Click-cleavable ADC Linkers

Linker technologies used in current clinical antibody–drug conjugates (ADCs) generally rely on intracellular processes – and therefore require ADC internalization – to liberate the cytotoxic payload. This limits the type of receptors that can be targeted with ADC therapies, especially in solid tumors. The recent emergence of the field of bioorthogonal chemistry has led to the development of several reactions that afford in vivo linker cleavage through reaction with an exogenous molecule, the activator, which is administered separately. This click-cleavable ADC approach allows the targeting of ADCs to noninternalizing targets in the tumor microenvironment, affording a strong bystander effect and greatly expanding the target scope for ADCs. The first and still most commonly applied bioorthogonal cleavage reaction with in vivo potential is pyridazine elimination (also known as click-to-release), which occurs upon the reaction of a trans-cyclooctene (TCO) with a tetrazine, which led to the first click-cleavable ADC technology. In this chapter, we will describe the development of TCO and tetrazine ADC linkers and activators, and we will compare the pyridazine elimination reaction to other bioorthogonal approaches for the triggered release of cytotoxins from ADCs.