Trigger-Responsive Gene Transporters for Anticancer Therapy

Abstract

In the current era of gene delivery, trigger-responsive nanoparticles for the delivery of exogenous nucleic acids, such as plasmid DNA (pDNA), mRNA, siRNAs, and miRNAs, to cancer cells have attracted considerable interest. The cationic gene transporters commonly used are typically in the form of polyplexes, lipoplexes or mixtures of both, and their gene transfer efficiency in cancer cells depends on several factors, such as cell binding, intracellular trafficking, buffering capacity for endosomal escape, DNA unpacking, nuclear transportation, cell viability, and DNA protection against nucleases. Some of these factors influence other factors adversely, and therefore, it is of critical importance that these factors are balanced. Recently, with the advancements in contemporary tools and techniques, trigger-responsive nanoparticles with the potential to overcome their intrinsic drawbacks have been developed. This review summarizes the mechanisms and limitations of cationic gene transporters. In addition, it covers various triggers, such as light, enzymes, magnetic fields, and ultrasound (US), used to enhance the gene transfer efficiency of trigger-responsive gene transporters in cancer cells. Furthermore, the challenges associated with and future directions in developing trigger-responsive gene transporters for anticancer therapy are discussed briefly.

Keywords: anti-cancer; cationic polymer; gene delivery; glutathione; magnetic field; non-viral; photothermal; trigger-responsive; ultrasound.

Figure 1

Schematic representation of gene transporter delivery into cancer cells: Gene transporters are uptaken by the cells via various pathways and then are accumulated in the early endosome (pH 6.3). Later, they are transported to the late endosome before entering the lysosome. However, via the proton sponge effect, they escape from the late endosome and later release their nucleic acid cargo (silencing RNA or plasmid DNA). The purple, orange, and green stars indicate the rate-limiting barriers, and the red star indicates the fate of an ineffective gene transporter in the lysosome. This scheme was drawn with help of Inkscape and www.mindthegraph.com icons.

Figure 2

Overview of the triggered release of nucleic acids inside cells. Enz-TGR: Enzyme-triggered gene release, L-TGR: Light-triggered gene release, US-TGR: Ultrasound-triggered gene release, and M-TGR: Magnetic-triggered gene release. Scheme was drawn with the help of Inkscape and www.mindthegraph.com icons.

Figure 3

Peroxidase enzyme-triggered gene release: (a) Schematic diagram showing that the matrix metalloproteinase-2 (MMP2) enzyme-mediated cleavage of the linker leads to the release of Polyethylene glycol (PEG) and exposure of the cell penetrating peptide (CPP), facilitating the tumor-specific cellular uptake of DNA cargo and (b) drug and gene delivery strategy using an MMP2-sensitive peptide linking the PEG and polyethylenimine (PEI)-lipids. Reproduced with permission from [56]. Biomaterials, Elsevier, 2017.

Figure 4

Glutathione enzyme-triggered gene release: (a) Schematic diagram of glutathione enzymes (glutathione cycle) involved in gene transporter disulfide bond breakage. Px Enz: Peroxidase enzyme, Rd Enz: Reductase enzyme. (b) Schematic illustration of the synthesis of the branched modified R9 (B-mR9) cell penetrating peptide (CPP) and construction of pDNA and siRNA polyplexes. Reproduced with permission from [97]. Journal of Controlled Release, Elsevier 2017.

Figure 5

Photothermally mediated gene delivery using metal-based gene transporters: (a) Schematic illustration of the design of GNRs-siRNA in an improved PTT platform; (b) GNRs-siRNA inhibited tumor growth in a xenograft model after irradiation with an 810 nm laser: (a,b) Mean tumor growth percent of different treatment in the xenograft model; (c) Immunochemistry of BAG3 expression and (d) TUNEL assay showing apoptotic cells in tumors after 24 h. Reproduced with permission from [117]. Biomaterials, Elsevier 2017.

Figure 6

Photothermally mediated gene delivery using carbon-based gene transporters: (a) Illustration of the synthesis of and photothermal combined gene therapy achieved by polyethylenimine (PEI)-grafted oxidized mesoporous carbon nanospheres (OMCN), (b) the application of which resulted in tumor growth inhibition after irradiation, which led to release of the plasmid ING4 (pING4) complexed with the PEI–grafted OMCN and its expression in breast cancer tumor-bearing nude mice: (a,b) mean body weight and relative tumor volume of treated mice; (c,d) survival curves and tumor images of mice on 30th day post injection with different treatments. Reproduced with permission from [120]. Biomaterials, Elsevier 2017.