Nanoparticle-based delivery of small interfering RNA: challenges for cancer therapy

Abstract

During recent decades there have been remarkable advances and profound changes in cancer therapy. Many therapeutic strategies learned at the bench, including monoclonal antibodies and small molecule inhibitors, have been used at the bedside, leading to important successes. One of the most important advances in biology has been the discovery that small interfering RNA (siRNA) is able to regulate the expression of genes, by a phenomenon known as RNA interference (RNAi). RNAi is one of the most rapidly growing fields of research in biology and therapeutics. Much research effort has gone into the application of this new discovery in the treatment of various diseases, including cancer. However, even though these molecules may have potential and strong utility, some limitations make their clinical application difficult, including delivery problems, side effects due to off-target actions, disturbance of physiological functions of the cellular machinery involved in gene silencing, and induction of the innate immune response. Many researchers have attempted to overcome these limitations and to improve the safety of potential RNAi-based therapeutics. Nanoparticles, which are nanostructured entities with tunable size, shape, and surface, as well as biological behavior, provide an ideal opportunity to modify current treatment regimens in a substantial way. These nanoparticles could be designed to surmount one or more of the barriers encountered by siRNA. Nanoparticle drug formulations afford the chance to improve drug bioavailability, exploiting superior tissue permeability, payload protection, and the "stealth" features of these entities. The main aims of this review are: to explain the siRNA mechanism with regard to potential applications in siRNA-based cancer therapy; to discuss the possible usefulness of nanoparticle-based delivery of certain molecules for overcoming present therapeutic limitations; to review the ongoing relevant clinical research with its pitfalls and promises; and to evaluate critically future perspectives and challenges in siRNA-based cancer therapy.

Keywords: biological barriers; cancer therapy; clinical trials; delivery strategies; nanoparticles; small interfering RNA.

Figure 1

RNAi/miRNA pathway schematization and major challenges for naked siRNA delivery in vivo. (A) Schematization of RNA interference (RNAi): non-translated double-stranded RNA (dsRNA) molecules called small interfering RNA (siRNA), of exogenous or endogenous origin, post-transcriptional regulate gene-expression through a sequence specific degradation of target messenger RNA (mRNA). [1] Longer siRNA molecules (dark green) are cleaved by the nuclease Dicer and [2] incorporated into a multiprotein RNA-inducing silencing complex (RISC). [2] The duplex RNA is unwound leaving the anti-sense strand (light green). [3] to guide RISC to complementary mRNA (red) for subsequent endonucleolytic cleavage and gene silencing. [4] Short hairpin RNAs (shRNA) (violet) are sequences of RNA encoded by specific genes; they are introduced into the nucleus, transcribed and transported into the cytoplasm where they follow the same fate of siRNA. (B) miRNA processing: microRNA (blue) are considered as the “endogenous substrate” of the RNAi machinery. They are trascribed by RNA-Pol III in long primary transcripts (pri-miR), then processed within the nucleus into precursor miRNA (pre-miRNA) by the RNase III enzyme Drosha-DGCR8. Pre-miRNA hairpins are exported from the nucleus in a process involving the nucleo-cytoplasmic shuttle Exportin-5 (Exp.5). In cytoplasm, the pre-miRNA hairpin is cleaved by Dicer and loaded into RISC as for siRNA. miRNAs often share only partial complementarity with target mRNAs, usually in the 3′UTR, acting mainly as a translational repressors. (C) “Naked” siRNA pitfalls. In the box are reported the major obstacles for therapeutic efficacy of siRNA without modifications (“naked”). See text for details.

Figure 2

Encapsulation technologies for siRNA delivery in vivo: nanoparticles strategies and advantages. Readapted from Shim et al.

Figure 3

Obstacles of Nanoparticles-based siRNA delivery in vivo. After administration into blood circulation the siRNA-nanoparticles (A) must avoid rapid degradation by plasma components (eg, cellular and humoral arm of the immune system) and sequester by negatively charged serum protein. (B) Then they need to escape renal filtration and/or clearance by the reticuloendothelial system (RES). (C) To reach the target cells they must overcome the capillary endothelium through an extravasation process and (D) overcome the extracellular matrix (ECM): a dense network of polysaccharides and fibrous proteins, rich in macrophages, which can obstacle the transport of nanoparticles. (E) Furthermore these particles must be taken up into the cells, usually bound to cellular receptors and transported into the cytoplasm through a receptor mediated endocytosis process. (F) Inside the cells the particles need to escape the endosome; (G) thus unpackage and release the siRNA to the RNA interference (RNAi) machinery.