antibody–drug conjugate linker
The linker is the chemical bridge that ties the cytotoxic payload to the antibody in an antibody-drug conjugate, and it quietly decides whether the whole drug works. It has two opposite jobs to balance: hold the toxin firmly while the conjugate travels through the bloodstream, then let go and release the toxin once the antibody has delivered it to a tumor cell. A leash that is too loose drops its cargo early; one that is too tight never lets it loose.
Linkers come in two broad styles. Cleavable linkers carry a built-in trigger — a bond that is snipped by an enzyme inside the tumor cell, or that breaks in the acidic interior of the cell's recycling compartments — releasing free payload on arrival. Non-cleavable linkers have no such trigger; instead, the antibody is digested down to a fragment that still carries the payload, which then does its damage. The choice shapes how much toxin escapes, how it behaves, and how it affects neighboring cells.
Linker design is one of the most consequential and difficult parts of building an ADC. Premature release in the blood causes the very off-tumor toxicity the ADC was meant to avoid, while overly stable linkers can blunt potency. The chemistry of how the payload is attached and how many copies ride per antibody also affects the conjugate's stability, solubility, and safety.
Brentuximab vedotin uses an enzyme-cleavable linker that is cut inside the target cell to release its microtubule-poison payload, whereas trastuzumab emtansine uses a non-cleavable linker.
Two ADCs illustrating cleavable versus non-cleavable linker strategies.
How much payload a cleavable linker lets diffuse out of the dying cell governs the "bystander effect," where freed toxin also kills nearby tumor cells that the antibody did not directly reach — helpful against patchy tumors but a source of off-tumor toxicity.