Current Topics in Medicinal Chemistry - Volume 6, Issue 9, 2006

Over the last decade, small molecule nucleoside and nucleotide drugs have attracted much attention due to a remarkable increase for the treatment of viral diseases. Currently more than 20 drugs fall into this category. Nucleoside antiviral drugs are pro-drugs in that they are actively transpored into cells and then activated by cellular kinases to the nucleotide triphosphate (NTP). This NTP is now able to competitively inhibit the enzyme or, more commonly, act as a substrate and be incorporated into the growing polynucleotide chain subsequently prevents (chain-terminates) the further replication of the viral genome. Extensive research efforts have been directed toward the development of nucleoside therapeutics, structural modified nucleoside inhibitors can vary in either (or both) the ribose or base portion of the molecule. In this issue, two chapters describe the development of anti-HBV nucleosides and the chemistry of 7-deazapurine ribonucleosides. RNA interference (RNAi) is a new field to describe the use of small inhibitory double-stranded RNA (siRNA) to target for degradation sequence-specific cellular mRNAs, and as a result to silencing gene expression. With the more recent development of RNAi in mammalian systems, investigators are not only dissecting gene function but also attempting the development of new therapeutic approaches in human genetics and/or infectious diseases. The main challenge for translating the experimental success of siRNA into clinical applications is how to solve the problems of the stability of siRNA in blood and the delivery to target. Three research groups are invited to review the recent developments in this area. Thanks to Current Topics of Medicinal Chemistry for this special issue which makes us to share these very interesting topics.

Chronic hepatitis B virus (HBV) infection affects about 400 million people worldwide. The development of nucleoside analogs that inhibit HBV polymerase provides an important approach for treating HBV infection. The approval of lamivudine, adefovir and entecavir represents a cornerstone of hepatitis B therapy. However, the challenges from the resistance and the off-therapy viral rebound are still unmet, and there is a need of developing new therapeutic agents. This review will discuss the structure-activity relationship of the most significant anti-HBV nucleoside analogs and the latest development in the field.

This review reports on the synthesis of 7-deazapurine ribonucleosides, including C-nucleosides, 2'-C-methyl derivatives and L-enantiomers. It covers the various aspects of convergent nucleoside synthesis such as the Schiff base procedure, the fusion reaction, the metal salt procedures, the Silyl-Hilbert-Johnson reaction, and the nucleobase anion glycosylation. The review discusses the scope and limitations of glycosylation reactions performed on 7-deazapurines. Peracylated ribose derivatives were now employed in the glycosylation, which overcome difficulties reported earlier.

siRNA has become an indispensable tool for functional characterization of genes. It has also demonstrated tremendous potential as a new generation of drug candidates. Although the technology works very well to a great panel of cells in vitro, it is still a challenge to translate the success into in vivo target validation easiness and, even more difficult, into therapeutic applications. With a number of chemically modified compounds under initial clinical trial from several commercial entities, the interests in chemical modification of siRNA have become heightened. In this review we have tried to touch on most of the chemical modifications of RNA that have been tested in the siRNA landscape, but maintained a focus on the backbone modifications, and 2'-modifications on the ribose ring. It is anticipated that more modifications and more systematic comparisons between different modifications will be performed to draw more educated conclusions over some of the modifications.

RNA interference (RNAi) is the process of using specific sequences of double-stranded RNA (dsRNA) to knock down the expression level of sequence-homologous genes. Such ability of small interfering RNA (siRNA) in mammalian cells will undoubtedly revolutionize the study of functional genomics, the discovery of drug targets and even the treatment of human diseases. In this review we briefly describe the history of RNAi discovery, the RNAi mechanism and the general guideline for siRNA design as well as various methods for siRNA production and delivery. We also introduce the potential applications of siRNA, inducible siRNA and siRNA library in speeding up basic biomedical research and in acting as potential therapeutic agents for treatment of numerous human diseases.

Short interfering RNAs (siRNAs) are 21-23 nt long double-stranded oligoribonucleotides which in mammalian cells exhibit a potency for sequence-specific gene silencing via an RNA interference (RNAi) pathway. It has been already proven that exogenous, chemically synthesized siRNA molecules are effective inhibitors of gene expression and are widely applied for analysis of protein function and proteomics-based target identification. Moreover, since their discovery siRNA molecules have been implemented as potential candidates for therapeutic applications. Variously modified siRNA molecules containing sugar modifications (2'-OMe, -F, -O-allyl, -amino, orthoesters and LNA analogues), internucleotide phospodiester bond modifications (phosphorothioates, boranophosphates), base modifications (s2U) as well as 3'-terminal cholesterol-conjugated constructs were investigated as potential candidates for effective inhibition of gene expression. This chapter reviews an impact of chemical and structural modifications of siRNA molecules on their serum and thermal stability, cellular and in vivo activity, cellular uptake, biodistribution and cytotoxicity. Functional analysis of chemically modified siRNA molecules allows for better understanding of the mechanism of the RNA interference process as well as demonstrates immense efforts in optimizing in vivo potency of siRNA molecules for RNAi-based drug design.