Endogenous stimuli-responsive linkers in nanoliposomal systems for cancer drug targeting

There are various drug delivery systems (DDSs) among which nanoliposomal formulations are among the most prominent. Despite the superiority of nanoliposomal DDSs compared to conventional drug delivery methods, recent reports have claimed that they can deliver small amounts of the injected dose to target site by passive targeting. However, our understanding of tumor microenvironment features, including dysregulation of pH, the high intracellular concentration of glutathione, change in the amount and expression of some enzymes, reactive oxygen species, hypoxia, and ATP concentrations, has driven the scope of research into the use of these endogenous stimuli for a design of smart linkers. These linkers optimize the release of payloads in favorable target sites and avoid premature releasing in non-favorable off-target sites. In this review, we discuss particular linkers, which are able to respond to the specific endogenous conditions, and could be used in nanoliposomal DDSs, based on pathophysiological changes that occur in tumors. Furthermore, structural and chemical properties of these linkers and other potential linkers, which could be used in nanoliposomal DDSs, have been reviewed.