Table 2. Summary of enzyme-responsive peptide linkers in PDC constructions.

Type of enzyme	Representative peptides	Cleavage site	Key features
Cathepsin B	Val-Cit	Tumor endosomes	• Clinically validated
	Val-Ala		• Heterogeneous payload release
			• Strong hydrophobicity leads to easy aggregation
	GFLG	Tumor endosomes	• Enhanced enzyme accessibility
			• Sustained intracellular release
			• Increased molecular weight and synthetic complexity
MMPs	GPLGIAG	Tumor extracellular microenvironment	• High specificity for MMP-2/9
	PLGLAG		• High off-target risk in non-tumor tissues
	PVGLIG
PSA	SSKYQSL	Prostate tumor extracellular microenvironment	• PSA enzyme selectivity
			• Prone to being affected by PSA expression levels
			• Restrict in prostate cancer treatment