**6. Concluding remarks**

NA-based gene silencing techniques have been successfully used in drug development. The major progress on ASON research is the chemical modifications and ligand conjugation to enhance drug stability and efficacy of delivery. The emergence of RNAi-mediated gene silencing techniques further provided new hope for this regard. Basically, siRNA silencing techniques can be used against any viral infection. Two major obstacles must, however, be overcome before it can become a broadly applicable standard therapy: the question of their specificity and efficient delivery to target cells. As siRNA can potentially cause off-site targeting and activate the immune system, minimizing the undesired effects must be considered in the drug design. Immense efforts have been undertaken to develop carrier system with which siRNAs can be delivered to their target cells. Despite great advances in the last years, further developments are still required to get systematically applied siRNAs to their required sites of action. Here, viral vectors systems for shRNA expression cassettes offer additional options for efficient and organ-specific delivery. However, this approach must be first overcome the reservations based on the negative experience with gene therapy. As discussed earlier, pRNA is a promising vehicle for targeted delivery of NA-based therapeutic molecules. For treatment of myocarditis, a myocardium-specific ligand such as peptides from the CVB3 antireceptor protein or RNA apatamers of cardiomyocytes should be identified, which will be used to replace folic acid on the pRNA vector.

Very recent advances in the understanding of miRNA biology and particularly their association with the molecular pathogenesis of a variety of diseases have served as a theoretical basis for drug development. On the one hand, miRNA, as one of the key factors for regulation of viral replication, tissue tropism and latency, are the ideal targets for inhibition. In this regard, construction of mRNAs that contain multiple tandem binding sites of a given miRNA may be useful to produce decoys or "miRNA sponges" to inhibit the function of a specific miRNA. In addition, chemically synthesized antisense RNA oligomers ('antagomirs') targeting a miRNA of interest could be also a promising approach to inhibit miRNA activity (Ebert et al., 2007; Krutzfeldt et al., 2005). Other strategies include i) overexpression of specific miRNAs using an expression vector to achieve a long-term effect of reversing the imbalance of miRNA expression caused by infections, and ii) introduction of pre-miRNA mimetics for transient replacement of a down-regulated miRNA. On the other hand, miRNA can serve as a useful tool for therapy. Since miRNA is tolerable to target mutation at its center region, application of multiple artificial miRNAs to target the 3'UTR and/or other regions of CVB3 RNA may improve the drug resistance. Given the immense interest in NA-based drug research and the rapid progress made in this field and other areas such as nano-biotechnology for drug delivery, the coming years are likely to see an increasing range of clinical applications, particularly for the RNAi-based drug candidates. The realization of the potential of NA-based therapies to address human viral pathogens suggests that this field has a very promising future.
