) by introducing a disulfide bond and expressed it using Pichia pastoris. The mut10 variant exhibited enhanced thermal stability, resistance to enzymatic degradation, and reduced aggregation in comparison to the original CH2 domain. However, it was less stable than mut20 (L242C/K334C), which is the variant prepared in a previous study (Gong et al., J. Biol. Chem., 2009). A more advanced mutant, mut25, was created by combining mut10 and mut20. Mut25 artificially contains two disulfide bonds. The new mutant, mut25, showed enhanced thermal stability, increased resistance to enzymatic digestion, and reduced aggregation in comparison to mut20. According to our knowledge, mut25 achieves an unprecedented level of stability among the humanized whole CH2 domains that have been reported so far. Mut25 has the potential to serve as a new platform for antibody therapeutics due to its ability to reduce immunogenicity by decreasing aggregation.

Cetuximab-based PROteolysis targeting chimera for effectual downregulation of NSCLC with varied EGFR mutations

PROteolysis Targeting Chimeras (PROTACs) showed tremendous therapeutic potential in degrading several oncoproteins including undruggable proteins. PROTACs are bifunctional molecules where one-part binds to target protein while the other end recruits protein degradation machinery. With the unveiling advancements in the field of PROTACs, we explored a combinatorial approach by developing antibody-based PROTAC (ABTAC) which may effectively degrade one of the key oncoprotein driving proliferation and progression of cancer - Epidermal growth factor receptor (EGFR). The objective of current research was to synthesize and characterize an EGFR degrading ABTAC for the treatment of non-small cell lung cancer (NSCLC). Cetuximab and pomalidomide (E3 ligase recruiting ligand) were conjugated using lysine conjugation and copper free azide-alkyne cycloaddition (CuAAC) click chemistry. Analytical characterization using reverse-phase liquid chromatography and mass spectrometry suggested conjugation of five E3-ligase inhibitor molecules/antibody. Nearly 10-30 folds reduction in IC50 was observed with ABTAC in HCC827 (EGFR sensitive) and H1650 (EGFR resistant) cells compared to cetuximab. Multicellular 3D spheroid assay strongly suggested that ABTAC induced significant apoptosis and also inhibited cell proliferation compared to control and antibody alone. Circular dichroism and surface plasmon resonance (SPR) confirmed minor alterations in the structure and receptor binding efficacy of the antibody post-conjugation.

Studying Intermolecular Interactions in an Antibody-Drug Conjugate Through Chemical Screening and Computational Modeling

Antibody-drug conjugates (ADCs) combine the selectivity of antibodies with the cytotoxicity of drug payloads to yield highly targeted and potent therapeutics. Owing to the need to chemically modify residues for attachment of the payload and their more complex structure compared to either component alone, ADCs can present additional challenges related to stability of the final drug product. Here, we report for the first time the use of high-throughput experimental screens and computational techniques to tune the conformational and colloidal behavior of a monomethyl auristatin F-based ADC. The ADC, which exhibits high opalescence with strongly attractive protein-protein interactions, is transformed into a more stable structure by experimentally traversing a library of more than ∼100 formulations. A significant reduction in turbidity and increase in diffusion interaction parameter is observed by varying properties such as pH and ionic strength. Computational modeling rationalized these changes and pointed to the presence of attractive electrostatic interactions between ADC molecules facilitated by the drug payload and histidine residues. Taken together, the experimental and computational work presented provides a general roadmap of studies to perform during ADC development to find stable formulations, while the mechanistic learnings can be applied towards the design and stabilization of other IgG1-based ADCs.

Stepping forward in antibody-drug conjugate development

Antibody-drug conjugates (ADCs) are cancer therapeutic agents comprised of an antibody, a linker and a small-molecule payload. ADCs use the specificity of the antibody to target the toxic payload to tumor cells. After intravenous administration, ADCs enter circulation, distribute to tumor tissues and bind to the tumor surface antigen. The antigen then undergoes endocytosis to internalize the ADC into tumor cells, where it is transported to lysosomes to release the payload. The released toxic payloads can induce apoptosis through DNA damage or microtubule inhibition and can kill surrounding cancer cells through the bystander effect. The first ADC drug was approved by the United States Food and Drug Administration (FDA) in 2000, but the following decade saw no new approved ADC drugs. From 2011 to 2018, four ADC drugs were approved, while in 2019 and 2020 five more ADCs entered the market. This demonstrates an increasing trend for the clinical development of ADCs. This review summarizes the recent clinical research, with a specific focus on how the in vivo processing of ADCs influences their design. We aim to provide comprehensive information about current ADCs to facilitate future development.

Fcγ Receptor-Dependent Internalization and Off-Target Cytotoxicity of Antibody-Drug Conjugate Aggregates

Purpose

Antibody-drug conjugates (ADCs), which are monoclonal antibodies (mAbs) conjugated with highly toxic payloads, achieve high tumor killing efficacy due to the specific delivery of payloads in accordance with mAbs’ function. On the other hand, the conjugation of payloads often increases the hydrophobicity of mAbs, resulting in reduced stability and increased aggregation. It is considered that mAb aggregates have potential risk for activating Fcγ receptors (FcγRs) on immune cells, and are internalized into cells via FcγRs. Based on the mechanism of action of ADCs, the internalization of ADCs into target-negative cells may cause the off-target toxicity. However, the impacts of aggregation on the safety of ADCs including off-target cytotoxicity have been unclear. In this study, we investigated the cytotoxicity of ADC aggregates in target-negative cells.

Methods

The ADC aggregates were generated by stirring stress or thermal stress. The off-target cytotoxicity of ADC aggregates was evaluated in several target-negative cell lines, and FcγR-activation properties of ADC aggregates were characterized using a reporter cell assay.

Results

Aggregation of ADCs enhanced the off-target cytotoxicity in several target-negative cell lines compared with non-stressed ADCs. Notably, ADC aggregates with FcγR-activation properties showed dramatically enhanced cytotoxicity in FcγR-expressing cells. The FcγR-mediated off-target cytotoxicity of ADC aggregates was reduced by using a FcγR-blocking antibody or Fc-engineering for silencing Fc-mediated effector functions.

Conclusions

These results indicated that FcγRs play an important role for internalization of ADC aggregates into non-target cells, and the aggregation of ADCs increases the potential risk for off-target toxicity.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11095-021-03158-x.

Structural characterization of a monomethylauristatin-E based ADC that contains 8 drugs conjugated at interchain cysteine residues

Antibody-drug conjugates (ADCs) with a drug-to-antibody ratio (DAR) of 8 are attractive as therapeutic anti-cancer agents due to the higher levels of cytotoxic payload delivered to tumors. Biophysical characterization of a DAR 8 ADC fully conjugated at all interchain cysteine residues was carried out to determine if IgG1 interchain disulfide reduction and conjugation led to structural perturbations that impacted product stability. Comparisons between the DAR 8 ADC and the unconjugated parent antibody identified minor tertiary and quaternary structural changes localized to the C_L, C_H1, and C_H2 domains and C_H2-C_H3 domain interface. Stability studies of the DAR 8 ADC indicated that the structural changes had minimal impacts to product stability as demonstrated by low levels of fragmentation and aggregation under nominal storage and temperature stress stability conditions. Additionally, no detectable higher order structural changes were observed by CD or DSC in the DAR 8 ADC after 3 months at (25 °C) stability conditions. The structural and stability results support the developability of DAR 8 ADCs fully conjugated to interchain cysteines residues with an optimized and clinically relevant second generation monomethylauristatin-E (MMAE) drug-linker.

Polyethylene glycol-based linkers as hydrophilicity reservoir for antibody-drug conjugates

Antibody-drug conjugates (ADCs) are an established therapeutic entity in which potent cytotoxic drugs are conjugated to a monoclonal antibody. In parallel with the great emphasis put on novel site-specific bioconjugation technologies, future advancements in this field also rely on exploring novel linker-drug architectures that improve the efficacy and stability of ADCs. In this context, the use of hydrophilic linkers represents a valid strategy to mask or reduce the inherent hydrophobicity of the most used cytotoxic drugs and positively impact the physical stability and in vivo performance of ADCs. Here, we describe the use of linkers containing monodisperse poly(ethylene glycol) (PEG) moieties for the construction of highly-loaded lysine-conjugated ADCs. The studied ADCs differ in the positioning of PEG (linear or pendant), the bonding type with the antibody (amide or carbamate), and the drug-to-antibody ratio (DAR). These ADCs were first evaluated for their stability in solution under thermal stress, showing that both the drug-linker-polymer design and the nature of the antibody-linker bonding are of great importance for their physical and chemical stability. Amide-coupled ADCs bearing two pendant 12-unit poly(ethylene glycol) chains within the drug-linker structure were the best performing conjugates, distancing themselves from the ADCs obtained with a conventional linear 24-unit PEG oligomer or the linker of Kadcyla®. The pharmacokinetic profiles of amide-linked ADCs, with a linear or pendant configuration of the PEG, were tested in mice in comparison to Kadcyla®. Total antibody pharmacokinetics paralleled the trends in aggregation tendency, with slower clearance rates for the ADCs based on the pendant drug-linker format. The above-mentioned findings have provided important clues on the drug-linker design and revealed that the positioning and configuration of a PEG unit have to be carefully tuned to achieve ADCs with improved stability and pharmacokinetics.

Localization of Therapeutic Fab-CHP Conjugates to Sites of Denatured Collagen for the Treatment of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation in synovial joints and protease-induced cartilage degradation. Current biologic treatments for RA can effectively reduce symptoms, primarily by neutralizing the proinflammatory cytokine TNFα; however, continued, indiscriminate overinhibition of inflammatory factors can significantly weaken the host immune system, leading to opportunistic infections and interrupting treatment. We hypothesize that localizing anti-TNFα therapeutics to denatured collagen (dCol) present at arthritic joints, via conjugation with collagen-hybridizing peptides (CHPs), will reduce off-site antigen binding and maintain local immunosuppression. We isolated the antigen-binding fragment of the clinically approved anti-TNFα therapeutic infliximab (iFab) and prepared iFab-CHP conjugates via lysine-based conjugation with an SMCC linker. After successful conjugation, confirmed by LC-MS, the binding affinity of iFab-CHP was characterized by ELISA-like assays, which showed comparable antigen binding relative to infliximab, comparable dCol binding relative to CHP, and the hybrid ability to bind both dCol and TNFα simultaneously. We further demonstrated localization of Fab-CHP to areas of high dCol in vivo and promising therapeutic efficacy, assessed by histological staining (Safranin-O and H&E), in a pilot mouse study.

Conjugation Ratio, Light Dose, and pH Affect the Stability of Panitumumab-IR700 for Near-Infrared Photoimmunotherapy

Near-infrared photoimmunotherapy (NIR-PIT), a newly developed cancer-cell-specific therapy, relies on a monoclonal antibody-photoabsorber conjugate (APC) and is based on a photoinduced ligand release reaction. Local exposure of the tumor to NIR light induces rapid immunogenic necrotic cell death. The molecular properties of APCs, including their stability and aggregation properties, have important implications for the long-term stability and shelf life. In this study, panitumumab was conjugated with IRDye700DX (IR700) as a model for other NIR-PIT agents. Higher IR700-to-mAb conjugation ratios correlated with increased in vitro cell death up to a ratio of 2.5 dye molecules per antibody. Conjugation ratios higher than 2.5 did not improve cell killing activity. APC aggregation was induced in a light-dose-dependent manner. A near-room-level light dose was sufficient to induce aggregation of APCs. Solvent pH lower than 4 induced aggregation, but higher pH did not induce aggregation. The IR700-to-mAb conjugation ratio, light irradiation dose, and solvent pH affect the APC stability and efficacy.

Robust Strategy for Antibody-Polymer-Drug Conjugation: Significance of Conjugating Orientation and Linker Charge on Targeting Ability

Antibody-drug conjugates have shown great promise in active targeting for cancer therapy. The existing chemical techniques for antibody conjugation generally lack efficiency or universality. In this article, a site-specific antibody conjugation was developed by using a mild reaction between a benzoboroxole (BB) functionality and cis-diol moiety of sugar units in the antibody fragment crystallizable region under neutral pH conditions. A BB/PEG/ICG-grafted poly(aspartic acid) comb-like functional polymer was first synthesized and conjugated with transferrin (Tf) to form a transferrin-polymer-drug conjugate [Tf-P(BB)], which showed 120% increase in HepG2 hepatoma (Tf receptor overexpression) cell uptake compared to a nontargeting protein-polymer-drug conjugate [HRP-P(BB)]. The universality of this method was further demonstrated by the enhanced uptake of trastuzumab (anti-Her2 antibody)-polymer-drug conjugates in MCF-7 (295%) and MDA-MB-435S (66.4%) (Her2 positive) cells. The positive charge of the linker had great influence on the targeting ability of the antibody-polymer-drug conjugates. The in vivo studies demonstrated the distinct targeting ability of Tf-P(BB) in the HepG2 xenograft tumor, and the tumor accumulation of the Tf-P(BB) testing group increased by 92% with respect to the control group [HRP-P(BB)]. More significantly, the HepG2 cell uptake amount of the antibody-oriented conjugate [Tf-P'(BB)] was 2.4-fold higher than that of the controlled group [Tf-P'(Hex)]. On the basis of this facile site-specific conjugation method, the conjugates are able to change the antibody species easily against various cancers, while maintaining the antibody integrity and targeting ability.

The uniqueness of flow in probing the aggregation behavior of clinically relevant antibodies

The development of therapeutic monoclonal antibodies (mAbs) can be hindered by their tendency to aggregate throughout their lifetime, which can illicit immunogenic responses and render mAb manufacturing unfeasible. Consequently, there is a need to identify mAbs with desirable thermodynamic stability, solubility, and lack of self-association. These behaviors are assessed using an array of in silico and in vitro assays, as no single assay can predict aggregation and developability. We have developed an extensional and shear flow device (EFD), which subjects proteins to defined hydrodynamic forces which mimic those experienced in bioprocessing. Here, we utilize the EFD to explore the aggregation propensity of 33 IgG1 mAbs, whose variable domains are derived from clinical antibodies. Using submilligram quantities of material per replicate, wide-ranging EFD-induced aggregation (9-81% protein in pellet) was observed for these mAbs, highlighting the EFD as a sensitive method to assess aggregation propensity. By comparing the EFD-induced aggregation data to those obtained previously from 12 other biophysical assays, we show that the EFD provides distinct information compared with current measures of adverse biophysical behavior. Assessing a candidate's liability to hydrodynamic force thus adds novel insight into the rational selection of developable mAbs that complements other assays.

Antibody Conjugates-Recent Advances and Future Innovations

Monoclonal antibodies have evolved from research tools to powerful therapeutics in the past 30 years. Clinical success rates of antibodies have exceeded expectations, resulting in heavy investment in biologics discovery and development in addition to traditional small molecules across the industry. However, protein therapeutics cannot drug targets intracellularly and are limited to soluble and cell-surface antigens. Tremendous strides have been made in antibody discovery, protein engineering, formulation, and delivery devices. These advances continue to push the boundaries of biologics to enable antibody conjugates to take advantage of the target specificity and long half-life from an antibody, while delivering highly potent small molecule drugs. While the "magic bullet" concept produced the first wave of antibody conjugates, these entities were met with limited clinical success. This review summarizes the advances and challenges in the field to date with emphasis on antibody conjugation, linker-payload chemistry, novel payload classes, absorption, distribution, metabolism, and excretion (ADME), and product developability. We discuss lessons learned in the development of oncology antibody conjugates and look towards future innovations enabling other therapeutic indications.

Conjugation of Emtansine Onto Trastuzumab Promotes Aggregation of the Antibody-Drug Conjugate by Reducing Repulsive Electrostatic Interactions and Increasing Hydrophobic Interactions

The impact of drug conjugation on intra- and intermolecular interactions of trastuzumab (TmAb) was determined by comparing the conformational and colloidal stabilities of TmAb and trastuzumab emtansine (T-DM1). In low ionic strength formulations, drug conjugation to native lysine residues of TmAb significantly reduced the repulsive electrostatic interactions between T-DM1 molecules. When these electrostatic interactions were screened in solutions with high ionic strength, intermolecular interactions between T-DM1 molecules were found to be more attractive than those between TmAb molecules. Drug conjugation lowered the colloidal stability of T-DM1 compared to TmAb, making T-DM1 more susceptible to agitation-induced aggregation. The presence of polysorbate-20 in the formulations inhibited aggregation of TmAb and T-DM1 induced by the hydrophobic air-water interface. Furthermore, the effect of increased hydrophobic interactions between T-DM1 molecules was studied by monitoring aggregation in TmAb and T-DM1 solutions that were incubated at 4°C, 25°C, and 50°C. Conjugating DM1 to TmAb increased the hydrophobicity of the molecule, and faster aggregation of T-DM1 at 50°C could be attributed to a temperature-dependent increase in hydrophobic interactions between T-DM1 molecules.

In-Depth Comparison of Lysine-Based Antibody-Drug Conjugates Prepared on Solid Support Versus in Solution

Antibody drug conjugates are a rapidly growing form of targeted chemotherapeutics. As companies and researchers move to develop new antibody–drug conjugate (ADC) candidates, high-throughput methods will become increasingly common. Here we use advanced characterization techniques to assess two trastuzumab-DM1 (T-DM1) ADCs; one produced using Protein A immobilization and the other produced in solution. Following determination of payload site and distribution with liquid chromatography-mass spectrometry (LC/MS), thermal stability, heat-induced aggregation, tertiary structure, and binding affinity were characterized using differential scanning calorimetry (DSC), dynamic light scattering (DLS), Raman spectroscopy, and isothermal titration calorimetry (ITC), respectively. Small differences in the thermal stability of the C_H2 domain of the antibody as well as aggregation onset temperatures were observed from DSC and DLS, respectively. However, no significant differences in secondary and tertiary structure were observed with Raman spectroscopy, or binding affinity as measured by ITC. Lysine-based ADC conjugation produces an innately heterogeneous population that can generate significant variability in the results of sensitive characterization techniques. Characterization of these ADCs indicated nominal differences in thermal stability but not in tertiary structure or binding affinity. Our results lead us to conclude that lysine-based ADCs synthesized following Protein A immobilization, common in small-scale conjugations, are highly similar to equivalent ADCs produced in larger scale, solution-based methods.