The Resurgence of Antibody Drug Conjugates: How ADCs are taking over oncology (again) and what we’ve learned about creating bioanalytical methods to support preclinical and clinical success.

02.26.26

Michelle Miller, PhD, Global Senior Scientific Director, Bioanalytical Services

What is an ADC?

Antibody Drug Conjugates (ADCs) consist of a monoclonal antibody linked to a cytotoxic payload. The antibody binds to a cancer antigen, resulting in the targeted delivery of the payload to the tumor site. This potent approach to immunotherapy is often referred to as a “magic bullet” and has become a highly successful approach in oncology. Key aspects of ADC design include the stability of the linker (stable versus cleavable) and the amount of drug per antibody (Drug-Antibody Ratio or DAR) in addition to the antibody specificity for the antigen of interest.

A Brief History of ADC Therapeutics:

ADCs have had a long, tenuous journey over the last forty years.  The first generation of ADCs reached the clinical trial stage in the 1980’s. These consisted mainly of murine antibodies and early variations were associated with high levels of off-target toxicity. It wasn’t until the year 2000 that the first ADC received FDA approval. Yet even this drug, Pfizer’s Mylotarg, had a problematic timeline, being withdrawn from the market in 2010 before re-approval in 2017.

In the last decade, cell therapies have been the new modality leading the way in the immunotherapy space but after a number of serious setbacks and significant manufacturing challenges for cell therapy products, ADCs began to return to the spotlight. Drug developers have continued to improve upon ADC design with better linker technologies, modified antibody structures, and an expanded selection of payload molecules contributing to more advanced ADC drug candidates.

In 2021 alone, 11 ADCs received approval from the FDA and there are currently an estimated 200 new ADCs in clinical trials. The global market for ADCs has grown dramatically and is projected to reach $32.11 billion USD by 2033.

Bioanalytical Testing Strategies for ADCs:

Testing of investigational ADCs in clinical studies requires specific solutions for several unique challenges. In addition to the clinical concerns associated with narrow therapeutic windows and low tumor penetration, the bioanalytical strategy for clinical sample analysis of ADCs requires careful planning.  In particular, the multidomain structure and the stability of the conjugated components can lead to complex pharmacokinetic measurements and necessitate strategic approaches to immunogenicity method development.

To overcome dose-limiting toxicities from premature payload release, the stability or target-specific cleavability of the linker is a major consideration in ADC design. Understanding the timing, efficiency, and location of linker cleavage is therefore a critical aspect of characterizing the drug absorption, distribution, metabolism, and excretion (ADME) profile. Distinct PK methods must be designed to detect the different forms of the drug including conjugated (antibody+linker+drug) and unconjugated (antibody or drug alone) variations. Multiple assays can be used together to form the full picture.

Common bioanalytical strategies include:

• Total Antibody – ligand binding assay; quantitates the antibody component of the ADC with reagents designed to capture/detect conjugated, partially unconjugated, or fully unconjugated forms
• Conjugated Antibody – ligand binding assay; quantitates antibody with at least one linker-drug component, can be designed to be DAR specific if appropriate reagents are available
• Unconjugated Drug – LC-MS; small molecule drug/payload not conjugated to the antibody
• Total Drug – LC-MS; incorporates a sample digestion to quantitate both unconjugated and conjugated drug

Anti-drug antibody (ADA) methods to monitor humoral immune responses against the ADC are a required part of bioanalytical testing. For most ADA assays, a bridging format is preferred which uses plate-bound ADC to capture any ADAs from patient samples and digoxin or ruthenium-labeled ADC to detect the bound ADAs. In this approach, antibodies directed against any of the three regions of the therapeutic can be detected in a single method. However, the multidomain structure of ADCs may necessitate additional characterization of ADA responses to better understand the immunogenic potential of each part separately.

Positive controls for these assays are a critical reagent used to demonstrate assay sensitivity, specificity, and precision during ADA assay development. The most common type of positive control is an anti-idiotypic antibody which targets the variable region of the drug antibody. However, if domain characterization studies are part of the immunogenicity strategy, additional antibodies that specifically bind to the payload and linker regions are required.

The Difference Experience Makes:

ADCs are a powerful tool in cancer therapy, combining the specificity of monoclonal antibodies with the potency of cytotoxic small molecules. The complex and heterogenic nature of these modalities requires specialized understanding and careful planning to ensure the appropriate solutions are in place for successful PK and immunogenicity measurements in support of clinical testing.

At Celerion, we have deep expertise in designing and implementing phase- and modality-appropriate PK and immunogenicity bioanalytical strategies. With over 50 years of experience in method development, validation, and sample analysis, our scientific teams provide unparalleled bioanalytical support across the pharmaceutical and biotech industries. Our GLP, GCP/GCLP, CLIA/CAP certified laboratories are equipped with state-of-the-art equipment and offer a full suite of bioanalytical services including LC-MS, MSD, and ELISA methods for ADC testing.