Enzyme-triggered, cell penetrating peptide-mediated delivery of anti-tumor agents.

Conventional chemotherapy has little or no specificity for cancer cells, normally resulting in low drug accumulation at the tumor region (inefficacy) and drug-induced severe side effects (toxicity). Nowadays, new strategies have been developed to improve both the targeting ability and cellular drug uptake using active targeting ligands and drug internalization agents, which could recognize and interact with specific receptors overexpressed on tumor cells and then trigger a drug internalization process by transporting the cargos into cells. Among those strategies, enzyme-triggered cell penetrating peptide (CPP)-mediated systems seem to be a feasible approach. The expression level of specific enzymes like proteases, esterases or glycosidases is often higher in tumor cells than in normal tissues, and such concentration gradients can be exploited as a tool for targeted cancer therapy. CPPs are known to be effective in promoting membrane transportation of the drug cargos, rendering a deeper tumor permeation that could further enhance the therapeutic efficacy of the delivered drug. An enzyme-triggered, CPP-mediated system would combine these advantages to yield a system with the enhanced tumor targeting ability and internalization efficiency and so far many systems have been successfully exploited and applied to cancer therapy. In this review, typical enzymes applied in cancer theranostic systems were firstly reviewed, followed by analyzing pros and cons of cell penetrating peptides. Most importantly, different types of applications of enzyme-triggered CPP-mediated systems in tumor imaging were illustrated. Finally, the drug loaded applications, i.e. enzyme-triggered CPP-mediated systems in drug delivery were reviewed.