Ionic α-helical polypeptides toward nonviral gene delivery

Design and applications of polymer-like peptides in biomedical nanogels

INTRODUCTION

Polymer nanogels are among the most promising nanoplatforms for use in biomedical applications. The substantial interest for these drug carriers is to enhance the transportation of bioactive substances, reduce the side effects, and achieve optimal action on the curative sites by targeting delivery and triggering the release of the drugs in a controlled and continuous mode.

AREA COVERED

The review discusses the opportunities, applications, and challenges of synthetic polypeptide nanogels in biomedicine, with an emphasis on the recent progress in cancer therapy. It is evidenced by the development of polypeptide nanogels for better controlled drug delivery and release, in complex in vivo microenvironments in biomedical applications.

EXPERT OPINION

Polypeptide nanogels can be developed by choosing the amino acids from the peptide structure that are suitable for the type of application. Using a stimulus - sensitive peptide nanogel, it is possible to obtain the appropriate transport and release of the drug, as well as to achieve desirable therapeutic effects, including safety, specificity, and efficiency. The final system represents an innovative way for local and sustained drug delivery at a specific site of the body.

Modification of KL4 Peptide Revealed the Importance of Alpha-Helical Structure for Efficient siRNA Delivery

A safe and effective delivery system is considered a key to the success of nucleic acid therapeutics. It has been reported that pulmonary surfactants or their components could facilitate the uptake of small interfering RNA (siRNA) into the lung epithelial cells. Previously, our group investigated the use of KL4 peptide, a synthetic cationic peptide that simulates the structural properties of surfactant protein B (SP-B), as siRNA delivery vector. Although KL4 peptide exhibits good in vitro siRNA transfection efficiency on lung epithelial cells, its therapeutic potential is limited by its poor aqueous solubility due to the presence of a high proportion of hydrophobic leucine residues. In this study, we aim to address the solubility issue, designing five different modified peptides by replacing the hydrophobic leucine with alanine or valine, and assess their potential as siRNA delivery vectors. While the modified peptides retain the overall cationic property, their siRNA binding is also affected and their transfection efficiency is inferior to the parent KL4 peptide. A closer examination of the conformation of these peptides by circular dichroism shows that substitution of leucine residues leads to the change of the secondary structure from α-helical content to either β-sheet or more disordered, β-turn conformations. Relatively conservative amino acid substitutions, in terms of hydrophobicity bulk, lead to substantial conformational alteration, heavily impacting siRNA binding and release, cellular uptake, and transfection efficiency. Although the peptide modification strategy employed in this study was unsuccessful in developing an improved version of KL4 peptide for siRNA delivery, it highlights the importance of the α-helical conformation for efficient siRNA transfection, providing useful insights for future development of peptide-based RNA delivery system.

Systemic siRNA delivery to tumors by cell-penetrating α-helical polypeptide-based metastable nanoparticles

Systemic, non-viral siRNA delivery for cancer treatment is mainly achieved via condensation by cationic materials (e.g., lipids and cationic polymers), which nevertheless, suffers from poor serum stability, non-specific tissue interaction, and unsatisfactory membrane activity against efficient in vivo gene knockdown. Here, we report the design of a metastable, cancer-targeting siRNA delivery system based on two functional polymers, PVBLG-8, a cationic, helical cell-penetrating polypeptide, and poly(l-glutamic acid) (PLG), an anionic random-coiled polypeptide. PVBLG-8 with rigid, linear structure showed weak siRNA condensation capability, and PLG with flexible chains was incorporated as a stabilizer which provided sufficient molecular entanglement with PVBLG-8 to encapsulate the siRNA within the polymeric network. The obtained PVBLG-8/siRNA/PLG nanoparticles (PSP NPs) with positive charges were sequentially coated with additional amount of PLG, which reversed the surface charge from positive to negative to yield the metastable PVBLG-8/siRNA/PLG@PLG (PSPP) NPs. The PSPP NPs featured desired serum stability during circulation to enhance tumor accumulation via the enhanced permeability and retention (EPR) effect. Upon acidification in the tumor extracellular microenvironment and intracellular endosomes, the partial protonation of PLG on PSPP NPs surface would lead to dissociation of PLG coating from NPs, exposure of the highly membrane-active PVBLG-8, and surface charge reversal from negative to positive, which subsequently promoted tumor penetration, selective cancer cell internalization, and efficient endolysosomal escape. When siRNA against epidermal growth factor receptor (EGFR) was encapsulated, the PSPP NPs showed excellent tumor penetration capability, tumor cell uptake level, EGFR silencing efficiency, and tumor growth inhibition efficacy in U-87 MG glioblastoma tumor spheroids in vitro and in xenograft tumor-bearing mice in vivo, outperforming the PSP NPs and several commercial reagents such as Lipofectamine 2000 and poly(l-lysine) (PLL). This study therefore demonstrates a facile and unique design approach of metastable and charge reversal NPs, which overcomes multiple biological barriers against systemic siRNA delivery toward anti-cancer treatment.

The Length of Hydrophobic Chain in Amphiphilic Polypeptides Regulates the Efficiency of Gene Delivery

The major challenges of non-viral carriers are low transfection efficiency and high toxicity. To overcome this bottleneck, it is very important to investigate the structure-property-function (transfection efficiency) relationships of polycations. Herein, different length hydrophobic poly(l-leucine) chains in amphiphilic polypeptides were precisely synthesized by α-amino acid N-carboxyanhydrides (NCA) ring-opening polymerization and these biocompatible polypeptides were chosen as a model to further examine the transfection in vitro. These polypeptides were characterized by nuclear magnetic resonance spectroscopy (NMR) and size exclusion chromatography (SEC). Agarose gel electrophoresis (AGE) was employed to validate the ability of DNA condensation and transmission electron microscopy (TEM) was used to observe the assemblies of polyplexes. Cytotoxicity was evaluated in COS-7 cell lines and transfection was performed in normal cell COS-7 and cancer cell Hep G2. The results showed that NCA monomers were prepared and the amphiphilic polypeptides, poly(lysine(CBZ))₅₀-block-poly(l-leucine)₁₀, poly(l-lysine(CBZ))₅₀-block-poly(l-leucine)₁₅, and poly(l-lysine(CBZ))₅₀-block-poly(l-leucine)₂₅, were successfully synthesized with controlled molecular weight and narrow distribution. After deprotection of CBZ, these materials can condense plasmid DNA into 100 nm nanoparticles and the cellular uptake of polyplexes was as fast as 30 min. The transfection data shown these materials had a good transfection efficiency comparing to polyethylenimine (Branched, 25 kDa) while they displayed ignored cytotoxicity. More importantly, we discovered the length of hydrophobic poly(l-leucine) in amphiphilic polypeptides steadily regulates gene delivery efficiency in two kinds of cells ranking poly(l-lysine)₅₀-block-poly(l-leucine)₂₅ > poly(l-lysine)₅₀-block-poly(l-leucine)₁₅ > poly(l-lysine)₅₀-block-poly(l-leucine)₁₀.

Orthogonally reactive amino acids and end groups in NCA polymerization

We summarize recent strategies for the synthesis of orthogonally reactive polypeptides and polypeptoids by direct and post-polymerization approaches.

Scaffold and scaffold-free self-assembled systems in regenerative medicine

Self-assembly in tissue engineering refers to the spontaneous chemical or biological association of components to form a distinct functional construct, reminiscent of native tissue. Such self-assembled systems have been widely used to develop platforms for the delivery of therapeutic and/or bioactive molecules and various cell populations. Tissue morphology and functional characteristics have been recapitulated in several self-assembled constructs, designed to incorporate stimuli responsiveness and controlled architecture through spatial confinement or field manipulation. In parallel, owing to substantial functional properties, scaffold-free cell-assembled devices have aided in the development of functional neotissues for various clinical targets. Herein, we discuss recent advancements and future aspirations in scaffold and scaffold-free self-assembled devices for regenerative medicine purposes. Biotechnol. Bioeng. 2016;113: 1155-1163. © 2015 Wiley Periodicals, Inc.