Stabilized Heptapeptide A7R for Enhanced Multifunctional Liposome-Based Tumor-Targeted Drug Delivery

^LA7R (ATWLPPR) is a heptapeptide with high binding affinity in vitro to vascular endothelial growth factor receptor 2 (VEGFR2) and neuropilin-1 (NRP-1) overexpressed on glioma, glioma vasculogenic mimicry and neovasculature. However, its tumor targeting efficacy is significantly reduced in vivo due to proteolysis in blood circulation. To improve the in vivo stability and targeting efficacy, the retro inverso isomer of ^LA7R (^DA7R) was developed for glioma-targeted drug delivery. ^DA7R was expected to have a similar binding affinity to its receptors in vitro (VEGFR2 and NRP-1), which was experimentally confirmed. In vivo, ^DA7R-modified liposomes achieved improved glioma-targeted efficiency than did ^LA7R-modified liposomes. After loading a chemotherapeutic agent (doxorubicin), ^DA7R-modified liposomes significantly inhibited subcutaneous model tumor in comparison to free doxorubicin, plain liposomes and ^LA7R-modified liposomes. In summary, the present study presented the potential of a proteolytically stable d-peptide ligand for in vivo tumor-targeted drug delivery.