ADC Manufacturing – A Comprehensive Guide to Drug Substance Manufacturing Services

• ADCs combine the targeting precision of antibodies with the potency of cytotoxic drugs. This approach has rapidly moved from concept to reality – by 2025 nearly 20 ADCs are approved.  
• Yet producing an ADC drug substance is far more complex than making a standard monoclonal antibody, because it requires chemically attaching a toxic small-molecule payload to a biologic. 

What Makes ADC Manufacturing Distinct from Conventional Biologics Production? 

Manufacturing an antibody-drug conjugate is inherently more complex than manufacturing a typical biologic alone. In conventional biologics production, the process centers on cultivating cells to express the protein and then purifying that single biomolecule. By contrast, ADC manufacturing involves multiple components and steps: the biologic (antibody) must first be produced, the highly potent drug-linker must be synthesized, and then the two are combined in an ADC conjugation process. In effect, two separate GMP manufacturing pathways converge into one final product. This multiplies the complexity at every stage, from production planning to quality control. 

One immediately apparent distinction is the extreme potency of ADC payloads. Traditional biologics like antibodies are not inherently toxic to handle, but ADC payloads are highly cytotoxic chemicals. As a result, ADC drug substance manufacturing must be conducted with extraordinary safety measures. Specialized high-containment facilities and protocols are required to protect workers and the environment from exposure. The compounds used can be dangerous upon direct contact or inhalation, so any contamination of people or cross-contamination of equipment must be strictly avoided. This is a far stricter level of hazard control than encountered in standard biologics production.¹ 

Another major difference is the heterogeneity of the product. In ADCs an antibody is conjugated randomly with toxic drug molecules, the result is a mixture of species with different drug-to-antibody ratios (DARs) and conjugation sites. Conventional lysine or cysteine conjugation methods produce ADC batches that are a heterogeneous blend of molecules with varying numbers of drugs attached, making their safety and efficacy less predictable. This contrasts sharply with most biologics, where each molecule is identical. The heterogeneity inherent to ADCs means that more complex analytics and stricter process controls are needed to ensure consistency. Indeed, industry trends are now shifting toward site-specific conjugation technologies to yield more uniform ADC products with controlled DAR and improved batch-to-batch consistency. Notably, the first site-specific ADC was approved in 2019, confirming the industry’s shift toward more homogeneous conjugates.² 

Key Stages of Drug Substance Manufacturing in ADC Production 

Despite its complexity, ADC drug substance manufacturing can be conceptually divided into a series of key stages, each requiring meticulous ADC process development. 

The process begins with the biologic component – producing the monoclonal antibody that will serve as the delivery vehicle. This is carried out using traditional biologics manufacturing techniques: cell culture or fermentation to express the antibody, followed by multiple purification steps (such as Protein A affinity and polishing chromatography) to yield a high-purity monoclonal antibody. The antibody drug substance is typically manufactured under GMP like any therapeutic mAb, often by a specialized biologics production team or facility. 

In parallel, the potent payload (typically a cytotoxic small-molecule drug) and its linker are synthesized through organic chemistry. This stage resembles a high-potency small-molecule pharmaceutical manufacturing process. It involves multi-step chemical synthesis and purification to produce a drug-linker intermediate that meets strict quality specifications (purity, potency, stability). Because these cytotoxic compounds require careful handling, this step is often conducted in contained chemistry labs or by specialist manufacturers equipped for high-potency APIs. The antibody and drug-linker are usually made separately and tested individually as GMP intermediates. 

Next comes the defining step of ADC manufacturing: chemically attaching the linker-payload to the antibody to create the conjugated molecule. The purified antibody and the drug-linker are brought together under specific reaction conditions (buffer, temperature, and a defined molar ratio) that facilitate conjugation. Common conjugation approaches include partial reduction of antibody disulfide bonds (to conjugate via cysteine thiols) or direct modification of surface lysine residues. The goal is to achieve the desired drug-to-antibody ratio and distribution without compromising the antibody’s integrity. This conjugation step is critically important because it ultimately determines the efficacy of the ADC product. The reaction parameters are carefully optimized to maximize conjugation yield while minimizing over-conjugation or aggregation. 

After the conjugation reaction, the crude ADC mixture contains the modified antibody along with undesired by-products (such as unreacted drug, unconjugated antibody, and aggregates). The nascent ADC must be purified, usually via a series of chromatographic steps designed to remove free drug and other impurities while retaining the conjugated antibody. Techniques like tangential flow filtration (TFF) may also be used for buffer exchange and concentration. The result is a purified bulk ADC drug substance meeting defined quality attributes. This bulk is often filtered for sterility and can be filled into sterile containers for storage.^3,4 

Critical Process Parameters and Control Strategies in ADC Manufacturing 

Given the complexity of ADC synthesis, a robust control strategy is essential to ensure each batch meets its quality targets. The conjugation reaction in particular is often highlighted as the central step that defines the final ADC product. In the chemistry, manufacturing, and controls (CMC) for ADCs, developers identify critical quality attributes (CQAs) and then determine the critical process parameters (CPPs) that influence those attributes. Perhaps the single most important parameter is the drug-to-antibody ratio achieved during conjugation. DAR directly impacts efficacy and safety: an insufficient drug load can make the ADC too weak against tumors, whereas an excessive drug load can alter the ADC’s pharmacokinetics or increase toxicity. Therefore, the conjugation reaction conditions are tightly controlled to consistently hit the desired DAR target.⁵ 

Process parameters such as the antibody concentration, drug-to-antibody feeding ratio, reaction temperature, pH, and reaction time must be optimized to drive the conjugation to the right extent. For example, in a cysteine-thiol conjugation, the degree of antibody reduction (number of disulfide bonds broken) is carefully tuned to expose a specific number of reactive thiols. Over-reduction could lead to structural instability or too many attachment sites, whereas under-reduction yields too few sites. Similarly, in lysine-based conjugation, the amount of linker-drug added relative to antibody and the reaction time are calibrated to achieve an average DAR in the intended range. In-process control testing is often implemented to monitor progress. If the DAR is trending too high or low, adjustments can be made to keep the batch within specifications.⁶ 

Additionally, any residual unconjugated drug or partially conjugated species must be minimized: using a slight excess of antibody (relative to drug) can help drive the reaction to consume most of the free drug, reducing leftover toxin that would later need removal. Post-conjugation, the purification strategy is an integral part of the control strategy as well. It reliably separates the desired ADC from free drug and impurities. 

Analytical and Quality Control Requirements for ADC Drug Substance 

The multifaceted nature of an ADC means its quality control strategy is correspondingly extensive. Both the biologic and the small-molecule aspects of the drug substance must be rigorously tested. In practice, an ADC undergoes a broader battery of analytical assays than a standard monoclonal antibody. A core focus of ADC analytics is confirming the drug-to-antibody ratio (DAR) and the distribution of drug loads in the product. Several complementary methods are employed. Another critical quality attribute is the level of residual unconjugated antibody (antibody that never got a payload) and other species.⁷ 

Importantly, biological activity must be verified as well. Even after conjugation, the antibody portion should retain the ability to bind its target antigen. Binding assays (such as ELISA or flow cytometry-based tests) are performed to confirm the ADC still recognizes the target. In addition, a cell-based cytotoxicity assay is often used to ensure that the ADC can indeed deliver its cell-killing effect to target cells.⁸ 

Critical quality attributes of ADC drug substances include the DAR (and its distribution), the amount of residual free drug, the amount of residual unconjugated antibody, and the presence of aggregates or charge variants, among others. Each of these must be quantified with sensitive, reliable methods. The data from all these assays are reviewed to ensure the ADC drug substance is consistent with the product’s specifications and regulatory expectations. Given the potency of ADCs, strict adherence to good laboratory practices and data integrity is also paramount during testing. In short, the analytical demands for ADCs are high, but they are necessary to guarantee that every batch of ADC released is safe, effective, and of the intended quality. 

Regulatory and GMP Considerations for ADC Drug Substance Manufacturing 

Antibody-drug conjugates straddle the boundary between biologics and small-molecule drugs, so regulatory expectations encompass elements of both. In general, authorities such as the FDA and EMA regulate ADCs as biological products, meaning they require a Biologics License Application (BLA) with the extensive CMC documentation typical for biologics. However, regulators will scrutinize the ADC’s chemical components and conjugation process in equal measure. This dual nature places a burden on manufacturers to meet the quality guidelines for biologics while also adhering to requirements for chemical drugs. It is not uncommon for regulatory review teams to involve both biologics and small-molecule specialists to adequately evaluate an ADC’s CMC package.⁹ 

A fundamental regulatory requirement is that the ADC drug substance be manufactured under strict GMP conditions. Given the potency of the payload, facilities must implement additional containment and safety measures beyond standard biologics suites. Dedicated production areas or single-use disposable equipment are often employed to prevent cross-contamination with other products. Indeed, only a limited number of sites worldwide are equipped for GMP ADC manufacturing, and these facilities are held to stringent controls for handling cytotoxic materials. Regulators expect to see evidence that the manufacturing environment and procedures effectively protect both product quality and worker safety. 

Selecting a Partner for ADC Drug Substance Manufacturing Services 

Faced with the complexity of ADC production, many biopharma companies decide to outsource this work to specialized contract manufacturers. In fact, it is estimated that the majority of ADC programs (on the order of 70-80%) rely on outsourcing for at least some manufacturing steps. Choosing the right CDMO is therefore a critical strategic decision. Only a few vendors have capabilities spanning all ADCs these areas, but those that do can greatly simplify the supply chain for their clients. By handling the biologics manufacturing services (antibody production), and conjugation process development in-house, a single CDMO can reduce the coordination burden on the sponsoring company.¹⁰ 

When evaluating potential partners, experience and safety record are paramount. ADC drug substance manufacturing demands niche expertise. The partner should have an analytical and quality capabilities. As described, ADCs require extensive analytical characterization; a capable biologics CDMO should offer state-of-the-art analytical labs familiar with DAR measurements, impurity analysis, and bioassays for conjugates. The quality systems must be robust. At Mabion, we are able to manage documentation and regulatory submissions for all aspects of ADCs. This gives our partners confidence that we understand the nuances of conjugation processes subject to regulatory oversight. 

Conclusion 

Selecting a partner for ADC drug substance manufacturing involves weighing technical capabilities, experience, capacity, and trust. Practical considerations like capacity, scalability, and communication come into play. ADC projects can quickly scale from small clinical batches to larger commercial demands if the product succeeds, so the chosen partner should have the capacity to scale up production (or clear plans to expand when needed). Close collaboration is also important. ADC development benefits from iterative feedback between chemistry and biologics teams, so a partner with strong project management and open communication practices can accelerate problem-solving. Some companies even engage a CDMO early, during process development, to leverage their expertise in designing a manufacturable ADC process. A good partner will essentially act as an extension of the sponsor’s team, providing specialized knowledge and infrastructure. The right CDMO can de-risk the program by providing end-to-end ADC drug substance manufacturing services with high quality and reliability. In a field as complex as ADCs, a strong partnership can make the difference between a smooth path to clinic or commercialization and costly setbacks. 

FAQ

Prepared by:

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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