ADC Linkers

ADC linkers are one of the three main components of antibody drug conjugates (ADCs) that connect an antibody to a payload via a chemical bond.

ADC linkers are a critical design element in antibody-drug conjugate development, directly influencing the stability, selectivity and therapeutic index of the final construct. The linker must maintain conjugate integrity in systemic circulation while enabling efficient payload release at the target site, whether through enzymatic cleavage, hydrolysis or reductive conditions within the tumour microenvironment. This balance between extracellular stability and intracellular lability remains one of the central challenges in ADC design.

Linkers are broadly classified as cleavable or non-cleavable. Cleavable linkers exploit physiological differences between plasma and intracellular compartments: acid-labile hydrazone linkers are cleaved at the low pH of the lysosome; disulfide linkers are reduced by elevated glutathione concentrations within the cell; protease-cleavable peptide linkers, typically valine-citrulline or valine-alanine dipeptides, are cleaved by cathepsin B. Non-cleavable linkers such as thioether-based systems rely on complete lysosomal degradation of the antibody to release an active metabolite, demanding a payload that retains activity following this process. The choice of linker class therefore depends heavily on the target biology, the payload and the intended mechanism of release.

The drug-to-antibody ratio (DAR) is determined in part by the attachment chemistry of the linker. Conventional maleimide-thiol conjugation to cysteine residues produces heterogeneous mixtures with variable DAR, which can compromise pharmacokinetic behaviour and tolerability. Site-specific conjugation technologies, including engineered cysteines, non-natural amino acid incorporation and enzymatic methods such as transglutaminase-mediated ligation, address this by delivering defined, homogeneous conjugates with improved therapeutic windows. Several approved ADCs illustrate these design principles: Kadcyla (ado-trastuzumab emtansine, Genentech/Roche) employs a non-cleavable SMCC linker; Adcetris (brentuximab vedotin, Seagen) uses a protease-cleavable valine-citrulline-PABC linker; Enhertu (trastuzumab deruxtecan, Daiichi Sankyo/AstraZeneca) incorporates a tetrapeptide cleavable linker with a self-immolative spacer.