siRNA delivery systems for cancer treatment

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Abstract

With increasing knowledge on the molecular mechanisms of endogenous RNA interference, small interfering RNAs (siRNAs) have been emerging as innovative nucleic acid medicines for treatment of incurable diseases such as cancers. Although several siRNA candidates for the treatment of ocular and respiratory diseases are undergoing clinical trials, there are challenges inherent in the further development of siRNAs for anti-cancer therapeutics, because systemic administration will be required in most cases. In addition to nonspecific off-target and immune stimulation problems, appropriate delivery remains a major hurdle. The technologies developed for delivery of nucleic acid medicines such as plasmid DNA and antisense oligonucleotides have paved the way to rapid progress for in vivo delivery of siRNAs. Here, we review various in vivo delivery strategies including chemical modification, conjugation, lipid-based techniques, polymer-based nanosystems, and physical methods. Moreover, the current progress in siRNA delivery systems for gynecologic, liver, lung, and prostate cancers is discussed.

Introduction

RNA interference (RNAi) is a biological mechanism whereby the presence of double-stranded RNA (dsRNA) interferes with the expression of a particular gene that shares a homologous sequence with the dsRNA. The RNAi machinery, first discovered in plants, was later demonstrated in the roundworm Caenorhabditis elegans by delivery of dsRNA using a microinjection technique. The introduction of dsRNA molecules could produce the interfering activity and result in the highly specific inhibition of complementary gene expression in C. elegans [1].

Recent studies have provided insights into the molecular mechanisms of RNAi, in which dsRNA induces the silencing of homologous mRNA. In the cytoplasm of mammalian cells, an enzyme known as Dicer initiates RNA silencing by breakdown of long dsRNA to generate small interfering RNA (siRNA) of about 21–23 nucleotides in length. The resulting siRNAs are incorporated into an RNA-induced silencing complex (RISC) and unwound into a single-stranded RNA (ssRNA), which is followed by the degradation of sense strand ssRNA [2]. The RISC containing ssRNA, a guide or antisense strand, looks for and binds to complementary mRNA molecules. The main components of the RISC complex are the Argonaute 2 protein, which is a member of the Argonaute family of proteins, responsible for mRNA degradation and ssRNA formation [3]. As a catalytic engine within RISC, Argonaute 2 (previously known as eIF2C2) facilitates guiding of the anti-sense ssRNA strand to complementary mRNA sequences and degrades target mRNAs through the PIWI domain of an Ago protein, a structural homolog of RNase H. ATP is not essential for the target cleavage reaction of RISC, although ATP increases the rate of endonuclease activity. The hydrolysis reaction involved in the breakdown of the target mRNA phosphodiester backbone requires a divalent metal ion (Mg2+) and releases the 5'-PO4 and 3'-OH groups [4], [5], [6], [7], [8].

Although siRNA is naturally generated from a long dsRNA, synthetic siRNA can affect RNAi. The introduction of an artificial siRNA of 21 nucleotides triggered gene silencing in mammalian cells [9]. siRNA molecules blocked specific expression of endogenous and heterologous genes in various mammalian cell lines [9]. Moreover, the multiple administrations of synthetic siRNAs achieved long-term silencing of target gene without disrupting the endogenous microRNA pathways [10].

Such work has generated much interest in the potential of synthetic siRNA as a key strategic molecule in biomedical research and in the development of innovative medicines. Indeed, siRNAs have been extensively used for the functional analysis of specific genes, especially genes overexpressed in cancer cells, and for the development of new therapeutics for various incurable diseases.

In this paper, we review therapeutic potentials of nonviral siRNA delivery strategies for treatment of various cancers.

Section snippets

siRNA in clinical trials

Given their specific and potent RNAi triggering activity, siRNAs are emerging as new generation biodrugs. Several studies have supported the therapeutic potential of siRNA. Viral mRNA-targeted siRNA has been shown to be effective in inhibiting different stages of the HIV virus life cycle [11]. The hydrodynamic injection of Fas-specific siRNA prevented liver failure in mice with chronic and severe autoimmune hepatitis [12]. Currently, several potential siRNA candidates are undergoing clinical

siRNA delivery systems for in vivo application

Although researchers and biotechnology companies have reported many siRNA vectors for delivery into the cytoplasm of cells, and although these are satisfactory for most in vitro applications, these delivery technologies are usually inappropriate for in vivo use [14]. Currently, siRNAs in clinical trials are directly administered to local target sites such as the eye and lung, thereby avoiding the complexity of systemic delivery. However, it is necessary to introduce siRNA by a systemic route to

Delivery of therapeutic siRNA in cancer

The RNAi phenomenon and siRNA have provided new opportunities for the development of innovative medicine to treat previously incurable diseases such as cancer. siRNA is of inherent potency because it exploits the endogenous RNAi pathway, allows specific reduction of disease-associated genes, and is applicable to any gene with a complementary sequence [100]. As cancer belongs to the category of genetic diseases, many important genes associated with various cancers have been discovered, their

Future prospects

siRNA therapeutics have several distinct advantages over traditional pharmaceutical drugs. RNAi is an endogenous biological process, so almost all genes can be potently suppressed by siRNA. The identification of highly selective and inhibitory sequences is much faster than the discovery of new chemicals, and it is relatively easy to synthesize and manufacture siRNA on a large scale [14].

Cancers are associated with abnormally high expression of a number of oncogenes. Interference in specific

Acknowledgements

This work was supported by the grants from the Ministry of Education, Science and Technology (F104AA010002-08A0101-00210, F104AA010003-08A0101-00310), the Ministry of Health and Welfare (A04-0041-B21004-07M4-00040B), and National Research Laboratory project.

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    This review is part of the Advanced Drug Delivery Reviews theme issue on “The Role of Gene– and Drug Delivery in Women's Health – Unmet Clinical Needs and Future Opportunities”.

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