Current Topics in Medicinal Chemistry - Volume 12, Issue 2, 2012

siRNAs can cleave complementary mRNAs. This ability to inhibit the translation of specific proteins offers important biomedical benefits from basic study of protein function through loss-of-function phenotypes up to therapeutic intervention in diseases resulting from expression of specific proteins [1]. When siRNA is considered as a drug molecule it has desirable and less attractive characteristics. One of the most appealing qualities is that, with the availability of the human genome sequence, siRNA design algorithms and oligonucleotide chemistry, interesting siRNA sequences can be readily identified and synthesized. This bypasses much of the need for lead compound identification and optimization that is a key process for traditional small molecular weight drugs. At the same time, siRNA molecules have very poor physicochemical characteristics to be drug molecules. Their nuclease sensitivity, molecular weight of approximately 14 kD and strong negative surface charge coupled to an intracellular site of activity make their application as drug molecules difficult [2]. In this special issue, we present a collection of papers that describe the possibilities of siRNA as a tool and of siRNA as a drug. Naturally, given the difficult physicochemical properties of the molecule itself, most attention in this section will be focused on the delivery. The paper of Zaffaroni and colleagues describes RNAi-based approaches to validate survivin and Apollon/BRUCE as new cancer therapeutic targets. It is an excellent example of the powerof siRNA as a tool in identification and functional characterization of new points for therapeutic intervention. Their results show that targeting the survivin pathway inhibits tumor growth by increasing apoptosis of cancer cells. Based on these findings, the clinical use of survivin-directed strategies is currently under investigation. Preliminary result on Apollon/BRUCE, indicate the induction of apoptosis in a similar manner, making this an interesting strategy for future exploration. The article by Ohrt et al. discusses the various pathways by which RNA can regulate gene expression and the intracellular location of this activity…..

The ability to evade apoptosis is one of the defining hallmarks of cancer. It enables the survival of cancer cells under abnormal growth stimulation and mediates their increased resistance to treatment with cytotoxic drugs and radiation. Therefore, antiapoptotic proteins that counteract apoptosis signaling represent promising new therapeutic targets to impair cancer cell growth and enhance treatment response. As soon as RNA interference (RNAi) was demonstrated in mammalian cells, it rapidly became an essential tool for gene knockdown in preclinical models, making it possible to define the role of specific genes in the onset and progression of cancer and explore their potential as therapeutic targets. The present review summarizes the findings from studies relying on the use of RNAi-based approaches to functionally validate two members of the inhibitors of apoptosis protein family, survivin and Apollon/BRUCE, as new cancer therapeutic targets. Results collected thus far indicate that targeting the survivin network efficiently inhibits tumor growth potential and increases spontaneous and treatment-induced apoptosis of cancer cells. Based on these findings, the applicability of survivin-directed strategies for the clinical treatment of human tumors is currently under investigation. As regards Apollon/ BRUCE, although very preliminary, results of RNAi-mediated gene knockdown point to the possibility to significantly impair tumor cell proliferation through the induction of apoptosis.

Several different pathways, generally termed RNA silencing pathways, utilize small RNA molecules guiding sequence-specific silencing effects of ribonucleoprotein effector complexes, traditionally termed RNA-induced silencing complex (RISC). Three RNA silencing pathways were recognized in mammalian cells: RNA interference (RNAi), where short RNAs produced from long double-stranded RNA guide cleavage of cognate mRNAs, microRNA (miRNA) pathway, where endogenously-encoded miRNAs typically induce translational repression, and piRNA pathway, where piRNAs (PIWI-associated RNAs) guide repression of repetitive sequences in the germline. Originally, RNAi and miRNA pathways were thought to act in the cytoplasm, however, there is a growing body of evidence that these pathways also have a nuclear component. This text reviews the current evidence concerning nuclear localization and function of miRNA and RNAi pathway components. We provide evidence that TRBP, Dicer and AGO2, proteins found in the RISC-loading complex (RLC) and RISC itself, are present in the nucleus. Nonetheless, fully functional RLC is not found in the nuclear compartment which is consistent with the recent findings obtained by Fluorescence Cross-Correlation Spectroscopy experiments illustrating that RISC is specifically loaded within the cytoplasm and shuttles subsequently between the nuclear and cytoplasmic compartment, thereby allowing small RNA gene regulation in both compartments. The function of nuclear TRBP and Dicer proteins remains elusive. We also discuss the consequences of nucleotide analogs introduced into siRNAs which can severely interfere with the natural cytoplasmic localization mediated by Exportin-5 which is required for efficient RISC loading in the cytoplasm.

Over the last decade, considerable effort has been put in the implementation of RNA interference (RNAi) as a treatment for various disorders. As RNAi occurs in the cytoplasm of cells, it is imperative that RNAi mediators such as small interfering RNA (siRNA) cross several extracellular and intracellular barriers to reach this site of action. Among the extensive range of proposed delivery systems for siRNA, matrix systems possess interesting properties to promote the delivery of siRNA to a target tissue. In this review, a number of recently developed matrix and hybrid systems for siRNA delivery are discussed.

A key hurdle for the further development of RNA interference (RNAi) therapeutics like small interfering RNA (siRNA) is their safe and effective delivery. Lipids are promising and versatile carriers because they are based on Nature's own building blocks and can be provided with properties which allow for protection of the siRNA, steric stabilization, targeting, membrane fusion and triggered drug release. At present a variety of lipid-based transfectants for siRNA delivery have been used for in vitro and in vivo purposes. The majority bears a cationic charge to electrostatically complex the siRNA into more hydrophobic lipoplexes, which promote passage of the siRNA across cellular membrane barriers, especially when lipids are added that facilitate membrane fusion. Despite these attractive features, siRNA delivery vehicles are facing a number of challenges such as the limited delivery efficiency in vivo, toxicity and non-specific stimulation of the immune system. To optimally design and tailor the lipidic systems for siRNA delivery, better insight is needed into the mechanisms of cell delivery. More specifically, further clarification is need regarding the nature of cell surface interactions, routes of internalization, passage of intracellular membranes, and mechanisms of immune activation. This review provides an overview of the main constituents currently employed in lipid-based siRNA carriers, and recent research into improvements of cell delivery. In addition, pitfalls related to immune activation and side effects are discussed, and possible ways to overcome them are highlighted.

RNA interference is a technique to induce sequence-specific gene silencing, but is hampered by inefficient delivery of its mediator, short interfering RNA, into target cells. This review describes recent advances in siRNA delivery using polymeric carrier systems. Structural variations that have been applied to these polymers for optimizing their intracellular trafficking are discussed, as well as strategies for stabilization and targeting to diseased tissues in vivo. Recent findings have highlighted safety issues that need to be taken into account in the design of nanoparticles for clinical application.