Synthesis and properties of siRNAs containing 5′-amino-2′,5′-dideoxy-2′α-fluororibonucleosides

Journey of siRNA: Clinical Developments and Targeted Delivery

Since the evolutionary discovery of RNA interference and its utilization for gene knockdown in mammalian cell, a remarkable progress has been achieved in small interfering RNA (siRNA) therapeutics. siRNA is a promising tool, utilized as therapeutic agent against various diseases. Despite its significant potential benefits, safe, efficient, and target oriented delivery of siRNA is one of the major challenges in siRNA therapeutics. This review covers major achievements in clinical trials and targeted delivery of siRNAs using various targeting ligand-receptor pair. Local and systemically administered siRNA drug candidates at various phases in clinical trials are described in this review. This review also provides a deep insight in development of targeted delivery of siRNA. Various targeting ligand-siRNA pair with complexation and conjugation approaches are discussed in this review. This will help to achieve further optimization and development in targeted delivery of siRNAs to achieve higher gene silencing efficiency with lowest siRNA dose availability.

Studies on Transcriptional Incorporation of 5'-N-Triphosphates of 5'-Amino-5'-Deoxyribonucleosides

In this study, several RNA polymerases were used for the first time to examine the possibility of transcriptional incorporation of 5’-N-triphosphates of 5’-amino-5’-deoxyribonucleosides (5’NH NTPs). The T3, T7, Sp6 and T7 Y639F RNA polymerases were employed to show that the full-length transcript cannot be synthesized. The results suggest that the application of 5’NH NTPs could decrease transcription reaction rates. What is more, the modification of transcription conditions had no influence on the rate of 5’NH NTPs incorporation. Based on experimental data it is postulated that 5’NH NTPs can be used as potential transcription inhibitors. Our findings expand the knowledge on suitable uses of the 5’-N-triphosphates of 5’-amino-5’-deoxyribonucleoside and the exact mechanism of transcriptional inhibition.

Efficient synthesis and cell-based silencing activity of siRNAS that contain triazole backbone linkages

An efficient synthesis of siRNAs modified at the backbone with a triazole functionality is reported. Through the use of 4,4'-dimethoxytrityl (DMT) phosphoramidite chemistry, triazole backbone dimers were site-specifically incorporated throughout various siRNAs targeting both firefly luciferase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene transcripts as representatives of an exogenous and endogenous gene, respectively. Following the successful silencing of the firefly luciferase reporter gene, triazole-modified siRNAs were also found to be capable of silencing GAPDH in a dose-dependent manner. Backbone modifications approaching the 3'-end on the sense strand were tolerated without compromising siRNA potency. This study highlights the compatibility of triazole-modified siRNAs within the RNAi pathway, and the modification's potential to impart favorable properties to siRNAs designed to target other endogenous genes.

Chemical modification of siRNA

The ability to manipulate the RNA interference (RNAi) machinery to specifically silence the expression of target genes could be a powerful therapeutic strategy. Since the discovery that RNAi can be triggered in mammalian cells by short double-stranded RNAs (small interfering RNA, siRNA), there has been a tremendous push by researchers, from academia to big pharma, to move siRNAs into clinical application. The challenges facing siRNA therapeutics are significant. The inherent properties of siRNAs (polyanionic, vulnerable to nuclease cleavage) make clinical application difficult due to poor cellular uptake and rapid clearance. Side effects of siRNAs have also proven to be a further complication. Fortunately, numerous chemical modification strategies have been identified that allow many of these obstacles to be overcome. This unit will present an overview of (1) the chemical modifications available to the nucleic acid chemist for modifying siRNAs, (2) the application of chemical modifications to address specific therapeutic obstacles, and (3) the factors that must be considered when assessing the activity of modified siRNAs.