**2.3 The linker**

The linker is the chemical bond that connects the cytotoxic payload to the antibody of ADCs. The linker is important to maintain stability of ADCs in plasma and to control the release of payload in the desired tumor site [6]. The linker can be cleavable or non-cleavable. Cleavable linkers are designed to be sensitive to the tumor environment where they can be chemically (hydrazone and disulfide based) or enzymatically degraded (glucuronide and peptide based) to release the payload [17]. Hydrazonebased linkers are acid sensitive or pH dependent [18]. These bonds are stable in plasma but hydrolyze in the lysosome and endosome where pH < 7 [18]. SG utilizes an acid sensitive, carbonate linker that is cleavable at low pH [19]. The most commonly utilized linker is the peptide linker, which is cleaved via lysosomal proteases such as cathepsin B that are typically overexpressed in cancer cells [20, 21]. This type of bond is employed in T-Dxd [22].

Non cleavable linkers such as thioether based linkers are more stable compared to cleavable linkers leading to less off-target toxicity. These bonds are not sensitive to the enzymatic and chemical environment of the tumor [23, 24]. When non-cleavable linkers are utilized, such as in T-DM1, the release of the cytotoxic agent takes place

after catabolism of the antibody component whereas enzymatic or chemical degradation of the linker releases the payload when cleavable linkers are utilized such as in T-Dxd and SG [25].
