Challenges and Solutions in ADC Development for Biologic Therapeutics

• Antibody-drug conjugate development is combining the specificity of monoclonal antibodies with the potency of cytotoxic agents. ADC development presents complex scientific, analytical, and regulatory challenges. 
• As of 2026, a total of 21 ADC drugs have been approved globally. There are more than 200 clinical-stage candidates in the global pipeline of ADCs that target more than 50 antigens. Notably, as shown in the table below, 41 ADCs have already progressed to Phase III clinical trials. Ten major targets are the focus of about 40% of these ADCs. 

Defining ADC Development in the Context of Biologic Therapeutics 

Antibody-drug conjugates (ADCs) are biologics that combine an antigen-selective monoclonal antibody with a cytotoxic payload via a chemical linker, aiming to increase tumor-selective drug delivery and thereby widen the therapeutic window compared with conventional systemic chemotherapy. ADC is one of the fastest growing areas in the field of anticancer drugs. The emergence of ADCs represents a huge advance for biopharma in ADC development of targeted therapies. Key challenges relate to how to design and combine the individual components of ADCs to achieve drugs with high efficacy, precision of release, and safety profile.¹ 

In ADC development, antibody selection begins upstream of chemistry. The target antigen must be sufficiently tumor-enriched, accessible to circulating IgG, and compatible with the intended linker-payload system. Because many ADCs ultimately rely on cellular uptake and intracellular processing, antigen internalization kinetics and trafficking routes become central to target antigen selection and validation. 

The general mechanism of action of antibody-drug conjugates is multi-step. After administration into the circulatory system (1), the drug recognizes a specific antigen on the surface of a cancer cell. It binds to it, forming an antigen–ADC complex (2). The entire complex is then internalized, primarily via receptor-mediated endocytosis, leading to the formation of an early clathrin-coated endosome (3). 

In ADC development, within the early endosome, ADCs may undergo transcytosis back to the extracellular space (4a) or proceed through endosomal maturation. The late endosome is characterized by an acidic environment, which promotes linker cleavage and the release of the cytotoxic drug (4b). The late endosome subsequently fuses with the lysosome (5), where the ADCs is exposed to proteolytic enzymes and an increasingly acidic environment, facilitating further release of the payload (6). 

The free drug exerts its cytotoxic effect through pathway specific to its mode of action. It can induce apoptosis by directly damaging DNA or by disrupting microtubules (7). Certain payloads – sufficiently hydrophobic to cross cell membranes – can also induce a bystander killing effect (8), eliminating neighboring cancer cells. This mechanism is particularly relevant in the treatment of solid tumors. 

Selecting Optimal Antibody Candidates During ADC Development 

Antibody selection should be coupled early to quantitative delivery/processing data, because the most effective antibody for a naked mAb is not always the best antibody for ADC development once intracellular trafficking and payload activation are considered. Optimal antibody candidates for ADC development therefore balance: 

• Sufficient affinity/avidity for durable tumor targeting 

• Enough dissociation to permit tissue penetration 

• Internalization/processing behavior that matches the payload release mechanism 

Higher antibody affinity may improve the precision of drug delivery; however, excessively strong binding can paradoxically reduce deep tumor penetration. This effect is particularly relevant in solid tumors with diffusion-limited transport, where the binding-site barrier phenomenon may occur, leading to reduced bystander killing.  

A well-documented example is the clinical analysis of anti-HER2 ADCs. Four drugs targeting this receptor have been approved worldwide: T-DM1 (Kadcyla, Trastuzumab emtansine), T-DXd (DS-8201, Enhertu, Trastuzumab deruxtecan), Disitamab vedotin and Trastuzumab rezetecan. In this model, antibodies with moderate affinity achieved the highest tumor accumulation, whereas those with the highest affinity demonstrated the lowest tumor accumulation and remained predominantly perivascular, with significantly shorter mean penetration distances from blood vessels. The same study also linked affinity to antigen-driven internalization and catabolism, describing scenarios in which internalization outpaces antibody dissociation, thereby increasing degradation and limiting the availability of diffusible antibodies for deeper tumor distribution.³ 

Advancements in antibody engineering in ADC development, particularly in the development of highly potent cytotoxic agents, along with the design of more stable linkers and site-specific bioconjugation techniques, have led to the creation of ADCs with improved safety profiles. Precisely delivered and highly effective payloads targeting cancer cells minimize off-target effects on healthy tissues. Clinical research ultimately resulted in the development of T-DM1, which became the first ADC approved for the treatment of solid tumors.⁴ 

Linker and Payload Design Challenges in ADC Development 

Payload selection ADC decisions are constrained by delivery physics. Only a finite number of antibodies reach any tumor cell, so the cytotoxin must typically be highly potent and suited to the target disease context. Clinically validated payload classes include microtubule inhibitors (e.g., auristatins, maytansinoids) and DNA-damaging or topoisomerase I inhibiting agents, and differences in mechanism of action can influence antitumor activity and toxicity spectra, shaping the therapeutic window. The payload’s physicochemical properties (notably hydrophobicity and membrane permeability) feed back into ADC stability, aggregation propensity, clearance, and bystander effects.⁵ 

Linker design must be stable enough in systemic circulation to prevent premature payload release (which would convert an ADC back into “systemic chemotherapy”). Linker chemistry, conjugation site, and release kinetics can shift not only the exposure of total antibody but also the exposure of conjugated drug and unconjugated payload, complicating exposure-response interpretation and bioanalytical strategy in ADC development. Payload potency and linker-triggered release directly affect pharmacokinetics. Increasing drug loading improved in vitro potency but accelerated clearance and reduced tolerability, yielding a narrower effective window at high drug loading.⁵ 

A prominent solution trend is to engineer linker-payload systems that preserve systemic stability while enabling effective tumor killing even under heterogeneous antigen expression. The DS-8201a developmental program was designed with a potent topoisomerase I inhibitor payload (DXd) and showed a strong bystander killing effect in vitro and in vivo, attributed to high membrane permeability of the released payload. It suppressed HER2-negative neighboring cells when co-located with HER2-positive cells, but did not impact distant HER2-negative tumors, supporting a mechanistically bounded bystander effect. Clinically, the superior PFS outcomes of T-DXd versus T-DM1 in DESTINY-Breast03 study are consistent with the idea that optimized linker–payload design can convert improved intratumoral payload delivery into major clinical benefit.^7,8 

Controlling Heterogeneity and Drug-to-Antibody Ratio in ADC Development 

Antibody homogeneity and compatibility with linker-payload chemistry and conjugation strategy are critical parameters in ADC development, as they improve product consistency and reduce the loss of potentially effective drug candidates. The majority of antibodies approved for use in ADCs are derived from three IgG isotypes (IgG1, IgG2, and IgG4), primarily due to their high avidity for target antigens and extended serum half-life. ⁵ 

In addition to antibody-related factors, heterogeneity in ADCs is strongly influenced by conjugation-driven variability, particularly in drug-to-antibody ratio (DAR).⁹ Understanding Drug-to-Antibody Ratio (DAR) distribution better requires connecting physical chemistry to disposition. Increased DAR often increases ADC surface hydrophobicity, which can accelerate plasma clearance and reduce exposure, undermining in vivo efficacy despite higher intrinsic cytotoxicity.¹⁰ From an ADC analytical characterization perspective, programs therefore rely on high-resolution mass spectrometry to quantify DAR profiles, and monitor lot-to-lot consistency. This analytical linkage between DAR distribution and pharmacokinetics is the basis for rational setting of release specifications and stability-indicating methods in biologics manufacturing.¹¹ 

ADCs with DARs of 2, 4, 6, and 8 can be produced with improved homogeneity compared to products obtained by lysine conjugation. This is historically most widely used conjugation method in commercial products¹² and remains relevant in ADC development. 

A strategy to reduce heterogeneity involves enhancing control over site-specific bioconjugation on the antibody, enabling tighter regulation of the DAR and improved safety margins.¹³ 

Regulatory Considerations and Risk Mitigation in ADC Development 

Regulatory expectations for ADC development reflect the modality’s dual nature: an ADC behaves like a monoclonal antibody in distribution and immunogenicity considerations, yet its clinical behavior is also shaped by small-molecule payload disposition and release. A current standard is FDA’s guidance on clinical pharmacology considerations for ADCs with cytotoxic small-molecule payloads, which highlights bioanalytical approaches, dosing strategies, exposure–response analysis, intrinsic factors, immunogenicity, and drug-drug interactions as key development considerations.¹⁴ 

Risk mitigation in ADC manufacturing begins by defining and controlling specific critical quality attributes that extend beyond those of monoclonal antibodies. ADCs require validated control of conjugation-related attributes such as DAR distribution, free (unconjugated) payload, residual linker-related impurities, deconjugation products, and stability mechanisms that can shift potency and safety over shelf life. 

FAQ

Prepared by:

Weronika Rabij

Team Leader, Process Biotechnologist

w.rabij@mabion.eu

Weronika Rabij

Team Leader, Process Biotechnologist

Weronika Rabij is a Process Biotechnologist in the Manufacturing Department at a Mabion Biologics CDMO, supporting the execution of bioprocesses used in the production of biologic medicines. Her role includes overseeing manufacturing operations, maintaining GMP-compliant documentation, and supporting process consistency, quality, and efficiency across production activities. She also collaborates with cross-functional teams to troubleshoot issues, support process improvements, and help ensure reliable manufacturing performance.

Jakub Knurek

Marketing Specialist

j.knurek@mabion.eu

Jakub Knurek

Marketing Specialist

Jakub Knurek is a marketing and business development expert specializing in CDMO operations with scientific and biopharmaceutical background. During the COVID-19 pandemic, he contributed to the development of the Novavax protein vaccine. As a side project, he runs his own pharma company.

As the author of numerous articles, he is recognized as an important voice of the younger generation of scientists. He is a member of Mabion’s representation at industry trade shows, including BIO International Convention 2025 in Boston, 2nd Global Biologics India 2025 in Hydrabad, CPHI 2024 in Milan, CPHI 2023 in Barcelona, Life Sciences Baltics 2023 in Vilnius. He actively participates in academic, business and industry discussions, such as the “Building the Future Pharma Workforce” panel at CPHI Milan 2024.

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