Table 3.

Different types of linkers used in peptide-drug conjugates.

Type of Linker	Name	Comment
pH-responsive	Acetal/Ketal bond	An acidic environment in endosomes
	Hydrazone bond
	Vinyl ether bond
Enzyme responsive	Gly-Phe-Leu-Gly	Cathepsin B
	Phe-Lys
	Val-Citrulline
Redox responsive	Disulfide bond	Reduction in endosomes by glutathione
Non-covalent interaction	Amido bond	The potential of modifying the particle’s surface peptide layer to stabilize the particle for systemic in vivo injection or for targeting opens fascinating possibilities for improving cargo trafficking
	Carbon chain
	Ether bond