Rethinking The Oligonucleotide Backbone: Why RNA Therapeutics May Need A New Molecular Design Framework

By David R. Tabatadze, Ph.D., president & CEO, ZATA Pharmaceuticals

The RNA therapeutics industry has achieved what many once considered impossible. Antisense oligonucleotides (ASOs), small interfering RNAs (siRNAs), and related nucleic acid modalities have evolved from academic concepts into clinically validated therapeutic platforms with dozens of approved medicines and hundreds of programs in development.

Yet despite decades of progress and more than $100 billion invested across the oligonucleotide sector, many of the same challenges that limited early programs continue to constrain modern drug development. Delivery remains difficult. Toxicity remains a concern. Off-target effects continue to complicate development. And many promising candidates fail because improvements in one performance characteristic often come at the expense of another.

The industry has become exceptionally skilled at optimizing sequences, conjugates, and formulations. What has received comparatively less attention is the structure that connects everything together: the oligonucleotide backbone itself. As RNA therapeutics continue to mature, backbone engineering may represent one of the largest remaining opportunities for expanding the design space available to drug developers.

The Backbone Problem Hidden In Plain Sight

Most oligonucleotide platforms share a common architectural challenge. Regardless of modality, nucleic acids possess highly charged phosphate backbones. These negative charges are essential to natural nucleic acid biology, but they also create many of the limitations therapeutic developers must overcome.

The negatively charged backbone contributes to:

• limited cellular uptake
• poor membrane permeability
• rapid clearance
• formulation complexity
• reliance on delivery vehicles
• unwanted protein interactions.

For decades, researchers have attempted to address these limitations through various chemical strategies.

Standard oligonucleotides preserve native phosphate chemistry but often require extensive optimization to achieve meaningful delivery and pharmacokinetic performance. Neutral backbone technologies such as peptide nucleic acids (PNAs) and morpholinos eliminate charge-related issues but introduce different challenges involving geometry, solubility, and biological compatibility.

Conjugate approaches have provided important advances by attaching targeting ligands, lipids, or other functional groups to existing oligonucleotide scaffolds. However, these modifications often improve only specific aspects of performance while leaving the fundamental properties of the backbone unchanged.

The result is a familiar pattern across RNA therapeutics: developers continually balance trade-offs among potency, stability, delivery, tolerability, and manufacturability.

Expanding The Optimization Space

One way to view the evolution of oligonucleotide therapeutics is as a progressive expansion of molecular design flexibility.

Early programs focused primarily on sequence selection. Later generations incorporated sugar modifications such as 2'-O-methyl and 2'-fluoro chemistry. Additional advances introduced phosphorothioate linkages, GalNAc conjugates, lipid nanoparticles, and increasingly sophisticated delivery systems.

Each innovation expanded the number of variables available to developers. Backbone engineering represents the next logical extension of this trend.

Rather than replacing existing chemistries, backbone engineering seeks to provide additional molecular control while preserving the properties that have already made modern oligonucleotide therapeutics successful. This approach is based on a simple principle: if the backbone contributes significantly to delivery, stability, protein interactions, and pharmacokinetics, then it should be considered a programmable design element rather than a fixed structural feature.

Moving Beyond Phosphorothioates

Phosphorothioate chemistry has become one of the foundational technologies in oligonucleotide drug development. Replacing a nonbridging oxygen atom with sulfur improves stability and enhances interactions with serum proteins, contributing to improved pharmacokinetics. However, phosphorothioates also introduce new challenges.

Protein binding can be beneficial in some contexts but problematic in others. Certain toxicities observed in oligonucleotide development have been associated with phosphorothioate-mediated interactions. In addition, phosphorothioate modifications create stereochemical complexity that can complicate optimization efforts.

The broader question is whether future generations of oligonucleotide therapeutics require more precise methods for controlling backbone behavior. Instead of accepting a fixed relationship between charge, stability, and protein binding, developers may increasingly seek technologies that allow these variables to be adjusted independently. Such control could enable a more rational balancing of efficacy, safety, and delivery performance.

Functionalizing The Backbone

Historically, most oligonucleotide functionalization strategies have focused on terminal attachments or external conjugates. An alternative approach is to directly introduce functional groups into the backbone itself.

Backbone-level functionalization creates opportunities to incorporate:

• fluorescent probes
• hydrophobic moieties
• delivery-enhancing groups
• imaging agents
• targeting elements
• reporter molecules.

Because these modifications can be introduced at specific locations within an oligonucleotide, they potentially provide a level of spatial control that is difficult to achieve through conventional end-modification strategies.

Importantly, preserving native base pairing and duplex geometry remains critical. Any backbone engineering strategy must maintain compatibility with Watson-Crick hybridization while avoiding disruption of the protein interactions required for biological activity. The goal is not simply to attach more chemistry to RNA molecules but to do so in ways that preserve the molecular recognition processes upon which oligonucleotide therapeutics depend.

The Promise Of Partial And Selective Backbone Neutralization

Among the most intriguing concepts in backbone engineering is selective charge neutralization.

The negative charge of oligonucleotides is simultaneously one of their greatest strengths and one of their greatest liabilities. Charge enables solubility and biological recognition. Yet it also contributes to poor membrane permeability and limits intracellular delivery.

Complete charge neutralization often creates new problems, including altered molecular geometry and reduced biological compatibility. Selective backbone neutralization offers a potentially more nuanced solution. Instead of removing all charges, specific backbone regions can be modified while preserving overall molecular architecture. In theory, this approach could enable developers to tune multiple performance characteristics simultaneously, including:

• cellular uptake
• serum stability
• biodistribution
• endosomal escape
• mismatch discrimination
• pharmacokinetic behavior.

This concept mirrors trends seen elsewhere in drug development, where precision tuning increasingly replaces broad structural changes.

Improving Specificity Through Molecular Design

As oligonucleotide therapeutics become more potent, specificity becomes increasingly important. Off-target activity remains one of the most persistent concerns across RNA modalities.

Traditional approaches focus heavily on sequence optimization. While sequence design remains essential, backbone architecture may also influence target selectivity.

Experimental data suggest that certain backbone modifications can improve mismatch discrimination, potentially allowing oligonucleotides to distinguish more effectively between intended and unintended targets. If these observations continue to hold across broader data sets, backbone engineering could become an additional tool for reducing off-target effects without sacrificing potency. For developers pursuing highly selective gene silencing strategies, this represents a particularly attractive possibility.

Preserving Manufacturability

Novel chemistry is only valuable if it can be translated into practical manufacturing processes. The history of oligonucleotide development contains numerous examples of promising molecular designs that ultimately failed because they could not be produced reliably at scale. Any next-generation backbone technology must therefore satisfy two requirements simultaneously: First, it must expand biological performance. Second, it must remain compatible with existing oligonucleotide manufacturing infrastructure. This is perhaps one of the most important considerations as the field evolves.

The future of RNA therapeutics will not be built entirely on disruptive replacement technologies. More likely, it will emerge through innovations that integrate seamlessly into established development and manufacturing workflows. Technologies that preserve amidite chemistry, automated synthesis, and scalable production may therefore have significant practical advantages.

The Broader Implications For RNA Therapeutics

The most important takeaway from backbone engineering is not any single modification strategy. It is the recognition that the optimization space for RNA therapeutics remains far larger than many assume.

For years, the industry's attention has focused primarily on sequence design, delivery vehicles, and conjugation strategies. Those areas will continue to be important. However, backbone chemistry may offer another dimension of control that has not yet been fully explored. Future oligonucleotide development may increasingly involve combinations of:

• advanced sequence design
• delivery engineering
• conjugate technologies
• computational optimization
• backbone-level molecular tuning.

Together, these approaches could create a new generation of therapeutics with improved potency, specificity, tolerability, and tissue access.

Conclusion

The evolution of oligonucleotide therapeutics has always been driven by chemistry. From the earliest antisense molecules to today's sophisticated siRNA platforms, each major advance has expanded the molecular toolkit available to developers. Backbone engineering represents the next potential expansion of that toolkit.

Rather than replacing established technologies, it offers an opportunity to augment them, providing new ways to tune molecular behavior while preserving the core architecture that has already proven clinically successful. As RNA therapeutics move into increasingly challenging disease areas and tissue targets, developers may find that future gains come not from entirely new modalities but from reimagining one of the oldest components of nucleic acid design: the backbone itself.

About The Author:

David R. Tabatadze, Ph.D., is president and CEO of ZATA Pharmaceuticals, where he leads the development of next-generation oligonucleotide chemistries and nucleic acid therapeutics. Trained under the late Paul Zamecnik, Tabatadze has spent more than two decades advancing innovations in oligonucleotide backbone engineering, antisense technologies, and gene-silencing platforms. His work focuses on developing chemically modified RNA and DNA therapeutics designed to improve stability, cellular uptake, specificity, and therapeutic performance across ASO, siRNA, and gene-editing applications. Prior to joining ZATA, he held scientific leadership and research roles at Hybridon, Viral Inactivation Technologies, and Palomar Medical Technologies. A frequent speaker at industry conferences, including TIDES USA, Tabatadze is recognized for his contributions to the evolution of oligonucleotide chemistry and the future of RNA-based medicines.