A framework for evaluating unexpected reactions in antibody-drug conjugates during clinical development: a case study of a promiscuous payload

Antibody-drug conjugates (ADCs) are complex biotherapeutic molecules, comprising an antibody moiety and a small molecule linker-drug. The product quality of an ADC is controlled through monitoring of critical quality attributes to ensure safety and efficacy. During development, unexpected changes may occur in the product quality of an ADC, for example under stress or physiological conditions. This should prompt a systematic evaluation to determine the cause and impact on product quality. A structured approach for such an evaluation is not well established. Here, a strategy for the detection, identification, and assessment of unexpected reactions in an ADC during clinical development is presented encompassing physicochemical and functional analytical characterization, mechanistic confirmation, and evaluation of the relevance in vivo. The strategy is illustrated by a real-world example of an ADC in clinical development, SYD1875, exhibiting an unexpected decrease in biological activity following 3 months of storage at the stressed condition. Comprehensive analytical characterization revealed an intramolecular cross-link between the linker-drug and a glutamic acid in the light chain of the antibody. Mechanistic studies confirmed that this cross-link abolishes the cytotoxicity of the ADC. Subsequent bioanalytical and pharmacokinetic investigations demonstrated that, although the cross-link is also formed in vivo, its overall impact on systemic exposure was limited. The generalizable framework described in this report aims at facilitating the evaluation of anomalies in the product quality of an ADC and subsequent (clinical) risk assessment, thereby supporting decision making.