Linker Design in Antibody-Drug Conjugates: Balancing Stability and Drug Release.

Antibody-drug conjugates (ADCs) have emerged as a powerful class of targeted therapeutics in many clinical areas, such as in oncology. Despite their efficacy, the onset of adverse events has been a major drawback in their clinical use. Among other explanations, the clinical performance of the ADCs has been associated with the chemistry of the linker connecting the antibody and payload. Linkers determine plasma stability, intracellular activation, and payload diffusibility, thereby influencing the therapeutic index, off-tumour toxicity, and by-stander activity. Mechanistic insights increasingly show that linker-payload properties govern catabolite permeability and intratumoral distribution, particularly in antigen-heterogeneous settings. Current developments include enzyme-cleavable and tumour-selective linkers, polarity-modulating masking strategies, alternative self-immolative spacers, and dual-trigger systems designed to enhance selectivity and decouple efficacy from toxicity. In parallel, linker behaviour intersects with broader mechanisms of tumour resistance. This review focuses on understanding these processes, which are essential for designing the next generation of linkers capable of improving stability, safety, and long-term therapeutic effectiveness across diverse tumour contexts.