Process Development of Site-Specific Antibody-Drug Conjugates.

Antibody-drug conjugates (ADCs) combine the targeting specificity of monoclonal antibodies with cytotoxic small-molecule payloads, enabling selective delivery of highly potent therapeutics. As the field has advanced, conjugation strategy has become a primary driver of product quality, manufacturability, and development risk. Site-specific approaches include engineered cysteine residues, non-canonical amino acid (ncAA) incorporation, and enzyme-mediated conjugation. These platforms enable defined drug-to-antibody ratios (DAR) and improved product homogeneity, shifting control to molecular design and necessitating integrated process strategies across upstream, downstream, and conjugation to ensure consistent execution at scale. This chapter presents a practical framework for the process development and characterization of site-specific ADCs across the development lifecycle. Upstream, recovery, downstream, conjugation, and post-conjugation purification are treated as an integrated system, where variability at each stage has the potential to influence performance and quality of the final drug substance conjugate. Analytical strategies applied across these stages provide the link between process parameters and critical quality attributes, including DAR distribution, aggregation, and residual impurities. The transition from early-phase to late-stage development is considered in the context of increasing process understanding, definition of control strategies, and regulatory expectations. Considerations for process intensification, advanced analytics, and process analytical technologies (PAT) are also discussed, with emphasis on how early development decisions and process knowledge define commercial manufacturing strategy and execution.