Engineering Better ADCs, Part 2: Characterization, CMC, And More

By Robert Dream

In the first article of this two-part article series on engineering better ADCs, I covered MoA, conjugation chemistry and site-specific conjugation, the drug-to-antibody ratio, linker technology, and payload technology. In this second article, I cover antigen selection and tumor biology, pharmacokinetics and pharmacodynamics, analytical characterization, manufacturing and CMC, toxicity and safety, resistance, limitations to tackle, and next-generation ADC technology.

Antigen Selection And Tumor Biology

An ADC cannot be optimized solely by improving its chemistry. The biology of the target antigen is equally important. A suitable target should ideally exhibit:

High tumor expression + low normal-tissue expression + surface accessibility + appropriate internalization + favorable trafficking

However, perfect tumor-specific expression is uncommon.

Normal tissues may express low levels of the same antigen, resulting in on-target/off-tumor toxicity. Additionally, tumors are biologically heterogeneous, meaning that antigen expression can vary between patients, between tumors in the same patient, or even between cells within a single tumor. This heterogeneity is one reason why bystander-capable ADCs can be attractive in certain settings.

Pharmacokinetics And Pharmacodynamics

ADC pharmacology is more complicated than that of either a conventional monoclonal antibody or a conventional small molecule drug.¹

Several molecular species may coexist in vivo:

1. Intact ADC
2. Deconjugated antibody
3. Partially deconjugated ADC species
4. Free payload
5. Payload-containing metabolites
6. Antibody degradation products

The concentrations and time courses of these species can differ substantially.

Important pharmacokinetic parameters include:

• maximum concentration
• area under the concentration–time curve
• clearance
• volume of distribution
• half-life
• exposure to total antibody
• exposure to conjugated antibody
• exposure to free payload.

The FDA specifically emphasizes the need for appropriate bioanalytical characterization and exposure–response evaluation during ADC development.

Analytical Characterization Of ADCs

Because ADCs are structurally heterogeneous and chemically complex, comprehensive analytical characterization is essential. Important analytical techniques include:

1. Mass spectrometry: LC–MS and high-resolution mass spectrometry can characterize:

• molecular mass
• DAR distribution
• conjugation sites
• linker stability
• drug-related species.

2. Hydrophobic interaction chromatography (HIC): This is frequently used to assess ADC DAR distributions because increasing payload loading often changes hydrophobicity.

3. Size-exclusion chromatography (SEC): This is used to evaluate:

• aggregates
• monomer content
• high-molecular-weight species.

4. UV/visible spectroscopy: Absorbance measurements can provide estimates of antibody and payload concentrations and can contribute to DAR determination.

5. Capillary electrophoresis: Various electrophoretic methods can assess molecular heterogeneity, fragmentation, and conjugation-related changes.

6. Binding assays: Antigen-binding assays are necessary to determine whether conjugation has compromised antibody recognition.

7. Cell-based assays: Functional assays evaluate:

• target-dependent cytotoxicity
• internalization
• payload activity
• bystander killing
• resistance mechanisms.

The analytical strategy must therefore connect molecular structure → physicochemical properties → biological activity.

Manufacturing And Chemistry, Manufacturing, And Controls

ADC manufacturing presents challenges beyond those encountered with conventional monoclonal antibodies.² The process generally involves:

Antibody production → purification → conjugation → removal of unreacted components → purification/formulation → analytical characterization

Important manufacturing considerations include:

• reproducibility of conjugation
• control of DAR
• removal of free payload
• removal of unconjugated linker
• control of aggregates
• control of degradation products
• batch-to-batch consistency
• sterility and bioburden
• stability during storage.

Site-specific conjugation can potentially simplify some aspects of product characterization because the resulting molecular population is more controlled. Nevertheless, every conjugation technology introduces its own process development and scale-up requirements. Recent reviews emphasize that conjugation strategy must be considered not only from a chemical perspective but also from chemistry, manufacturing, and controls (CMC) and clinical perspectives.

Toxicity And Safety

The fundamental goal of an ADC is to increase the therapeutic index of a highly potent cytotoxic drug by directing it preferentially toward tumor cells. However, ADCs are not completely tumor-specific. Toxicity can arise through several mechanisms:

1. On-target/off-tumor toxicity: The target antigen may be expressed at lower levels in normal tissues.

2. Premature payload release: Instability of the linker can lead to free payload entering systemic circulation.

3. Bystander toxicity: Membrane-permeable payloads can affect neighboring cells, including normal cells.

4. Antibody-mediated toxicity: The antibody component itself can interact with normal tissues or immune pathways.

5. Payload-related toxicity: Once released, the payload behaves according to its pharmacological mechanism and may cause characteristic toxicities.

Thus, successful ADC development requires optimization of the entire molecular system rather than merely increasing payload potency. Linker stability, conjugation site, DAR, antigen biology, and payload properties must all be considered simultaneously.

Resistance To ADCs

Despite their sophistication, ADCs can encounter multiple resistance mechanisms.³ These include:

• loss or downregulation of the target antigen
• altered antigen internalization
• changes in intracellular trafficking
• impaired lysosomal processing
• reduced linker cleavage
• altered drug efflux
• mutations in the payload target
• enhanced cellular survival pathways
• tumor heterogeneity.

ADC resistance is therefore a multilevel phenomenon extending from the cell surface to intracellular drug action. Recent literature describes ADC activity as involving several interconnected processes, including target recognition, internalization, intracellular payload release, and bystander effects.

Overcoming resistance may involve:

• alternative targets
• different payload classes
• improved bystander-capable payloads
• novel linkers
• bispecific antibodies
• combination therapy
• sequential use of ADCs with different mechanisms.

The Limitations Of ADC Technology For The Industry To Tackle

Despite significant progress, several limitations remain.

Biological limitations:

• Tumor heterogeneity
• Insufficient antigen expression
• Antigen loss
• Poor tumor penetration
• Variable internalization

Chemical limitations:

• Linker instability
• Payload hydrophobicity
• Limited payload compatibility
• Conjugation heterogeneity

Manufacturing limitations:

• Complex purification
• DAR control
• Free-drug removal
• Aggregation
• Scale-up of site-specific processes

Clinical limitations:

• Off-target toxicity
• Drug resistance
• Limited therapeutic window for some constructs
• Patient-to-patient variability

Consequently, ADC development is fundamentally an optimization problem involving multiple interacting variables rather than a simple antibody-plus-drug formulation exercise.

Next-Generation ADC Technology

The next generation of ADCs is moving beyond the classical three-component model toward increasingly sophisticated molecular architectures.

1. Highly site-specific ADCs: Site-specific conjugation is expected to remain a major area of development because controlled conjugation can reduce molecular heterogeneity and improve control over critical quality attributes.

2. Novel payloads: New payload classes are expanding the biological mechanisms available for ADC therapy, including novel DNA-damaging and topoisomerase-targeting agents.

3. Improved linkers: Future linkers are being designed to improve:

• plasma stability
• intracellular release
• payload solubility
• bystander activity
• therapeutic index.

Linker chemistry remains one of the central determinants of ADC performance.

4. Bispecific and biparatopic ADCs: Bispecific and biparatopic antibody formats may allow recognition of multiple epitopes or antigens, potentially improving tumor selectivity and overcoming antigen heterogeneity.

5. Alternative antibody formats: Smaller antibody fragments and engineered antibody formats may improve tumor penetration, although reduced systemic half-life and altered pharmacology must be carefully considered.

6. Data-driven ADC design: Computational modeling and machine learning are increasingly being investigated for predicting relationships among antibody sequence, antigen properties, linker structure, payload chemistry, DAR, and biological activity. Recent research has specifically explored machine learning approaches for predicting ADC activity from integrated antibody, linker, payload, and DAR information.

ADC Development As An Integrated Design Problem

A major conceptual shift in ADC science is the recognition that ADC performance cannot be predicted from any single component. The therapeutic outcome can be viewed as:

Antigen biology × antibody properties × conjugation site × DAR × linker chemistry × payload potency × intracellular trafficking × pharmacokinetics × tumor microenvironment

This means that the optimal antibody is not necessarily the antibody with the highest affinity, the optimal payload is not necessarily the most potent drug, and the optimal DAR is not necessarily the highest DAR. Instead, successful ADC development requires balanced optimization of the entire molecular system.

This is particularly important for site-specific conjugation. Although controlled conjugation can produce homogeneous molecules, the chosen conjugation site can affect antibody structure, antigen binding, linker accessibility, payload exposure, stability, and pharmacokinetics. Consequently, site selection itself should be considered a drug design variable rather than merely a manufacturing parameter.

Future Perspectives

The future of ADC technology is likely to be characterized by increasingly precise control over molecular architecture. Several areas are particularly promising.⁴

First, site-specific conjugation is likely to become increasingly important as developers seek homogeneous products with predictable DAR and improved developability.

Second, linker innovation will continue to determine how effectively payload release can be controlled. The ideal linker should remain stable during systemic circulation while efficiently releasing the pharmacologically active species at the desired site.

Third, payload diversification will enable ADCs to exploit mechanisms beyond traditional microtubule inhibition and may provide opportunities to overcome resistance.

Fourth, antibody engineering may improve tumor penetration, internalization, antigen selectivity, and pharmacokinetics.

Finally, the integration of computational modeling, structural biology, high-throughput screening, and machine learning may enable more rational ADC design. Rather than optimizing antibodies, linkers, payloads, and conjugation strategies independently, future development may use integrated predictive models to identify combinations with the greatest probability of achieving a favorable therapeutic index.

Conclusion

Antibody–drug conjugates represent one of the most sophisticated applications of modern bioconjugation chemistry and targeted drug delivery. Their fundamental concept is straightforward — use an antibody to deliver a potent cytotoxic agent to a selected cell population — but successful implementation requires the integration of antibody engineering, tumor biology, medicinal chemistry, linker design, conjugation chemistry, analytical science, pharmacology, and pharmaceutical manufacturing.

The three major components of an ADC — the antibody, linker, and payload — each contribute distinct but interconnected functions. The antibody provides target recognition, the linker regulates stability and drug release, and the payload produces the desired pharmacological effect. The development of controlled and site-specific conjugation technologies has substantially improved the homogeneity and reproducibility of ADCs compared with many conventional random conjugation approaches. Strategies including engineered cysteine conjugation, glycan-directed conjugation, enzyme-mediated conjugation, and non-canonical amino acid incorporation provide increasingly precise control over conjugation site and DAR.

Nevertheless, ADCs remain complex therapeutic systems with significant biological and technological challenges. Target heterogeneity, premature drug release, off-target toxicity, resistance, aggregation, and manufacturing complexity continue to limit some programs. The future of the field will therefore depend not simply on producing more potent ADCs but on achieving rational control of the complete ADC architecture.

Ultimately, next-generation ADC technology is moving toward highly homogeneous, site-specific, pharmacologically optimized molecules in which antibody, linker, payload, conjugation site, and DAR are deliberately engineered as a single therapeutic system. These advances have the potential to expand the therapeutic window of cytotoxic drugs and establish ADCs as an increasingly important platform for precision cancer therapy.

References:

1. U.S. Food and Drug Administration. Clinical Pharmacology Considerations for Antibody-Drug Conjugates: Guidance for Industry. March 2024.
2. A review of conjugation technologies for antibody drug conjugates. Recent review addressing nonspecific, site-specific, and selective conjugation approaches, including their CMC considerations.
3. Resistance to antibody–drug conjugates: A review. Recent review addressing ADC mechanisms of action and mechanisms of therapeutic resistance.
4. Advances and Future Directions in Antibody-Drug Conjugates: From Paradigm Shifts to Data-Driven Design. Recent review covering target selection, antibody engineering, linker and payload development, site-specific conjugation, resistance, and computational approaches.

About The Author:

Robert Dream is a recognized industry leader with over 35 years of experience in the life sciences sector, including executive leadership roles. He has successfully led projects, optimized processes, and scaled products by leveraging operational excellence and deep technological expertise. Business-minded and strategically focused, Dream brings functional knowledge across manufacturing, supply chain, and regulatory domains. His background includes extensive hands-on and senior executive experience in therapeutic biotechnology and biological product manufacturing at world-leading organizations. A prolific contributor to the industry, Dream has authored numerous articles, industry guidances, and delivered many presentations.