**8. Concluding remarks**

cient success suggests that further investigation of allosteric regulation is essential to advance

Aptamers, single-stranded deoxyribonucleic acid or ribonucleic acid oligonucleotides, are generated by an *in vitro* selection process called SELEX (systematic evolution of ligands by exponential enrichment). They can bind their target molecules with high specificity and selectivity, indicating the probable therapeutic and diagnostic applications for diseases like cancer, inflammatory diseases, etc. [70, 71]. Because aptamers contain some advantages over antibodies and other conventional small-molecule therapeutics, such as high specificity, flexible modification, and low adverse effect, they have been shown as a valuable substi‐ tute to protein antibodies [72]. Moreover, the strategies developed to chemically modify backbone can further improve affinity and bioavailability of aptamers [73]. Higher affinity and specificity could be simultaneously achieved by the genetic-algorithms-based ISM (in

The properties above have paved the way to further studies on introduction of aptamers to preclinical and clinical applications. Based on previous data showing antitumor activity of AS1411, a first-in-class quadruplex DNA aptamer targeting nucleolus, a phase II trial found that AS1411 appears to have dramatic and durable responses in enrolled patients with metastatic renal cell carcinoma, even though about 34% participants have AS1411-related mild adverse events [75]. Malik *et al.* further discovered that AS1411-linked gold nanospheres (AS1411-GNS) could markedly promote superior cellular uptake by cancer cells and increase antiproliferative/cytotoxic effects, with no signs of toxicity [76]. Likewise, other clinical trials on aptmers targeting FIX (Coagulation Factor IX) [77], vWF (von Willebrand factor) [78], and TFPI (tissue factor pathway inhibitor) [79] respectively, all show that aptamers are well tolerated, safe, and represent a new promising target therapy. However, as for some side effects, further clinical investigations are warranted to better define the clinical indications,

Unlike antisense oligonucleotide approaches that target mRNA, decoys are short, doublestranded DNA molecules that compete with specific binding sites of transcription factors to prevent their binding at target promoters, in order to inhibit gene expression at pretranscrip‐ tion level. Since decoys are DNA, they are more stable and easy to handle than RNA-based intervention strategies [80]. Some methods, including the locked nucleic acid (LNA) intro‐ duced at the 3'-end [81] and chimeric decoys containing discrete binding sites [82], can increase decoys nuclease resistance and specificity. So far, numerous of studies have indicated that decoys are suited for novel potential therapeutic for combating cancer [80] and infectious

the drug development.

148 Nucleic Acids - From Basic Aspects to Laboratory Tools

silico maturation) [74].

**7. Decoys**

safety, efficacy, and optimal dosing strategy.

**6. Aptamers**

Gene-targeting strategies based on nucleic acid have opened a new era with the development of potent and effective gene intervention techniques, such as DNAzymes, ribozymes, siRNA, ASOs, aptamers, decoys, etc. It is demonstrated that these technologies have versatility and potency in disrupting pathophysiologically important pathways by silencing the target gene with relative specificity *in vivo* and *in vitro*. Numerous investigative works by several labora‐ tories have been made in these fields. Although some clinical trials have proved the effective‐ ness of these techniques, only a few antisense drugs have been approved by the FDA for clinical purposes. The main difficulties on the way to develop successful nucleic acid drugs are as follows: how to ensure efficient and controlled delivery, prolonged target-specific action, and no adverse effects. If the challenges outlined above can be overcome, these molecules would prove to be valuable agents for economical and practical new therapies for diseases in the near future.
