Bispecific Antibodies and Antibody Drug Conjugates: More Complex and Targeted Biologic Therapeutics

Bispecific Antibodies

Bispecific antibodies are a relatively new class of targeted antibody therapeutics that can bind to two different antigens. There are over 150 bispecific antibodies in clinical pipelines, ranging from small monovalent binding fragments to IgG-like larger molecules. Asides from cancer, they are also studied for the treatment of osteoporosis, hemophilia, and autoimmune diseases.

Picture 1. Mechanism of action of catumaxomab, showing the bispecific regions binding to a tumor cell receptor and a T cell receptor. The Fc region is additionally bound to a Fc receptor. Source: Catumaxomab - Wikipedia

The double specificity means that these special antibodies can bind to cancer cells and can also bind to other cells, most commonly to T-cells. Theoretically, this method is superior as it allows for three mechanisms of action, as compared to the one or two mechanisms of action for regular monoclonal antibodies.

Antibody Drug Conjugates

Antibody drug conjugates (ADCs) are a class of therapeutics that co-leverages the selectivity of antibodies and the cytotoxicity of small molecule drugs, creating a payload delivery system that is more targeted and precise than traditional methods. This class of therapeutics has received significant interest since it was first synthesized decades ago, given its promise of delivering potent payloads at small quantities and minimizing the side effects to the patient. All but three of the ten FDA-approved ADCs have come in the past four years.

Picture 2. A simple representation of antibody drug conjugates in action, with a monoclonal antibody – linker - cytotoxin drug conjugation, attached to cancer cells. Source: Expanding the Horizon of Antibody-Drug Conjugates for Cancer Treatment

Antibody drug conjugates are manufactured by attaching the cytotoxic drug to the monoclonal antibody with a linker. Linkers are made of short-chained structural motifs such as peptides or disulfides. The ADC can either have a non-cleavable linker, where the entire system is the drug, or a cleavable linker, which allows enzymes in the cancer cells to separate the cytotoxic payload upon delivery.

With several approvals in oncology, ADCs have a bright future ahead, with several small biotechs expanding the payload-delivery potential to autoimmune diseases such as rheumatoid arthritis or even to deliver antibiotics.

Addressing Binding Affinity and Kinetics

Binding affinity and kinetics are a set of parameters that describe how strongly a protein binds to its ligands, and how fast a protein binds and unbinds to its ligands. Due to their more complex structure, both bispecific antibodies and antibody drug conjugates bring new challenges when understanding their binding affinity and kinetics.

When considering bispecific antibodies, the interdependence of their two antigen binding sites, plus the Fc region, make it important to have specific assays and data processing techniques that account for bivalent analytes. While when considering ADCs, even minor changes to the conjugations and linkers have been shown to alter the binding kinetics of the protein-ligand binding events, and therefore make full characterization important in both the development of the conjugation chemistry and in drug development.

Surface Plasmon Resonance (SPR) and Isothermal Calorimetry (ITC) are two complementary protein-ligand binding assay techniques that address these challenging antibodies. SPR is the gold standard label-free kinetics tool and ITC is the gold standard label-free affinity tool.

It is useful to measure using the SPR and ITC techniques for the following:

Binding affinity under native conditions
Binding kinetic on-rates and off-rates
Screening for potential candidates
Epitope-mapping and epitope-binning
Measuring immunogenic response from anti-drug antibodies (ADA)
Characterizing bivalent interaction of two antigen binding sites in bispecific antibodies
Characterizing drug-linker conjugates
Comparability studies

About the author

James Hsu joined DKSH in 2019 and is part of the Business Development, Business Unit Technology team in Taiwan. In this role, he is responsible for growing the life sciences and scientific instrumentations business. His previous experience was accumulated in the bustling Asian genomics and proteomics sector, where he worked on bringing a digital PCR startup to market. James graduated from the University of California, San Diego.