Know your oligo mod: 2ʹ-MOE

The "Know your oligo mod" series continues with a deep dive into the 2'-O-methoxyethyl (2'-MOE) modification, a major advance in oligonucleotide therapeutic technology.  

The 2' position on the sugar moiety of nucleotides is a prime location for chemical modification, playing a pivotal role in antisense technology and genome-based drug discovery. The 2'-MOE modification replaces the 2'-OH in ribose with an O-methoxyethyl group, enhancing the attributes of therapeutic oligonucleotides, especially antisense oligonucleotides (ASOs). 

Enhancing drug efficacy with 2'-MOE modifications 

2ʹ-MOE is known as a second-generation antisense modification, along with 2'-O-methyl (2ʹ-OMe) and 2ʹ-Fluoro (2'-F) (figure 1). Most approved oligo therapeutics incorporate these 2ʹ modifications in their structure, increasing resistance to nucleases without compromising their ability to hybridise with complementary RNA targets. 

Figure 1: Types of 2' modifications in second-generation ASOs: 2ʹ-MOE, 2ʹ-OMe and 2ʹ-F. 

The 2'-MOE modification is particularly effective in increasing affinity, specificity and stability. As such, it is commonly incorporated in oligonucleotides used for posttranscriptional gene silencing, particularly ASOs. 

There are several key properties of MOE-modified oligonucleotides that make this a popular approach for developing therapeutics, including: 

• Nuclease resistance: Resistance to nuclease degradation is essential for the effectiveness of oligonucleotide-based therapies. A landmark study in 1995 by Pierre Martin demonstrated the superior stability of 2ʹ-MOE-modified oligonucleotides compared to unmodified and OMe-modified counterparts in the same positions.¹ 2ʹ-MOE increases nuclease resistance by replacing the nucleophilic 2ʹ-hydroxyl group of unmodified RNA, enhancing the stability of oligos in vivo.¹ 
• Binding affinity: The 2' modifications, in general, increase the binding affinity of oligos by enhancing the thermodynamic stability of nucleic acid duplexes. This is because of the 3'-endo pucker conformation (RNA-like) of the ribose, which is strongly preferred by A-form RNA duplexes.² The rigidity of this conformation is attributed to gauche effects and extensive hydration.³ 2'-MOE increases ΔTm by 0.9 to 1.6 °C per modification. This effect is similar to the 2ʹ-OMe modification but less than the 2ʹ-F modification, which offers a ΔTm increase of 2.5 °C per modified nucelotide.³ 
• Increased specificity: Alongside high affinity, antisense oligonucleotides must exhibit high specificity, differentiating their target mRNA sequence from similar, non-target sequences, to ensure low cytotoxicity. Notably, MOE-modified oligonucleotides were shown to lose affinity in the presence of mismatches or bulges compared to unmodified deoxyoligonucleotides, demonstrating their enhanced specificity.³ 
• Increased lipophilicity: The 2ʹ-modification alters the lipophilicity of oligonucleotides, which is crucial for favourable pharmacokinetic properties. The lipophilicity of the modification is associated with improved protein binding/absorption properties and decreased solubility in vivo.¹ However, it's noteworthy that third-generation therapeutic oligonucleotides can feature lipophilic conjugates like cholesterol and palmitic acid, enhancing cell penetration and tissue uptake by facilitating passage through lipid membranes. Alternatively, an N‑acetylgalactosamine (GalNAc) ligand can help to increase penetration of liver cells. 
  
  

2'-MOE in drug development  

2ʹ-MOE is the most common 2ʹ modification used in therapeutic ASOs. These second-generation ASOs enabled higher potency, longer tissue half-lives, and reduced pro-inflammatory effects in vivo.³ Currently, 2ʹ-MOE is used in seven approved ASO drugs (see table 1) and many others in clinical trials.⁴ 
 

Table 1: Approved therapeutic ASOs.⁵ 

Steric-blocking ASOs

Steric-blocking ASOs prevent other molecules from binding to the target RNA. They are also used to hinder protein-RNA interactions between splicing machinery and pre-mRNA, selectively altering the splicing of specific transcripts. Since 2ʹ-ribose modifications are not compatible with RNase H activity, steric-blocking ASOs are fully modified at the 2ʹ position to avoid degradation of target RNA, with 2ʹ-MOE being the most widely employed modification. For example, Spinraza (nusinersen) is an all-PS/MOE-modified splice-switching ASO approved for spinal muscular atrophy, which modulates gene splicing of the survival motor neuron 2 (SMN2).⁴ 

Gapmers

Gapmers are another class of antisense oligonucleotides that silence genes by forming a DNA-RNA duplex and triggering RNase H to cleave the target mRNA. Gapmers have a central sequence of PS-DNA nucleotides ("DNA gap") complementary to the target RNA, with flanking modified RNA residues on either side to protect the DNA gap from nuclease degradation and improve binding affinity.⁶ Interestingly, 2'-MOE gapmers have a better safety profile than those with locked nucleic acid modifications, which can cause hepatotoxicity. These favourable properties have led to several 2ʹ-MOE gapmers being approved, including Kynamro (mipomersen), Tegsedi (inotersen) and Wainua (eplontersen).⁴ Kynamro was just the second ASO to be approved, 14 years after the first, demonstrating the potential for 2ʹ-MOE gapmers in this burgeoning field. 

siRNA

Another therapeutic area of interest is siRNA; although 2ʹ-MOE is not used in any approved siRNA drugs, that may change in the future. siRNA works by combining with argonaute-2 and other proteins to form the RNA-induced silencing complex (RISC), and 2ʹ-ribose modifications at specific positions can interfere with RISC loading and silencing activity. However, some studies have shown that MOE modification can increase the target-binding affinity and improve the nuclease stability of siRNAs. Song et al. found that adding 2′-MOE at the cleavage site improved both the specificity and silencing activity of siRNAs by facilitating the oriented RISC loading of the modified strand.⁷ Furthermore, Saito et al. reported dual sugar modification, i.e. combining 4ʹ-thioribonucleosides with 2ʹ-MOE in siRNA, offers synergistic benefits, boosting nuclease stability and prolonging silencing activity.⁸ 

2ʹ-MOE exemplifies how vital modification selection can be to the success of oligonucleotide delivery and functionality. It has helped to drive antisense technology, enabling life-extending drugs to reach approval after a long wait for a successful second generation of ASOs. 

LGC Biosearch Technologies™ offers 2'-MOE oligo modifications for research and clinical development of various types of therapeutic oligonucleotides. With our technical expertise and innovative toolbox solutions, simplify your path from drug discovery to commercialisation.  

Know your oligo mod series 

• Phosphorothioate bonds 
• ATTO dyes 
• 5-Nitroindole – a universal base oligo modification 
• BHQ™ (Black Hole Quencher™) non-fluorescent quenchers 
• Methylene Blue 
• Thio C6 linker
• 3' Spacer C3
  

References 

1. Martin P (1995) ‘Ein neuer Zugang zu 2′-O-Alkylribonucleosiden und Eigenschaften deren Oligonucleotide’, Helvetica Chimica Acta 78, 486–504.
2. Manoharan M (1999) "2′-Carbohydrate Modifications in Antisense Oligonucleotide Therapy: Importance of Conformation, Configuration and Conjugation." Biochimica Et Biophysica Acta (BBA) - Gene Structure and Expression 1489: 117-130. https://doi.org/10.1016/S0167-4781(99)00138-4
3. Hill AC and Hall J (2023) "The MOE Modification of RNA: Origins and Widescale Impact on the Oligonucleotide Therapeutics Field." Helvetica Chimica Acta 106, no. 3: e202200169. https://doi.org/10.1002/hlca.202200169
4. Egli M and Manoharan M. (2023) "Chemistry, Structure and Function of Approved Oligonucleotide Therapeutics." Nucleic Acids Research 51, no. 6: 2529-2573. https://doi.org/10.1093/nar/gkad067
5. Quemener, AM et al. (2021) "Small Drugs, Huge Impact: The Extraordinary Impact of Antisense Oligonucleotides in Research and Drug Development." Molecules 27, no. 2. https://doi.org/10.3390/molecules27020536
6. Marrosu E et al. (2017) "Gapmer Antisense Oligonucleotides Suppress the Mutant Allele of COL6A3 and Restore Functional Protein in Ullrich Muscular Dystrophy." Molecular Therapy - Nucleic Acids 8: 416-427. https://doi.org/10.1016/j.omtn.2017.07.006
7. Song X et al. (2017) "Site-Specific Modification Using the 2′-Methoxyethyl Group Improves the Specificity and Activity of SiRNAs." Molecular Therapy - Nucleic Acids 9: 242-250. https://doi.org/10.1016/j.omtn.2017.10.003
8. Gangopadhyay S and Gore KR (2022) "Advances in SiRNA Therapeutics and Synergistic Effect on SiRNA Activity Using Emerging Dual Ribose Modifications." RNA Biology 19, no. 1: 452-467. https://doi.org/10.1080/15476286.2022.2052641