Indotricarbocyanine-loaded AS1411 DNA aptamer- and TGN peptide-modified poly(ethylene glycol)-poly(ε-caprolactone) nanoparticles

Indotricarbocyanine (DiR)-loaded AS1411 DNA aptamer- and TGN peptide-modified poly(ethylene glycol)-poly(e-caprolactone) (PEG-PCL) nanoparticles, abbreviated as DiR-AsTNP, is a cascade delivery system synthesized by Gao et al. for brain glioma imaging and treatment (1).Effective treatment of brain tumors requires the therapeutic agents to penetrate both the blood–brain barrier (BBB) and the tumor cell barrier. The cascade targeting strategy is designed to meet this requirement, which relies on two specific ligands that are conjugated to a nanocarrier (1, 2). One ligand is designed to penetrate the BBB, and another is designed to target tumor tissues. With this strategy, Gao et al. developed a PEG-PCL cascade targeting delivery system that was modified with an angiopep-2 peptide to penetrate the BBB and an EGFP-EGF1 fusion protein to bind the brain neuroglial cells (2). Because of the promising results of this system, Gao et al. further developed another PEG-PCL system using TGN and AS1411 ligands (AsTNP) (1). TGN is a peptide of 12 amino acids (TGNYKALHPHNG), which was obtained through four rounds of in vivo phage display screening from a 12-mer peptide library (3). In vivo phage display is a technique that was first introduced by Pasqualini and Ruoslahti in 1996 (4). This technique has been used to screen tissue-specific peptides as targeting moieties for tumors and organs. The three-amino-acid sequence Arg-Gly-Asp is one of the most successful targeting ligands screened with the phage display technique (5). The TGN peptide has been shown to possess a superior BBB-penetrating ability compared with the native phage (3). AS1411 is a G-rich DNA aptamer that exhibits high binding affinity to nucleolin, a highly expressed protein in the plasma membrane of cancer cells including the C6 glioma cells (6, 7). Aptamers are short sequences of DNA or RNA that can bind to specific proteins via recognition of their three-dimensional structures (6). Aptamers are good candidates for targeted delivery because of their high stability, easy synthesis, and nonimmunogenicity (7). AS1411 has been used successfully to increase the anti-tumor efficacy of nanoparticles containing chemotherapeutic agents (7). In the AsTNP system, the TGN peptide served as the first-stage targeting ligand to transport the PEG-PCL nanoparticles through the BBB, and the AS1411 aptamer served as the second-stage ligand to deliver the system into the cancer cells after penetrating the BBB. An inhibitor of microtubule depolymerization, docetaxel (DTX), was then encapsulated within the AsTNP particles for therapeutic purpose (1, 8). A fluorescent dye (DiR or coumarin-6) was also encapsulated to track the behavior of AsTNP with optical imaging. The results obtained by Gao et al. showed that presence of both TGN and AS1411 was necessary, and the generated AsTNP had a superior BBB-penetration ability and high tumor accumulation (1). This chapter summarizes the data obtained with DiR-AsTNP.