Engineered Cell-Penetrating Peptides for Mitochondrion-Targeted Drug Delivery in Cancer Therapy.

Mitochondrion is a promising target in cancer therapy. However, gaining access to this organelle is difficult due to the obstacles to cross the complicated mitochondrial membrane. Cell-penetrating peptides (CPPs) with mitochondrion-targeting ability, named mitochondrion-targeting peptides (MTPs), are efficient tools to deliver exogenous therapeutics into mitochondria. Herein, we report several new MTPs, which can be readily synthesized via resin-based solid-phase peptide synthesis. In particular, MTP3 (compound 5), consisting of three positively charged arginines and two D- and L- alternating naphthylalanines, demonstrated excellent mitochondrion-targeting ability with high Pearson's correlation coefficient, suggesting that MTP3 has good potential for mitochondrion-targeted drug delivery. As proof-of-concept, the feasibility of MTP3 was validated by the preparation of a mitochondrion-targeting prodrug (compound 17, doxorubicin-based prodrug). This prodrug was subsequently confirmed to be specifically transported to the mitochondria of tumour cells, where it was able to release the native doxorubicin upon intracellular GSH activation, leading to mitochondrial depolarization and eventually cell death. Importantly, compound 17 showed good cytotoxicity against human tumour cells while negligible toxicity towards normal cells, indicating its potential as a potent mitochondrial medicine for targeted cancer therapy. Our study thus opens a way for engineered CPPs to be used to deliver bioactive cargos in mitochondrion-targeted cancer therapy.