Processes for Constructing Homogeneous Antibody Drug Conjugates

Antibody drug conjugates (ADCs) are synthesized by conjugating a cytotoxic drug or "payload" to a monoclonal antibody. The payloads are conjugated using amino or sulfhydryl specific linkers that react with lysines or cysteines on the antibody surface. A typical antibody contains over 60 lysines and up to 12 cysteines as potential conjugation sites. The desired DAR (drugs/antibody ratio) depends on a number of different factors and ranges from two to eight drugs/antibody. The discrepancy between the number of potential conjugation sites and the desired DAR, combined with use of conventional conjugation methods that are not site-specific, results in heterogeneous ADCs that vary in both DAR and conjugation sites. Heterogeneous ADCs contain significant fractions with suboptimal DARs that are known to possess undesired pharmacological properties. As a result, new methods for synthesizing homogeneous ADCs have been developed in order to increase their potential as therapeutic agents. This article will review recently reported processes for preparing ADCs with improved homogeneity. The advantages and potential limitations of each process are discussed, with emphasis on efficiency, quality, and in vivo efficacy relative to similar heterogeneous ADCs.