A modular framework for antibody-drug conjugate biomarkers.

The availability of approved antibody-drug conjugates (ADCs) has rapidly expanded across oncology indications, yet biomarker development remains dominated by semi-quantitative immunohistochemistry. Robust biomarkers are needed to improve patient selection, guide therapeutic sequencing and identify resistance mechanisms that inform rational combinations. ADC activity depends on target antigen expression, linker chemistry and payload characteristics all within the context of the tumour microenvironment. Consequently, treatment response cannot be predicted solely by target expression. A more comprehensive biomarker framework is required that accounts for antigen abundance and accessibility, intracellular trafficking and linker cleavage, and tumour susceptibility to the payload. Furthermore, biomarkers of toxicity have been largely overlooked, even though toxicity can be equally determinant of clinical utility. In this Review, we outline a modular framework for ADC biomarker development organized around these components. We distinguish established evidence from emerging strategies and discuss technologies that could enable assessment of these determinants. We consider how modular biomarkers might inform patient selection, therapeutic sequencing, rational combinations, dose optimization and regulatory strategy while highlighting the various challenges that must be addressed before broad clinical adoption. Harmonized development across trials, tumour types and geographical regions will be essential to move biomarker development beyond empirical target-expression thresholds.