ADC manufacturing is an emerging and highly complex biopharmaceutical technology that has attracted significant attention from innovative biopharmaceutical companies and CDMO procurement professionals. Antibody–drug conjugates (ADCs) are a class of precision therapeutics in which highly potent small-molecule cytotoxic drugs are chemically linked to tumor-targeting antibodies. The design concept originates from the "magic bullet" theory, combining the specificity of monoclonal antibodies with the powerful cytotoxicity of highly toxic drugs, enabling safe delivery of the drug to tumor cells and selective release of the toxic component. As an increasing number of ADCs enter clinical development, a thorough understanding of ADC manufacturing processes, key quality attributes, and regulatory requirements is critical for biopharmaceutical developers.

What Is Antibody Drug Conjugate Manufacturing? A Technical Overview

The manufacturing of ADCs is a highly interdisciplinary process that integrates biopharmaceutical production, small-molecule chemistry, and advanced analytical technologies. The ADC manufacturing process includes several key steps, such as monoclonal antibody expression and purification, synthesis of highly potent payloads, and chemical linker conjugation. Compared with traditional biologics manufacturing (such as unconjugated monoclonal antibodies), ADC production must additionally address the safe synthesis of highly potent active pharmaceutical ingredients (HPAPIs) and the special requirements of conjugation conditions, resulting in higher process complexity and more stringent quality control. Due to the involvement of highly potent small molecules and the complexity of conjugation technologies, many companies choose to outsource ADC projects to experienced CDMOs to reduce risk and accelerate product development.

Core Components of ADCs: Antibody, Linker, and Payload

ADCs are typically composed of three core components: a targeting antibody, a chemical linker, and a cytotoxic payload.

l The antibody component is responsible for recognizing and binding to target antigens on the surface of tumor cells, and its specificity and affinity determine the targeting capability of the ADC;

l The payload is a highly potent small-molecule cytotoxic agent that exerts its toxic effect after internalization into the target cell;

l The linker chemically connects the payload to the antibody and is usually designed to remain stable in systemic circulation while releasing the drug only inside tumor cells.

For example, linkers may be designed to hydrolyze under low-pH conditions or to be cleaved by specific intracellular enzymes, ensuring that the drug is released at the desired site. Ideal payload characteristics include extremely high potency, low immunogenicity, and the presence of reactive functional groups for linker attachment. Common payloads include microtubule inhibitors (such as MMAE and DM1) and DNA-damaging agents (such as acridine-based conjugates), whose cytotoxicity is typically 100–1000 times greater than that of conventional chemotherapeutic drugs.

In summary, antibody selection must balance stability, internalization rate, and immunogenicity; linker design must balance in vivo stability with efficient intracellular release; and payloads must possess sufficient potency while allowing safe and controlled conjugation.

How ADC Manufacturing Differs from Traditional Biologics Production?

ADC manufacturing differs significantly from traditional monoclonal antibody production and presents additional challenges. First, in addition to standard antibody expression and purification, ADCs require the separate synthesis of highly potent cytotoxic payloads and the design of suitable linkers for conjugation. This means that ADC projects demand coordinated development of both macromolecules (antibodies) and small molecules (payloads), as well as expertise in chemical conjugation and toxicological evaluation.

Moreover, due to the extreme toxicity of payloads, ADC processes must be carried out in closed systems or multi-level containment environments to ensure operator safety. From a quality control perspective, beyond routine assessments of protein purity and activity, unique parameters such as the drug-to-antibody ratio (DAR) must be carefully monitored. As a result of this high technical complexity, many developers choose to collaborate with specialized ADC CDMOs and outsource their projects to accelerate clinical development and commercialization.

The Complete ADC Manufacturing Process Workflow

The ADC manufacturing workflow typically includes the following major stages: monoclonal antibody production and purification, synthesis and handling of cytotoxic payloads, linker design and chemical conjugation, purification of conjugates and removal of free drug, and final formulation and fill-finish. Each stage is closely interconnected and collectively determines product quality and accessibility. The key process steps of each stage are described below:

1. Monoclonal Antibody Production and Purification

Upstream cultivation: High-quality antibody genes must first be obtained and stably expressed. Cell lines (typically CHO cells) are expanded from seed cultures in the laboratory and then scaled up to large-scale fermentation in bioreactors. During this process, culture media, feeding strategies, temperature, and pH conditions must be optimized to achieve high antibody yields and consistent quality.

Downstream purification: After fermentation, cell debris is removed through centrifugation and filtration, followed by sequential chromatographic purification. Common purification schemes include Protein A affinity chromatography to capture antibodies, followed by ion-exchange and hydrophobic interaction chromatography to remove host-cell impurities, and final sterile filtration to obtain purified antibodies. The high-purity antibody produced at this stage serves as the starting material for subsequent conjugation.

Antibody modification: In ADC processes, purified antibodies often require chemical or enzymatic modification, such as introducing reactive functional groups at glycan moieties or specific sites on the antibody to enable subsequent payload conjugation. Strict process monitoring is required throughout this stage to ensure antibody stability and preservation of biological activity.

2. Cytotoxic Payload Synthesis and Handling

Cytotoxic payloads (HPAPIs) are typically extremely potent small-molecule compounds with special requirements for synthesis and handling. Payloads may be produced through chemical synthesis or semi-synthetic routes and are often synthesized and purified in dedicated high-containment facilities. These synthetic routes must be carefully designed, optimized, and validated. The final payload must meet very high purity standards (typically >99%) and contain the functional groups required for conjugation, while transportation and storage must be strictly controlled to prevent exposure. Throughout the entire process, operator safety is of paramount importance, requiring multiple protective measures such as ventilation control, contained operations, and specialized training.

3. Linker Design and Conjugation Chemistry

The role of the linker is to stably attach the payload to the antibody while allowing drug release in the target intracellular environment. Linkers are generally classified as cleavable or non-cleavable. Cleavable linkers utilize mechanisms such as acid-labile bonds, disulfide bonds, or peptide sequences that are cleaved under specific intracellular conditions to release the drug; non-cleavable linkers (such as cyclohexane maleimide structures) require complete lysosomal degradation of the antibody to release the active metabolite. Linkers are often further modified with hydrophilic groups (such as PEG) to improve ADC solubility and stability.

In terms of conjugation chemistry, traditional methods rely on covalent attachment to cysteine thiols or lysine amines on antibodies, which results in relatively random conjugation. In recent years, site-specific conjugation technologies have emerged, enabling payload attachment at predefined antibody sites to produce more homogeneous products. Common strategies include engineering antibodies to introduce additional cysteine residues (such as THIOMAB technology), enzymatic conjugation using native glycans or specific peptide sequences, and bioorthogonal conjugation using non-natural amino acids. For example, GlycoConnect™ technology first employs endoglycosidases to trim antibody glycans and uses glycosyltransferases to introduce azide functional groups; subsequently, a metal-free cyclooctyne–azide "click" reaction is used to conjugate a cyclooctyne-containing drug linker to the antibody. This two-step enzymatic–click approach enables the production of highly homogeneous ADC molecules.

4. Purification of ADCs and Removal of Free Drug

After chemical conjugation, the reaction mixture contains unconjugated antibodies (DAR 0), conjugates with different DAR values, free drug, and linker-related impurities. To obtain high-purity ADC products, multiple downstream purification methods are employed. For example:

Hydrophobic interaction chromatography (HIC): Separates ADC species with different DAR values based on hydrophobicity differences. HIC effectively distinguishes unconjugated antibodies (low hydrophobicity) from high-DAR species (high hydrophobicity) and is commonly used for DAR distribution analysis.

l Ion exchange chromatography (IEC): Adjusts charge distribution to help remove charged variants and improve overall protein purity.

l Gel filtration/size-exclusion chromatography (SEC): Removes or analyzes high-molecular-weight aggregates, ensuring aggregate levels meet specifications.

l Ultrafiltration and diafiltration (TFF): Used to concentrate ADC products and perform buffer exchange while removing non-aqueous impurities and low-molecular-weight substances.

Through these multidimensional purification processes, free toxins, unconjugated antibodies, and other small-molecule impurities are removed, yielding ADC formulations that meet quality standards.

5. Final Formulation, Fill-Finish, and Storage

After purification, ADCs must undergo final formulation and sterile fill-finish to be prepared as drug products. During formulation, appropriate buffer systems, pH values, and excipients are selected based on the physicochemical properties of the ADC (such as hydrophobicity and isoelectric point) to enhance solubility and stability. Common approaches include adding low concentrations of organic solvents or surfactants, as well as anti-aggregation agents and antioxidants, to prevent aggregation or degradation of hydrophobic ADCs. Following formulation, the product is sterilized through 0.22 μm filtration and filled under aseptic conditions. Glass vials or prefilled syringes are commonly used, and fill-finish equipment and filters are subjected to integrity testing and sterility assurance to ensure compliance with GMP requirements. After filling, ADC formulations may be stored frozen or refrigerated, with storage conditions and shelf life determined based on stability and real-time stability studies.