Dimeric camptothecin-loaded RGD-modified targeted cationic polypeptide-based micelles with high drug loading capacity and redox-responsive drug release capability.

Camptothecin (CPT) is a broad spectrum anticancer drug, but its application is limited due to the poor water solubility, lactone ring instability, and low drug loading potential. In this study, biocompatible cationic polypeptide-based micelles were developed to deliver dimeric CPT (DCPT) with the aim of overcoming the above-mentioned obstacles and achieving favorable therapeutic effects. Cationic polypeptide poly-lysine-block-poly-leucine (PLys-b-PLeu) was fabricated via the ring-opening polymerization of N-ε-carbobenzoxy-l-lysine (ε-Lys(Z)) and l-leucine (Leu) and further grafted with polyethylene glycol (PEG) and an arginine-glycine-aspartic acid (RGD) peptide. DCPT was synthesized by reacting CPT and 2-hydroxyethyl disulfide, and micelles were prepared using a dialysis method. The obtained DCPT-loaded RGD-PEG-g-poly-l-lysine-b-poly-l-leucine (DRPPP) micelles showed a high encapsulation efficiency of 89.7% and a high drug loading capacity of 46.1%. In addition, the DRPPP micelles remained stable under physiological conditions (PBS at a pH of 7.4) but showed rapid release when triggered by a reductive environment (PBS at a pH of 7.4 with 10 mM dithiothreitol). Compared to micelles without RGD decoration, the DRPPP micelles exhibited an increased cellular uptake through RGD targeting and were internalized into cells via caveolae-mediated endocytosis and macropinocytosis. Furthermore, the DRPPP micelles exerted an enhanced cytotoxicity against MDA-MB-231 cells compared to MCF-7 cells, which expressed less αβ receptors. Besides, the DRPPP micelles induced cell apoptosis and caused a decrease of mitochondrial membrane potential. These results indicate that dimeric camptothecin-loaded cationic polypeptide-based micelle is a promising strategy for cancer therapy.