**6. RNAi as therapeutic in clinical application**

In terms of possible clinical applications, a key difference between siRNA and miRNA is that an siRNA specifically targets a single site on a unique mRNA, and initiates inhibition

**Figure 2.** microRNA-mRNA interaction network.

of expression of a single target gene. In contrast, an miRNA molecule usually has multiple mRNA targets and regulates the activity of a number of genes (**Figure 2**). For an mRNA to be recognized by an miR, just a short binding sequence is necessary rather than the entire nucleotide sequence of siRNA. In contrast, the siRNA must be entirely complementary with its target mRNA to enable it to initiate RNAi. However, miRNAs partially complement and bind to multiple mRNAs to inhibit their expression. In addition, their mechanism(s) of action are quite different: siRNAs are known to cleave mRNAs, but miRNAs in contrast act to inhibit the translation of mRNAs. This leads to the inevitable conclusion that scientists will have to develop techniques for targeting specific cells, thus avoiding the down-regulation of the genes expressed by normal cells. The objective will be the delivery of siRNAs and miRNAs to specific targets exclusively in tumor cells. Taken together, three key procedures must be undertaken before RNAi treatment can be given to cancer patients in the clinic, namely: (1) the target gene(s) driving the development of a specific cancer need to be unequivocally identified; (2) an siRNA that specifically targets a particular gene must be fabricated and (3) the man-made siRNA must be capable of being delivered into the target cell cytoplasm [6].

About 20 clinical trials have been initiated to study the great potential of siRNA- and miRNAbased therapy. Only 1 miRNA therapeutic drug, SPC3649, an inhibitor of miR-122, known on the market as miravirsen, has entered clinical trial [20]. A number of other miRNA-based therapeutic agents are currently in the preclinical stage of testing with the aim of introducing the most promising candidates into clinical trials in the near future. At present, many siRNA-based drugs have begun clinical trials for many researchers believe that miRNAs will become a unique category for RNAi-based therapy [20, 21]. The actions of miRNAs are similar to those of siRNAs regarding the post-transcriptional silencing of genes. This chapter has reviewed the biological features of miRNAs and described them as endogenous short RNAs that interact with Argonaute proteins and regulate the expression of many genes. While both siRNA- and miRNA are important for gene regulation at the translational level, endogenous siRNAs help to maintain the stability of the genome. Both are single-stranded and have been shown to be associated with RISC complex components. However, there are fundamental differences between the siRNA and miRNA mechanisms of action. miRNAs most often attach to 8 nucleotides that comprise the 5′ end to enable them to bind to the target mRNA sequences and thus use their inhibitory activity to limit the translation processes. On the other hand, siRNAs use nearly all of their full sequences to identify a particular target and thus mediate cleavage of the targeted mRNA [20]. Taken together, both siRNA and miRNA based therapies are currently under pre-clinical development as breast cancer treatment options.
