Why do some antibody-drug conjugates use cleavable linkers and others non-cleavable ones, and how does the 'bystander ef

The choice between cleavable and non-cleavable linkers in antibody-drug conjugates (ADCs) is a strategic design decision based on how the drug is intended to kill the target cancer cell and whether it can influence neighboring cells.

Cleavable Linkers

Cleavable linkers are designed to break down under specific conditions—typically within the lysosome of the target cell. This can happen through enzymatic cleavage (like cathepsin B), acidic pH, or the presence of reducing agents like glutathione.

• The Benefit: Once the linker is cleaved, the payload (the toxic drug) is released in its original, potent form.
• The Bystander Effect: Because the released payload is often a small, hydrophobic molecule, it can easily diffuse across the cell membrane after the target cell dies. This allows the drug to exit the target cell and enter neighboring tumor cells, even if those cells do not express the target antigen. This "bystander effect" is highly beneficial in heterogeneous tumors where the drug may not reach every single cell directly.

Non-Cleavable Linkers

Non-cleavable linkers do not rely on environmental triggers to break. Instead, the entire antibody-linker-drug complex must be degraded by the cell's own internal machinery (specifically lysosomal proteases that degrade the antibody protein itself).

• The Benefit: Because the linker does not break, the payload is released as a metabolite attached to a single amino acid (usually cysteine or lysine from the antibody). This structure is often charged or more polar, making it difficult for the payload to pass through the cell membrane.
• The Bystander Effect: Because the payload remains trapped inside the target cell due to its polarity/charge, it cannot leak out to affect neighboring cells. This minimizes "off-target" toxicity to healthy cells surrounding the tumor, as the drug is effectively "locked" within the cell that initially internalized it.

How the Choice is Made

The decision comes down to a balance between efficacy and safety:

1. Choose Cleavable if the tumor is highly heterogeneous or has poor vascularization. You want the bystander effect to ensure the drug spreads through the tissue to kill cells that the antibody might not reach.
2. Choose Non-Cleavable if the goal is to increase the therapeutic index (safety). If the target antigen is also present on essential healthy tissues at low levels, using a non-cleavable linker keeps the toxicity strictly confined to the target cells, reducing the risk of damaging nearby healthy bystander cells.

In short, cleavable linkers act like a "grenade" that affects the immediate area, while non-cleavable linkers act like a "precision strike" confined to a single house.