An emerging playbook for antibody-drug conjugates: lessons from the laboratory and clinic suggest a strategy for improving efficacy and safety

Predictive and Prognostic Biomarkers and Tumor Antigens for Targeted Therapy in Urothelial Carcinoma

Urothelial carcinoma (UC) is the fourth most prevalent cancer amongst males worldwide. While patients with non-muscle-invasive disease have a favorable prognosis, 25% of UC patients present with locally advanced disease which is associated with a 10-15% 5-year survival rate and poor overall prognosis. Muscle-invasive bladder cancer (MIBC) is associated with about 50% 5 year survival when treated by radical cystectomy or trimodality therapy; stage IV disease is associated with 10-15% 5 year survival. Current therapeutic modalities for MIBC include neoadjuvant chemotherapy, surgery and/or chemoradiation, although patients with relapsed or refractory disease have a poor prognosis. However, the rapid success of immuno-oncology in various hematologic and solid malignancies offers new targets with tremendous therapeutic potential in UC. Historically, there were no predictive biomarkers to guide the clinical management and treatment of UC, and biomarker development was an unmet need. However, recent and ongoing clinical trials have identified several promising tumor biomarkers that have the potential to serve as predictive or prognostic tools in UC. This review provides a comprehensive summary of emerging biomarkers and molecular tumor targets including programmed death ligand 1 (PD-L1), epidermal growth factor receptor (EGFR), human epidermal growth factor receptor 2 (HER2), fibroblast growth factor receptor (FGFR), DNA damage response and repair (DDR) mutations, poly (ADP-ribose) polymerase (PARP) expression and circulating tumor DNA (ctDNA), as well as their clinical utility in UC. We also evaluate recent advancements in precision oncology in UC, while illustrating limiting factors and challenges related to the clinical application of these biomarkers in clinical practice.

Incidence of antibody-drug conjugate-related fatigue in patients with breast cancer: A systematic review and meta-analysis

BACKGROUND

Numerous studies have reported the efficacy of antibody-drug conjugates (ADCs) for treating breast cancer. However, during cytotoxic drug treatment, long-term disabling fatigue is common. Moreover, studies in the relevant literature have indicated that fatigue can significantly increase the incidence of depression and sleep disorders. Therefore, this meta-analysis aims to evaluate the incidence of fatigue in breast cancer survivors treated with ADCs.

METHODS

PubMed, EMBASE, Web of Science, and Cochrane Library databases were systematically searched for articles and conference abstracts published before March 16, 2023. Further, two authors independently extracted data from the included studies. The primary outcome of this study was the incidence of all-grade fatigue caused by the use of ADCs in patients with breast cancer. Finally, a random-effects model was used to calculate the incidence and 95% confidence intervals (CIs) of the outcome.

RESULTS

Overall, 7963 patients from 31 studies were included in this meta-analysis to assess the incidence of fatigue caused by the use of approved and marketed ADCs in patients with breast cancer. Notably, the incidence of all-grade fatigue during ADC monotherapy was 39.84% (95% CI, 35.09%-44.69%). In subgroup analyses, among ADCs, the incidence of trastuzumab deruxtecan-induced fatigue was the highest, with an all-grade fatigue incidence of 47.05% (95% CI, 42.38%-51.75%). Meanwhile, the incidence of trastuzumab emtansine (T-DM1)-induced all-grade fatigue was 35.17% (95% CI, 28.87%-41.74%), which was the lowest among ADCs. Further, the incidence of all-grade fatigue due to sacituzumab govitecan was 42.82% (95% CI, 34.54%-51.32%), which was higher than that due to T-DM1. Moreover, the incidence of fatigue was higher with T-DM1 combination therapy than with monotherapy.

CONCLUSIONS

Clinicians have highlighted the high incidence of ADC-related fatigue and its negative impact on patients' physical and mental health, making fatigue an important research variable. The results of this study will further contribute to a comprehensive understanding of ADCs, which have some clinical importance and are of great benefit to patients with breast cancer.

Targeted therapies in bladder cancer: signaling pathways, applications, and challenges

Bladder cancer (BC) is one of the most prevalent malignancies in men. Understanding molecular characteristics via studying signaling pathways has made tremendous breakthroughs in BC therapies. Thus, targeted therapies including immune checkpoint inhibitors (ICIs), antibody-drug conjugates (ADCs), and tyrosine kinase inhibitor (TKI) have markedly improved advanced BC outcomes over the last few years. However, the considerable patients still progress after a period of treatment with current therapeutic regimens. Therefore, it is crucial to guide future drug development to improve BC survival, based on the molecular characteristics of BC and clinical outcomes of existing drugs. In this perspective, we summarize the applications and benefits of these targeted drugs and highlight our understanding of mechanisms of low response rates and immune escape of ICIs, ADCs toxicity, and TKI resistance. We also discuss potential solutions to these problems. In addition, we underscore the future drug development of targeting metabolic reprogramming and cancer stem cells (CSCs) with a deep understanding of their signaling pathways features. We expect that finding biomarkers, developing novo drugs and designing clinical trials with precisely selected patients and rationalized drugs will dramatically improve the quality of life and survival of patients with advanced BC.

Antibody Drug Conjugates in Bladder Cancer: Current Milestones and Future Perspectives

OPINION STATEMENT

Over the last several years, the treatment landscape of urothelial carcinoma has witnessed an unprecedented expansion of therapeutic options including checkpoint inhibitors, tyrosine kinase inhibitors, and antibody drug conjugates (ADC). Early trial data has shown that ADCs are safer and potentially effective treatment options in advanced bladder cancer as well as in the early disease. In particular, enfortumab-vedotin (EV) has shown promising results with a recent cohort of a clinical trial demonstrating that EV is effective as neoadjuvant monotherapy as well as in combination with pembrolizumab in metastatic setting. Similar promising results have been shown by other classes of ADC in other trials including sacituzumab-govitecan (SG) and oportuzumab monatox (OM). ADCs are likely to become a mainstay treatment option in the urothelial carcinoma playbook as either a monotherapy or combination therapy. The cost of the drug presents a real challenge, but further trial data may justify the use of the drug as mainstay treatment.

Resistance to antibody-drug conjugates in breast cancer: mechanisms and solutions

Antibody-drug conjugates (ADCs) are a rapidly developing therapeutic approach in cancer treatment that has shown remarkable activity in breast cancer. Currently, there are two ADCs approved for the treatment of human epidermal growth factor receptor 2-positive breast cancer, one for triple-negative breast cancer, and multiple investigational ADCs in clinical trials. However, drug resistance has been noticed in clinical use, especially in trastuzumab emtansine. Here, the mechanisms of ADC resistance are summarized into four categories: antibody-mediated resistance, impaired drug trafficking, disrupted lysosomal function, and payload-related resistance. To overcome or prevent resistance to ADCs, innovative development strategies and combination therapy options are being investigated. Analyzing predictive biomarkers for optimal therapy selection may also help to prevent drug resistance.

Development of peptide affinity ligands for the purification of polyclonal and monoclonal Fabs from recombinant fluids

Engineered multi-specific monoclonal antibodies (msAbs) and antibody fragments offer valuable therapeutic options against metabolic disorders, aggressive cancers, and viral infections. The advancement in molecular design and recombinant expression of these next-generation drugs, however, is not equaled by the progress in downstream bioprocess technology. The purification of msAbs and fragments requires affinity adsorbents with orthogonal biorecognition of different portions of the antibody structure, namely its Fc (fragment crystallizable) and Fab (fragment antigen-binding) regions or the C_H1-3 and C_L chains. Current adsorbents rely on protein ligands that, while featuring high binding capacity and selectivity, need harsh elution conditions and suffer from high cost, limited biochemical stability, and potential release of immunogenic fragments. Responding to these challenges, we undertook the de novo discovery of peptide ligands that target different regions of human Fab and enable product release under mild conditions. The ligands were discovered by screening a focused library of 12-mer peptides against a feedstock comprising human Fab and Chinese hamster ovary host cell proteins (CHO HCPs). The identified ligands were evaluated via binding studies as well as molecular docking simulations, returning excellent values of binding capacity (Q_max ∼ 20 mg of Fab per mL of resin) and dissociation constant (K_D = 2.16·10^-6 M). Selected ligand FRWNFHRNTFFP and commercial Protein L ligands were further characterized by measuring the dynamic binding capacity (DBC_10%) at different residence times (RT) and performing the purification of polyclonal and monoclonal Fabs from CHO-K1 cell culture fluids. The peptide ligand featured DBC_10% ∼ 6-16 mg/mL (RT of 2 min) and afforded values of yield (93-96%) and purity (89-96%) comparable to those provided by Protein L resins.

Antibody-drug conjugates targeting TROP-2: Clinical development in metastatic breast cancer

Antibody drug conjugates (ADCs) combine the potent cytotoxicity of chemotherapy with the antigen -specific targeted approach of antibodies into one single molecule. Trophoblast cell surface antigen 2 (TROP-2) is a transmembrane glycoprotein involved in calcium signal transduction and is expressed in multiple tumor types. TROP-2 expression is higher in HER2-negative breast tumors (HR+/HR-) and is associated with worse survival. Sacituzumab govitecan (SG) is a first-in-class TROP-2-directed ADC with an anti-TROP-2 antibody conjugated to SN-38, a topoisomerase inhibitor via a hydrolysable linker. This hydrolysable linker permits intracellular and extracellular release of the membrane permeable payload enabling the "bystander effect" contributing to the efficacy of this agent. There was significant improvement in progression free survival (PFS) and overall survival (OS) with SG versus chemotherapy in pretreated metastatic triple negative breast cancer (TNBC), resulting in regulatory approval. Common adverse events (AE) reported were neutropenia and diarrhea. SG also demonstrated clinical activity versus chemotherapy in a phase III trial of HR+/HER2-metastatic breast cancer (MBC) and is under evaluation in first-line metastatic and early stage TNBC as well. Datopotamab deruxtecan (Dato-DXd) is a TROP-2 ADC that differs from SG in that it has a cleavable tetrapeptide linker and a more potent topoisomerase inhibitor payload. This construct is highly stable in circulation with a longer half-life than SG, and undergoes cleavage in presence of intracellular lysosomal proteases. Dato-DXd demonstrated preliminary efficacy in unselected metastatic TNBC, with common AEs of low-grade nausea and stomatitis. Dato-DXd is being investigated in phase III studies in metastatic TNBC and HR+/HER2- MBC. These novel TROP-2 ADCs have the potential to deliver enhanced efficacy with reduced toxicity in MBC and possibly in early stage breast cancer (EBC).

Her-2 Targeted Therapy in Advanced Urothelial Cancer: From Monoclonal Antibodies to Antibody-Drug Conjugates

Metastatic urothelial cancer, associated with a poor prognosis, is still major cause of cancer-related death, with scarce options of effective treatment after progression to platinum-based chemotherapy and immunotherapy. The human epithelial growth factor receptor 2 (Her-2) has been identified as a new therapeutic target in medical oncology. However, despite the encouraging results in breast and gastric cancers, clinical trials with anti-Her-2 monoclonal antibodies and tyrosine-kinase inhibitors have shown limited efficacy of this strategy in urothelial tumors. Notably, more favorable data have been recently shown that antibody-drug conjugates are currently emerging as a novel promising approach for Her-2 targeted therapy in advanced urothelial cancer.

An update on antibody-drug conjugates in urothelial carcinoma: state of the art strategies and what comes next

In recent years, considerable progress has been made in increasing the knowledge of tumour biology and drug resistance mechanisms in urothelial cancer. Therapeutic strategies have significantly advanced with the introduction of novel approaches such as immune checkpoint inhibitors and Fibroblast Growth Factor Receptor inhibitors. However, despite these novel agents, advanced urothelial cancer is often still progressive in spite of treatment and correlates with a poor prognosis. The introduction of antibody–drug conjugates consisting of a target-specific monoclonal antibody covalently linked to a payload (cytotoxic agent) is a novel and promising therapeutic strategy. In December 2019, the US Food and Drug Administration (FDA) granted accelerated approval to the nectin-4-targeting antibody–drug conjugate, enfortumab vedotin, for the treatment of advanced or metastatic urothelial carcinomas that are refractory to both immune checkpoint inhibitors and platinum-based treatment. Heavily pre-treated urothelial cancer patients reported a significant, 40% response to enfortumab vedotin while other antibody–drug conjugates are currently still under investigation in several clinical trials. We have comprehensively reviewed the available treatment strategies for advanced urothelial carcinoma and outlined the mechanism of action of antibody–drug conjugate agents, their clinical applications, resistance mechanisms and future strategies for urothelial cancer.

Antibody-Drug Conjugates in Urothelial Carcinoma: A New Therapeutic Opportunity Moves from Bench to Bedside

Significant progress has been achieved over the last decades in understanding the biology and mechanisms of tumor progression in urothelial carcinoma (UC). Although the therapeutic landscape has dramatically changed in recent years with the introduction of immune checkpoint inhibitors, advanced UC is still associated with rapidly progressing disease and poor survival. The increasing knowledge of the pathogenesis and molecular pathways underlying cancer development and progression is leading the introduction of target therapies, such as the recently approved FGFR inhibitor Erdafitinib, or the anti-nectin 4 antibody drug-conjugate Enfortumab vedotin. Antibody drug conjugates represent an innovative therapeutic approach that allows the combination of a tar get-specific monoclonal antibody covalently conjugated via a linker to a cytotoxic agent (payload). UC is a perfect candidate for this therapeutic approach since it is particularly enriched in antigen expression on its surface and each specific antigen can represent a potential therapeutic target. In this review we summarize the mechanism of action of ADCs, their applications in localized and metastatic UC, the main mechanisms of resistance, and future perspectives for their use in clinical practice.

Recent advances in the design of antimicrobial peptide conjugates

Antimicrobial peptides (AMPs) are ubiquitous host defense peptides characterized by antibiotic activity and lower propensity for developing resistance compared to classic antibiotics. While several AMPs have shown activity against antibiotic-sensitive...

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.

Targeted Mass Spectrometry-Based Approach for the Determination of Intrinsic Internalization Kinetics of Cell-Surface Membrane Protein Targets

Successful development of targeted therapeutics aimed at the elimination of diseased cells relies on the target properties and the therapeutics that target them. Currently, target properties have been evaluated through antibody-dependent semiquantitative approaches such as flow cytometry, Western blotting, or microscopy. Since antibodies can alter target properties following binding, antibody-dependent approaches provide at best skewed measurements for target intrinsic properties. To circumvent, here we attempted to develop an antibody-free targeted mass spectrometry-based (ATM) strategy to measure the surface densities and the intrinsic rates (K_int) of CD38 internalization in multiple myeloma cell lines. Using cell-surface biotinylation in conjunction with differential mass tagging to separate inward CD38 molecules from the outbound and nascent ones, the ATM approach revealed diversities in measured CD38 K_int values of 0.239 min^-1 S.E. ± 0.076, 0.109 min^-1 S.E. ± 0.032, and 0.058 min^-1 S.E. ± 0.001 for LP1, NCIH929, and MOLP8 cell lines, respectively. Together with CD38 surface densities, intrinsic K_int values aligned well with the tumor penetration model and supported the outcomes for tumor regression in mouse xenografts upon drug treatment. Additionally, the ATM approach can evaluate molecules with fast K_int as we determined for CTLA4 protein. We believe that the ATM approach has the potential to evaluate diverse cell-surface targets as part of the pharmacological assessment in drug discovery.

Improvement of Biophysical Properties and Affinity of a Human Anti-L1CAM Therapeutic Antibody through Antibody Engineering Based on Computational Methods

The biophysical properties of therapeutic antibodies influence their manufacturability, efficacy, and safety. To develop an anti-cancer antibody, we previously generated a human monoclonal antibody (Ab417) that specifically binds to L1 cell adhesion molecule with a high affinity, and we validated its anti-tumor activity and mechanism of action in human cholangiocarcinoma xenograft models. In the present study, we aimed to improve the biophysical properties of Ab417. We designed 20 variants of Ab417 with reduced aggregation propensity, less potential post-translational modification (PTM) motifs, and the lowest predicted immunogenicity using computational methods. Next, we constructed these variants to analyze their expression levels and antigen-binding activities. One variant (Ab612)—which contains six substitutions for reduced surface hydrophobicity, removal of PTM, and change to the germline residue—exhibited an increased expression level and antigen-binding activity compared to Ab417. In further studies, compared to Ab417, Ab612 showed improved biophysical properties, including reduced aggregation propensity, increased stability, higher purification yield, lower pI, higher affinity, and greater in vivo anti-tumor efficacy. Additionally, we generated a highly productive and stable research cell bank (RCB) and scaled up the production process to 50 L, yielding 6.6 g/L of Ab612. The RCB will be used for preclinical development of Ab612.

Unlocking the potential of antibody-drug conjugates for cancer therapy

Nine different antibody-drug conjugates (ADCs) are currently approved as cancer treatments, with dozens more in preclinical and clinical development. The primary goal of ADCs is to improve the therapeutic index of antineoplastic agents by restricting their systemic delivery to cells that express the target antigen of interest. Advances in synthetic biochemistry have ushered in a new generation of ADCs, which promise to improve upon the tissue specificity and cytotoxicity of their predecessors. Many of these drugs have impressive activity against treatment-refractory cancers, although hurdles impeding their broader use remain, including systemic toxicity, inadequate biomarkers for patient selection, acquired resistance and unknown benefit in combination with other cancer therapies. Emerging evidence indicates that the efficacy of a given ADC depends on the intricacies of how the antibody, linker and payload components interact with the tumour and its microenvironment, all of which have important clinical implications. In this Review, we discuss the current state of knowledge regarding the design, mechanism of action and clinical efficacy of ADCs as well as the apparent limitations of this treatment class. We then propose a path forward by highlighting several hypotheses and novel strategies to maximize the potential benefit that ADCs can provide to patients with cancer.

Targeting a Radiosensitizing Antibody-Drug Conjugate to a Radiation-Inducible Antigen

PURPOSE

We recently discovered that anti-TIP1 antibody activates endocytosis in cancer cells, which facilitates retention of antibody and dissociation of a conjugated drug. To improve the pharmacokinetics and cancer specificity of radiosensitizing drugs, we utilized antibody-drug conjugates (ADCs) that bind specifically to radiation-inducible antigen, TIP1, on non-small cell lung cancer (NSCLC). This approach exploits the long circulation time of antibodies to deliver a radiosensitizing drug to cancer each day during radiotherapy.

EXPERIMENTAL DESIGN

Antibodies to TIP1 were prioritized based on affinity, cancer-specific binding, and internalization. The lead antibody, 7H5, was conjugated with a cytotoxic drug MMAE because of its ability to radiosensitize cancer. Cytotoxicity, colony formation, and tumor growth studies were performed with 7H5-VcMMAE in combination with radiation.

RESULTS

7H5 showed a high affinity to recombinant TIP1 protein and radiation-inducible TIP1 on the cancer cell surface. 7H5 undergoes endocytosis in NSCLC cells in vitro. We obtained an average drug-to-antibody ratio (DAR) of 4.25 for 7H5-VcMMAE. A 70% reduction in viable cells was observed following 7H5-VcMMAE treatment compared with 7H5 alone in both A549 and H1299 cells. 7H5-VcMMAE sensitized NSCLC cells to radiation, thereby significantly decreasing the surviving fraction. The ADC combined with radiation showed a prolonged delay in tumor growth and improved survival in A549 and H1299 tumor models.

CONCLUSIONS

Targeting radiation-inducible TIP1 with a radiosensitizing ADC is a promising strategy to enhance the therapeutic efficacy of NSCLC. This novel approach of targeting with ADCs to radiation-inducible antigens will lead to clinical trials in lung cancer patients treated with radiotherapy.

Antibody-drug conjugates: an evolving approach for melanoma treatment

Melanoma continues to be an aggressive and deadly form of skin cancer while therapeutic options are continuously developing in an effort to provide long-term solutions for patients. Immunotherapeutic strategies incorporating antibody-drug conjugates (ADCs) have seen varied levels of success across tumor types and represent a promising approach for melanoma. This review will explore the successes of FDA-approved ADCs to date compared to the ongoing efforts of melanoma-targeting ADCs. The challenges and opportunities for future therapeutic development are also examined to distinguish how ADCs may better impact individuals with malignancies such as melanoma.

Small peptide diversification through photoredox-catalyzed oxidative C-terminal modification

A photoredox-catalyzed oxidative decarboxylative coupling of small peptides is reported, giving access to a variety of N,O-acetals. They were used as intermediates for the addition of phenols and indoles, leading to novel peptide scaffolds and bioconjugates. Amino acids with nucleophilic side chains, such as serine, threonine, tyrosine and tryptophan, could also be used as partners to access tri- and tetrapeptide derivatives with non-natural cross-linking.

Homogenous Scavenging Resolves Low-Purification Yield/Selectivity Caused by Secondary Binding of Protein-A to Antigen-Binding Antibody Fragments

Antigen-binding fragments of antibodies are biotechnologically useful agents for decorating drug delivery systems, for blocking cell-surface receptors in cell culture, for recognizing analytes in biosensors, and potentially as therapeutics. They are typically produced by enzymatic digestion of full antibodies and isolated from the undesirable fragment crystallizable (Fc) by affinity chromatography using Protein-A columns. However, while Protein-A has a strong "classical" interaction with Fc fragments, it can also more weakly bind to an "alternative" site on the heavy chain variable region of antigen-binding fragments. As such, purifying small amounts of antibody fragments by Protein-A chromatography can result in low yield. Moreover, loading larger amounts of antibody fragments onto a Protein-A column can result in poor separation, because of competition of Fc and antigen-binding fragments for immobilized Protein-A. This study demonstrates that Protein-A-based homogeneous scavenging resolves this issue by precisely controlling the stoichiometry of Protein-A to Fc fragments, something that is not possible for conventional flow-type systems, such as affinity chromatography.

Bio-orthogonal Supramolecular Latching inside Live Animals and Its Application for in Vivo Cancer Imaging

Here, we demonstrate a supramolecular latching tool for bio-orthogonal noncovalent anchoring of small synthetic molecules in live animal models using a fully synthetic high-affinity binding pair between cucurbit[7]uril (CB[7]) and adamantylammonium (AdA). This supramolecular latching system is small (∼1 kDa), ensuring efficient uptake into cells, tissues, and whole organisms. It is also chemically robust and resistant to enzymatic degradation and analogous to well-characterized biological systems in terms of noncovalent binding. Occurrence of fluorescence resonance energy transfer (FRET) between cyanine 3-CB[7] (Cy3-CB[7]) and boron-dipyrromethene 630/650X-AdA (BDP630/650-AdA) inside a live worm (Caenorhabditis elegans) indicates efficient in situ high-affinity association between AdA and CB[7] inside live animals. In addition, selective visualization of a cancer site of a live mouse upon supramolecular latching of cyanine 5-AdA (Cy5-AdA) on prelocalized CB[7]-conjugating antibody on the cancer site demonstrates the potential of this synthetic system for in vivo cancer imaging. These findings provide a fresh insight into the development of new chemical biology tools and medical therapeutic systems.

Antibody-Drug Conjugates Targeting the Urokinase Receptor (uPAR) as a Possible Treatment of Aggressive Breast Cancer

A promising molecular target for aggressive cancers is the urokinase receptor (uPAR). A fully human, recombinant antibody that binds uPAR to form a stable complex that blocks uPA-uPAR interactions (2G10) and is internalized primarily through endocytosis showed efficacy in a mouse xenograft model of highly aggressive, triple negative breast cancer (TNBC). Antibody-drug conjugates (ADCs) of 2G10 were designed and produced bearing tubulin inhibitor payloads ligated through seven different linkers. Aldehyde tag technology was employed for linking, and either one or two tags were inserted into the antibody heavy chain, to produce site-specifically conjugated ADCs with drug-to-antibody ratios of either two or four. Both cleavable and non-cleavable linkers were combined with two different antimitotic toxins—MMAE (monomethylauristatin E) and maytansine. Nine different 2G10 ADCs were produced and tested for their ability to target uPAR in cell-based assays and a mouse model. The anti-uPAR ADC that resulted in tumor regression comprised an MMAE payload with a cathepsin B cleavable linker, 2G10-RED-244-MMAE. This work demonstrates in vitro activity of the 2G10-RED-244-MMAE in TNBC cell lines and validates uPAR as a therapeutic target for TNBC.

Peptide-Cleavable Self-immolative Maytansinoid Antibody-Drug Conjugates Designed To Provide Improved Bystander Killing

A new type of antibody–drug conjugate (ADC) has been prepared that contains a sulfur-bearing maytansinoid attached to an antibody via a highly stable tripeptide linker. Once internalized by cells, proteases in catabolic vesicles cleave the peptide of the ADC’s linker causing self-immolation that releases a thiol-bearing metabolite, which is then S-methylated. Conjugates were prepared with peptide linkers containing only alanyl residues, which were all l isomers or had a single d residue in one of the three positions. A d-alanyl residue in the linker did not significantly impair a conjugate’s cytotoxicity or bystander killing unless it was directly attached to the immolative moiety. Increasing the number of methylene units in the maytansinoid side chain of a conjugate did not typically affect an ADC’s cytotoxicity to targeted cells but did increase bystander killing activity. ADCs with the highest in vitro bystander killing were then evaluated in vivo in mice, where they displayed improved efficacy compared to previously described types of maytansinoid conjugates.

HER2-Overexpressing/Amplified Breast Cancer as a Testing Ground for Antibody-Drug Conjugate Drug Development in Solid Tumors

Efficacy data from the KATHERINE clinical trial, comparing the HER2-directed antibody-drug conjugate (ADC) ado-trastuzumab emtansine (T-DM1) to trastuzumab in patients with early-stage HER2-amplified/overexpressing breast cancer with residual disease after neoadjuvant therapy, demonstrates superiority of T-DM1 (HR for invasive disease or death, 0.50; P < 0.001). This establishes foundational precedent for ADCs as effective therapy for treatment of subclinical micrometastasis in an adjuvant (or post-neoadjuvant) early-stage solid tumor setting. Despite this achievement, general principles from proposed systems pharmacokinetic modeling for intracellular processing of ADCs indicate potential shortcomings of T-DM1: (i) C _max limited by toxicities; (ii) slow internalization rate; (iii) resistance mechanisms due to defects in intracellular trafficking [loss of lysosomal transporter solute carrier family 46 member 3, (SLC46A3)], and increased expression of drug transporters MDR1 and MRP1; and (iv) lack of payload bystander effects limiting utility in tumors with heterogeneous HER2 expression. These handicaps may explain the inferiority of T-DM1-based therapy in the neoadjuvant and first-line metastatic HER2⁺ breast cancer settings, and lack of superiority to chemotherapy in HER2⁺ advanced gastric cancer. In this review, we discuss how each of these limitations is being addressed by manipulating internalization and trafficking using HER2:HER2 bispecific or biparatopic antibody backbones, using site-specific, fixed DAR conjugation chemistry, and payload swapping to exploit alternative intracellular targets and to promote bystander effects. Newer HER2-directed ADCs have impressive clinical activity even against tumors with lower levels of HER2 receptor expression. Finally, we highlight ongoing clinical efforts to combine HER2 ADCs with other treatment modalities, including chemotherapy, molecularly targeted therapies, and immunotherapy.

David vs. Goliath: The Structure, Function, and Clinical Prospects of Antibody Fragments

Since the licensing of the first monoclonal antibody therapy in 1986, monoclonal antibodies have become the largest class of biopharmaceuticals with over 80 antibodies currently approved for a variety of disease indications. The development of smaller, antigen binding antibody fragments, derived from conventional antibodies or produced recombinantly, has been growing at a fast pace. Antibody fragments can be used on their own or linked to other molecules to generate numerous possibilities for bispecific, multi-specific, multimeric, or multifunctional molecules, and to achieve a variety of biological effects. They offer several advantages over full-length monoclonal antibodies, particularly a lower cost of goods, and because of their small size they can penetrate tissues, access challenging epitopes, and have potentially reduced immunogenicity. In this review, we will discuss the structure, production, and mechanism of action of EMA/FDA-approved fragments and of those in clinical and pre-clinical development. We will also discuss current topics of interest surrounding the potential use of antibody fragments for intracellular targeting and blood-brain barrier (BBB) penetration.

A chemoselective strategy for late-stage functionalization of complex small molecules with polypeptides and proteins

Conjugates between proteins and small molecules enable access to a vast chemical space that is not achievable with either type of molecule alone; however, the paucity of specific reactions capable of functionalizing proteins and natural products presents a formidable challenge for preparing conjugates. Here we report a strategy for conjugating electron-rich (hetero)arenes to polypeptides and proteins. Our bioconjugation technique exploits the electrophilic reactivity of an oxidized selenocysteine residue in polypeptides and proteins, and the electron-rich character of certain small molecules to provide bioconjugates in excellent yields under mild conditions. This conjugation chemistry enabled the synthesis of peptide-vancomycin conjugates without the prefunctionalization of vancomycin. These conjugates have an enhanced in vitro potency for resistant Gram-positive and Gram-negative pathogens. Additionally, we show that a 6 kDa affibody protein and a 150 kDa immunoglobulin-G antibody could be modified without diminishing bioactivity.

Enzymatic strategies for (near) clinical development of antibody-drug conjugates

Target-specific killing of tumor cells with antibody-drug conjugates (ADCs) is an elegant concept in the continued fight against cancer. However, despite more than 20 years of clinical development, only four ADC have reached market approval, while at least 50 clinical programs were terminated early. The high attrition rate of ADCs may, at least in part, be attributed to heterogeneity and instability of conventional technologies. At present, various (chemo)enzymatic approaches for site-specific and stable conjugation of toxic payloads are making their way to the clinic, thereby potentially providing ADCs with increased therapeutic window.

Bioprocess development of antibody-drug conjugate production for cancer treatment

Antibody-drug conjugate (ADC) is a class of targeted cancer therapies that combine the advantages of monoclonal antibody (mAb)'s specific targeting and chemotherapy's potent cytotoxicity. The therapeutic effect of ADC is significantly affected by its bioproduction process. This study aims to develop an effective ADC production process using anti-HER2 mAb-drug as a model therapeutic. First, a high titer (>2 g/L) of mAb was produced by Chinese hamster ovary cells from fed-batch cell culture. Both live-cell confocal microscopy imaging and flow cytometry analysis demonstrated that the produced mAb and ADC had strong and specific binding to HER2+ cell line BT474. Second, various conjugation conditions of mAb and drug, including linker selection, ratio of drug and mAb, and conjugation approaches, were investigated to improve the production yield and product quality. Finally, the ADC structure and biological quality were evaluated by SDS-PAGE and anti-breast cancer toxicity study, respectively. The ADC with integral molecular structure and high cytotoxicity (IC50 of 1.95 nM) was produced using the optimized production process. The robust bioproduction process could guide the development of ADC-based biopharmaceuticals.

Heterozygous deletion of chromosome 17p renders prostate cancer vulnerable to inhibition of RNA polymerase II

Heterozygous deletion of chromosome 17p (17p) is one of the most frequent genomic events in human cancers. Beyond the tumor suppressor TP53, the POLR2A gene encoding the catalytic subunit of RNA polymerase II (RNAP2) is also included in a ~20-megabase deletion region of 17p in 63% of metastatic castration-resistant prostate cancer (CRPC). Using a focused CRISPR-Cas9 screen, we discovered that heterozygous loss of 17p confers a selective dependence of CRPC cells on the ubiquitin E3 ligase Ring-Box 1 (RBX1). RBX1 activates POLR2A by the K63-linked ubiquitination and thus elevates the RNAP2-mediated mRNA synthesis. Combined inhibition of RNAP2 and RBX1 profoundly suppress the growth of CRPC in a synergistic manner, which potentiates the therapeutic effectivity of the RNAP2 inhibitor, α-amanitin-based antibody drug conjugate (ADC). Given the limited therapeutic options for CRPC, our findings identify RBX1 as a potentially therapeutic target for treating human CRPC harboring heterozygous deletion of 17p.

Antineoplastic Agents. 605. Isoquinstatins

In order to further explore quinoline-type structural modification of the powerful anticancer drug dolastatin 10, an Indian Ocean sea hare constituent and parent molecule of the very successful antibody drug conjugate (ADC) Adcetris, our recent quinstatin study has been extended by replacing the quinoline ring with an isoquinoline. The resulting isoquinstatins (4-6) were modified to N-terminal desmethylisoquinstatins (7-9) and, in turn, bonded to appropriate linker units to give linker-desmethylisoquinstatin conjugates 11-13 in preparation for eventual monoclonal antibody attachment. Comparison of the new isoquinstatins with their quinstatin counterparts against six human cancer cell lines indicated the isoquinstatins to have GI₅₀ values that were comparable to or somewhat higher than those of the isomeric quinstatins. However, desmethylisoquinstatin 5 (7) was significantly more potent than its desmethylquinstatin 5 analogue. When evaluated against quinstatin 8, its isoquinstatin 8 (6) counterpart was somewhat less potent. In general, the isoquinstatins evaluated proved to be quite strong cancer cell growth inhibitors.

Nanobodies: Chemical Functionalization Strategies and Intracellular Applications

Nanobodies can be seen as next-generation tools for the recognition and modulation of antigens that are inaccessible to conventional antibodies. Due to their compact structure and high stability, nanobodies see frequent usage in basic research, and their chemical functionalization opens the way towards promising diagnostic and therapeutic applications. In this Review, central aspects of nanobody functionalization are presented, together with selected applications. While early conjugation strategies relied on the random modification of natural amino acids, more recent studies have focused on the site-specific attachment of functional moieties. Such techniques include chemoenzymatic approaches, expressed protein ligation, and amber suppression in combination with bioorthogonal modification strategies. Recent applications range from sophisticated imaging and mass spectrometry to the delivery of nanobodies into living cells for the visualization and manipulation of intracellular antigens.

A Polar Sulfamide Spacer Significantly Enhances the Manufacturability, Stability, and Therapeutic Index of Antibody-Drug Conjugates

Despite tremendous efforts in the field of targeted cancer therapy with antibody–drug conjugates (ADCs), attrition rates have been high. Historically, the priority in ADC development has been the selection of target, antibody, and toxin, with little focus on the nature of the linker. We show here that a short and polar sulfamide spacer (HydraSpace™, Oss, The Netherlands) positively impacts ADC properties in various ways: (a) efficiency of conjugation; (b) stability; and (c) therapeutic index. Different ADC formats are explored in terms of drug-to-antibody ratios (DAR2, DAR4) and we describe the generation of a DAR4 ADC by site-specific attachment of a bivalent linker–payload construct to a single conjugation site in the antibody. A head-to-head comparison of HydraSpace™-containing DAR4 ADCs to marketed drugs, derived from the same antibody and toxic payload components, indicated a significant improvement in both the efficacy and safety of several vivo models, corroborated by in-depth pharmacokinetic analysis. Taken together, HydraSpace™ technology based on a polar sulfamide spacer provides significant improvement in manufacturability, stability, and ADC design, and is a powerful platform to enable next-generation ADCs with enhanced therapeutic index.

Challenges in Optimising the Successful Construction of Antibody Drug Conjugates in Cancer Therapy

Although considerable progress has been made in the field of cancer chemotherapy, there remains a significant unmet medical need, with a requirement to move away from traditional cytotoxics and explore novel, smarter chemotherapeutic approaches. One such example of the smart chemotherapy approach is antibody-drug conjugates (ADCs), which consist of an antibody that binds selectively to a cancer antigen linked to a cytotoxic agent. When developing an ADC, it may be necessary to produce a variety of constructs to fully assess the optimal configuration for the molecule. By testing ADCs prepared using a range of cytotoxic agents, linkers, or different antibodies, it is possible to fully assess the optimal approach for this treatment modality before advancing to the clinic. Since the development and approval of first-generation ADCs, significant improvements in development technology have occurred. Here, we consider the advances made within the field of ADCs, focusing on the development of EDO-B278 and EDO-B776, both of which have demonstrated efficacy in preclinical testing. Although some limitations remain in this field of development, the potential reduction in toxicity offered by ADCs justifies the investment in research to find workable solutions that could ultimately provide patients with superior outcomes.

One-pot N-glycosylation remodeling of IgG with non-natural sialylglycopeptides enables glycosite-specific and dual-payload antibody-drug conjugates

Chemoenzymatic transglycosylation catalyzed by endo-S mutants is a powerful tool for in vitro glycoengineering of therapeutic antibodies. In this paper, we report a one-pot chemoenzymatic synthesis of glycoengineered Herceptin using an egg-yolk sialylglycopeptide (SGP) substrate. Combining this one-pot strategy with novel non-natural SGP derivatives carrying azido or alkyne tags, glycosite-specific conjugation was enabled for the development of new antibody-drug conjugates (ADCs). The site-specific ADCs and semi-site-specific dual-drug ADCs were successfully achieved and characterized with SDS-PAGE, intact antibody or ADC mass spectrometry analysis, and PNGase-F digestion analysis. Cancer cell cytotoxicity assay revealed that small-molecule drug release of these ADCs relied on the cleavable Val-Cit linker fragment embedded in the structure. These results represent a new approach for glycosite-specific and dual-drug ADC design and rapid synthesis, and also provide the structural requirement for their biologic activities.

Leveraging Formylglycine-Generating Enzyme for Production of Site-Specifically Modified Bioconjugates

Enzymatic modification of proteins can generate uniquely reactive chemical functionality, enabling site-specific reactions on the protein surface. Formylglycine-generating enzyme (FGE) is one enzyme that can be exploited in this fashion. FGE binds its consensus sequence (CXPXR, known as the "aldehyde-tag") and converts the cysteine to a formylglycine (fGly). fGly-containing proteins contain a bioorthogonal aldehyde on their surface that can be modified selectively in the presence of the 20 canonical amino acids. Here, we describe protocols for the generation of a site-specifically modified protein, an antibody-drug conjugate (ADC), using aldehyde-tagging protocols and aldehyde-reactive conjugation chemistry.

Current progress in innovative engineered antibodies

As of May 1, 2017, 74 antibody-based molecules have been approved by a regulatory authority in a major market. Additionally, there are 70 and 575 antibody-based molecules in phase III and phase I/II clinical trials, respectively. These total 719 antibody-based clinical stage molecules include 493 naked IgGs, 87 antibody-drug conjugates, 61 bispecific antibodies, 37 total Fc fusion proteins, 17 radioimmunoglobulins, 13 antibody fragments, and 11 immunocytokines. New uses for these antibodies are being discovered each year. For oncology, many of the exciting new approaches involve antibody modulation of T-cells. There are over 80 antibodies in clinical trials targeting T cell checkpoints, 26 T-cell-redirected bispecific antibodies, and 145 chimeric antigen receptor (CAR) cell-based candidates (all currently in phase I or II clinical trials), totaling more than 250 T cell interacting clinical stage antibody-based candidates. Finally, significant progress has been made recently on routes of delivery, including delivery of proteins across the blood-brain barrier, oral delivery to the gut, delivery to the cellular cytosol, and gene- and viral-based delivery of antibodies. Thus, there are currently at least 864 antibody-based clinical stage molecules or cells, with incredible diversity in how they are constructed and what activities they impart. These are followed by a next wave of novel molecules, approaches, and new methods and routes of delivery, demonstrating that the field of antibody-based biologics is very innovative and diverse in its approaches to fulfill their promise to treat unmet medical needs.

Recent Developments in ADC Technology: Preclinical Studies Signal Future Clinical Trends

The antibody-drug conjugate (ADC) field is in a transitional period. Older approaches to conjugate composition and dosing regimens still dominate the ADC clinical pipeline, but preclinical work is driving a rapid evolution in how we strategize to improve efficacy and reduce toxicity towards better therapeutic outcomes. These advances are largely based upon a body of investigational studies that together offer a deeper understanding of the absorption, distribution, metabolism, and excretion (ADME) and drug metabolism and pharmacokinetics (DMPK) fates of both the intact conjugate and its small-molecule component. Knowing where the drug goes and how it is processed allows mechanistic connections to be drawn with commonly observed clinical toxicities. The field is also starting to consider ADC interactions with the immune system and potential synergistic therapeutic opportunities therein. In an indication of future directions for the field, antibody conjugates bearing non-cytotoxic small-molecule payloads are being developed to reduce side effects associated with treatment of chronic diseases. ADCs are not a magic bullet to cure disease. However, they will increasingly become valuable therapeutic tools to improve patient outcomes across a variety of indications.

The state-of-play and future of antibody therapeutics

It has been over four decades since the development of monoclonal antibodies (mAbs) using a hybridoma cell line was first reported. Since then more than thirty therapeutic antibodies have been marketed, mostly as oncology, autoimmune and inflammatory therapeutics. While antibodies are very efficient, their cost-effectiveness has always been discussed owing to their high costs, accumulating to more than one billion dollars from preclinical development through to market approval. Because of this, therapeutic antibodies are inaccessible to some patients in both developed and developing countries. The growing interest in biosimilar antibodies as affordable versions of therapeutic antibodies may provide alternative treatment options as well potentially decreasing costs. As certain markets begin to capitalize on this opportunity, regulatory authorities continue to refine the requirements for demonstrating quality, efficacy and safety of biosimilar compared to originator products. In addition to biosimilars, innovations in antibody engineering are providing the opportunity to design biobetter antibodies with improved properties to maximize efficacy. Enhancing effector function, antibody drug conjugates (ADC) or targeting multiple disease pathways via multi-specific antibodies are being explored. The manufacturing process of antibodies is also moving forward with advancements relating to host cell production and purification processes. Studies into the physical and chemical degradation pathways of antibodies are contributing to the design of more stable proteins guided by computational tools. Moreover, the delivery and pharmacokinetics of antibody-based therapeutics are improving as optimized formulations are pursued through the implementation of recent innovations in the field.

Antineoplastic Agents. 604. The Path of Quinstatin Derivatives to Antibody Drug Conjugates

To further evaluate the exceptional cancer cell growth inhibition by the quinstatins, of which one of the series, quinstatin 8, approaches the exceptional cytotoxic activity of the parent dolastatin 10 (1), four of the quinstatins have been converted to desmethyl derivatives. Three of the four (4, 5, and 8 [7b-d]) were next bonded to the linker (8) employed in the synthesis of the very successful and structurally related anticancer drug Adcetris (3). Owing to these structural modifications, a next step could be taken by bonding to a monoclonal antibody, thereby producing an antibody drug conjugate (ADC) related to Adcetris structurally but with the possibility of a wider spectrum of activity and utility.

Real-Time Analysis on Drug-Antibody Ratio of Antibody-Drug Conjugates for Synthesis, Process Optimization, and Quality Control

Drug-antibody ratio (DAR) of antibody-drug conjugates (ADCs) is important for their therapeutic efficacy and pharmacokinetics, therefore control on DAR in synthesis process is a key for ADC quality control. Although various analytical methods were reported, the real-time monitoring on DAR is still a challenge because time-consuming sample preparation is usually needed during the analysis. Antibody deglycosylation of ADC simplifies DAR measurement, however long-time PNGaseF digestion for deglycosylation hampers the real-time detection. Here, we report a rapid DAR analysis within 15 min by robust deglycosylation treatment and LC-MS detection that enables real-time DAR monitoring for optimization on ADC synthetic process. With this approach, we were able to screen suitable conjugation conditions efficiently and afford the ADCs with expected DARs. To the best of our knowledge, this is the first report on real-time DAR analysis of ADCs for conjugation optimization and quality control, compatible with random lysine-linked ADCs, glycosite-specific ADCs, and the complicated dual-payload ADCs.

Stable and Potent Selenomab-Drug Conjugates

Selenomabs are engineered monoclonal antibodies with one or more translationally incorporated selenocysteine residues. The unique reactivity of the selenol group of selenocysteine permits site-specific conjugation of drugs. Compared with other natural and unnatural amino acid and carbohydrate residues that have been used for the generation of site-specific antibody-drug conjugates, selenocysteine is particularly reactive, permitting fast, single-step, and efficient reactions under near physiological conditions. Using a tailored conjugation chemistry, we generated highly stable selenomab-drug conjugates and demonstrated their potency and selectivity in vitro and in vivo. These site-specific antibody-drug conjugates built on a selenocysteine interface revealed broad therapeutic utility in liquid and solid malignancy models.

Laboratory Scale Production and Purification of a Therapeutic Antibody

Ensuring the successful production of a therapeutic antibody begins early on in the development process. The first stage is vector expression of the antibody genes followed by stable transfection into a suitable cell line. The stable clones are subjected to screening in order to select those clones with desired production and growth characteristics. This is a critical albeit time-consuming step in the process. This protocol considers vector selection and sourcing of antibody sequences for the expression of a therapeutic antibody. The methods describe preparation of vector DNA for stable transfection of a suspension variant of human embryonic kidney 293 (HEK-293) cell line, using polyethylenimine (PEI). The cells are transfected as adherent cells in serum-containing media to maximize transfection efficiency, and afterwards adapted to serum-free conditions. Large scale production, setup as batch overgrow cultures is used to yield antibody protein that is purified by affinity chromatography using an automated fast protein liquid chromatography (FPLC) instrument. The antibody yields produced by this method can provide sufficient protein to begin initial characterization of the antibody. This may include in vitro assay development or physicochemical characterization to aid in the time-consuming task of clonal screening for lead candidates. This method can be transferable to the development of an expression system for the production of biosimilar antibodies.

Strain Prioritization and Genome Mining for Enediyne Natural Products

The enediyne family of natural products has had a profound impact on modern chemistry, biology, and medicine, and yet only 11 enediynes have been structurally characterized to date. Here we report a genome survey of 3,400 actinomycetes, identifying 81 strains that harbor genes encoding the enediyne polyketide synthase cassettes that could be grouped into 28 distinct clades based on phylogenetic analysis. Genome sequencing of 31 representative strains confirmed that each clade harbors a distinct enediyne biosynthetic gene cluster. A genome neighborhood network allows prediction of new structural features and biosynthetic insights that could be exploited for enediyne discovery. We confirmed one clade as new C-1027 producers, with a significantly higher C-1027 titer than the original producer, and discovered a new family of enediyne natural products, the tiancimycins (TNMs), that exhibit potent cytotoxicity against a broad spectrum of cancer cell lines. Our results demonstrate the feasibility of rapid discovery of new enediynes from a large strain collection.

Recent advances in microbial genomics clearly revealed that the biosynthetic potential of soil actinomycetes to produce enediynes is underappreciated. A great challenge is to develop innovative methods to discover new enediynes and produce them in sufficient quantities for chemical, biological, and clinical investigations. This work demonstrated the feasibility of rapid discovery of new enediynes from a large strain collection. The new C-1027 producers, with a significantly higher C-1027 titer than the original producer, will impact the practical supply of this important drug lead. The TNMs, with their extremely potent cytotoxicity against various cancer cells and their rapid and complete cancer cell killing characteristics, in comparison with the payloads used in FDA-approved antibody-drug conjugates (ADCs), are poised to be exploited as payload candidates for the next generation of anticancer ADCs. Follow-up studies on the other identified hits promise the discovery of new enediynes, radically expanding the chemical space for the enediyne family.

Antibody-drug conjugates: recent advances in conjugation and linker chemistries

The antibody-drug conjugate (ADC), a humanized or human monoclonal antibody conjugated with highly cytotoxic small molecules (payloads) through chemical linkers, is a novel therapeutic format and has great potential to make a paradigm shift in cancer chemotherapy. This new antibody-based molecular platform enables selective delivery of a potent cytotoxic payload to target cancer cells, resulting in improved efficacy, reduced systemic toxicity, and preferable pharmacokinetics (PK)/pharmacodynamics (PD) and biodistribution compared to traditional chemotherapy. Boosted by the successes of FDA-approved Adcetris^® and Kadcyla^®, this drug class has been rapidly growing along with about 60 ADCs currently in clinical trials. In this article, we briefly review molecular aspects of each component (the antibody, payload, and linker) of ADCs, and then mainly discuss traditional and new technologies of the conjugation and linker chemistries for successful construction of clinically effective ADCs. Current efforts in the conjugation and linker chemistries will provide greater insights into molecular design and strategies for clinically effective ADCs from medicinal chemistry and pharmacology standpoints. The development of site-specific conjugation methodologies for constructing homogeneous ADCs is an especially promising path to improving ADC design, which will open the way for novel cancer therapeutics.

Prospects and progress of antibody-drug conjugates in solid tumor therapies

INTRODUCTION

Antibody-drug conjugates (ADCs) for targeted chemotherapy have evolved in the past 2-3 decades to become a validated clinical cancer therapy modality. While considerable strides have been made in treating hematological tumors, challenges remain in the more difficult-to-treat solid cancers.

AREAS COVERED

The current model for a successful ADC uses a highly potent cytotoxic drug as the payload, with stringent linker requirements and limited substitutions. In solid tumor treatment, a number of ADCs have not progressed beyond Phase I clinical trials, indicating a need to optimize additional factors governing translational success. In this regard, insights from mathematical modeling provide a number of pointers relevant to target antigen and antibody selection. Together with the choice of targets, these can be expected to complement the gains made in ADC design towards the generation of better therapeutics.

EXPERT OPINION

While highly potent microtubule inhibitors continue to dominate the current ADC landscape, there are promising data with other drugs, linkers, and targets that suggest a more flexible model for a successful ADC is evolving. Such changes will undoubtedly lead to the consideration of new targets and constructs to overcome some of the unique natural barriers that impede the delivery of cytotoxic agents in solid tumor.