**1. Introduction**

The usage of nucleic acids in therapeutics is founded on the inhibition of RNA expression and on their capacity to modulate the expression of a target protein associate to a disease [1]. ASOs are typically modified synthetic nucleic acids. They are single stand oligonucleotide with 16-mer to 21-mer sequence with high affinity for a specific RNA target sequence, through base pairing [2]. The two general chemical classes of nucleic acids commonly used today are antisense oligonucleotides (ASOs) and siRNA. The first is single-stranded, and modulates RNA function either by degrading the RNA sequence in question due to enzyme RNase H activity, or by modifying splicing function thus affecting RNA metabolism; and the second is double-stranded synthetic oligonucleotides that degrade target RNA through an RNA-induced silencing complex (RISC). Importantly, both must reach the nucleus and/or cytoplasm to exert their activity, and thus must cross a biological membrane.

Although ASOs and siRNA share similarities, they are divergent on some points, and so, choosing ASO or siRNA strategy for gene targeting depends on the target gene. However, since ASOs are single stranded, as opposed to siRNA, they have lower cost of production. Moreover, it is easier to deliver ASOs *in vivo*, since they do not a vector and a simple chemical modification can increase their resistance

to nucleases, as opposed to siRNA that need a carrier. Finally, for *in vitro* studies, siRNA is considered a better technology, since it's relatively easier to obtain a potent siRNA since unmodified RNA works with high potency as opposed to ASOs.

In this review we will mainly focus on ASOs chemistry and mechanism of action.

Indeed, since Zamecnik in 1978 used an ASO-like unmodified DNA sequence in cell culture, notable progress has been made in ASO pharmacology [3]. Currently, the efficacy of different ASOs is being studied in many neurodegenerative diseases such as Huntington's disease, Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis but also in several cancer states. Numerous ASO-based therapeutics are being tested in clinical trials. In **Table 1** figures some of the ASO in clinical trial for cancer treatments. However, until now, only two ASOs have been approved by the US FDA to be used on humans, namely, fomivirsen (Vitravene),


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**Figure 1.**

*(6) activation of RNase H.*

*Antisense Oligonucleotides, A Novel Developing Targeting Therapy*

**2. ASO's design, chemistry and mechanism of action**

of a 5′-cap structure, splicing and polyadenylation.

splice modulators are active in the nucleus.

a first-generation ASO for cytomegalovirus (CMV) retinitis, and mipomersen (Kynamro), a second-generation ASO for homozygous familial hypercholesterolemia (HoFH) [4, 5], both developed by Isis Pharmaceuticals. They work via RNase

Two major mechanisms contribute to the antisense activity. The first is that most ASOs are designed to activate RNase H, which cleaves the RNA moiety of a DNA–RNA heteroduplex and therefore leads to degradation of the target mRNA in the nucleus and cytoplasm. In addition, ASOs that do not induce RNase H cleavage can be used to inhibit translation by steric blockade of the ribosome in the cytoplasm [6]. When the ASOs are targeted to the 5′-terminus, binding and assembly of the translation machinery can be prevented [7]. Most mammalian RNAs undergo multiple post-transcriptional processing steps in the cell nucleus including addition

Regulation of RNA processing is another efficient mechanism in which ASOs can be utilized to regulate gene expression. Studies have been published documenting that ASOs can be used to destabilize pre-mRNA [8] and to regulate RNA splicing [9]. Another viable approach to reversibly 'switch' protein function is alternative splicing that generates RNA encoding antagonistic proteins (**Figure 1**). Whether RNaseH activity takes place favorably in the cytoplasm or in the nucleus is not well documented. However, most studies suggest that the most part of the inhibition takes place in the cytoplasm. Controversially, ASOs that targets pre-mRNA and are

*ASOs mechanism of action. (1) In the absence of ASO, normal gene and protein expression is maintained. (2) Formation of ASO-mRNA heteroduplex in cytoplasm induces activation of RNase H, leading to mRNA degradation or (3) steric interference of ribosomal assembly. Alternatively, ASO can enter the nucleus and regulate mRNA maturation by (4) inhibition of 5*′ *cap formation, (5) inhibition of mRNA splicing and* 

*DOI: http://dx.doi.org/10.5772/intechopen.82105*

H-mediated cleavage of the targeted RNA.

**2.1 ASO's mechanism of action**

### **Table 1.**

*Registered clinical studies with ASO in cancer treatments in clinical trials.gov.*

a first-generation ASO for cytomegalovirus (CMV) retinitis, and mipomersen (Kynamro), a second-generation ASO for homozygous familial hypercholesterolemia (HoFH) [4, 5], both developed by Isis Pharmaceuticals. They work via RNase H-mediated cleavage of the targeted RNA.
