SCAD: Unlocking a New Era of Duplex RNA Delivery to the Brain and Beyond

Delivering on the RNA Promise — Beyond the Liver

Over the past decade, RNA therapeutics have reshaped modern medicine.
From small interfering RNAs (siRNAs) to antisense oligonucleotides (ASOs), these molecules can precisely silence or activate disease-associated genes.

Yet one challenge has persisted: delivery.

While GalNAc conjugation has enabled highly effective siRNA delivery to the liver, most extrahepatic tissues — especially the central nervous system (CNS) — remain out of reach due to biological barriers like the blood-brain barrier (BBB).
This has limited the potential of RNA therapeutics for neurological and systemic diseases.

Introducing SCAD: Smart Chemistry Aided Delivery

Researchers at Ractigen Therapeutics (Suzhou, China) have unveiled a breakthrough delivery platform called Smart Chemistry Aided Delivery (SCAD), reported in Molecular Therapy – Nucleic Acids (Cell Press).

SCAD represents a new class of modular chemical design that leverages the self-delivery characteristics of ASOs to transport duplex RNAs (siRNA, saRNA, miRNA) efficiently into difficult-to-reach tissues — including the brain.

At its core, SCAD attaches a non-targeting single-stranded accessory oligonucleotide (ACO) to one strand of a siRNA duplex.
This ACO acts as a chemical “booster,” enhancing plasma protein binding, improving cellular uptake, and promoting broad tissue distribution — without compromising gene-silencing potency.

How It Works: Modular Chemistry Meets Molecular Precision

Through extensive structure–activity relationship (SAR) optimization, the Ractigen team identified key factors behind SCAD’s performance:

2’-O-MOE (2'-O-methoxyethyl) and phosphorothioate (PS) modifications are critical for stability and protein affinity.

ACO length (optimal around 14 nucleotides) strongly influences biodistribution and potency.

Linear linkers (like S9) outperform cyclic linkers in maintaining activity.

Fine-tuning PS content helps balance CNS retention with safety.

This fully solid-phase synthetic design ensures scalability, manufacturing simplicity, and cost efficiency — vital advantages over antibody or lipid-based systems.

CNS Delivery: Durable Knockdown, Broad Reach

In rodent models, a single intracerebroventricular (ICV) dose of SCAD-siRNA achieved:

• Widespread biodistribution across cortex, hippocampus, cerebellum, and spinal cord
• Deep and sustained mRNA knockdown — up to 80% in multiple regions
• Extended duration of effect lasting over 5 months
• Minimal systemic exposure — rapid plasma clearance within 24 hours

Compared to conventional siRNAs, SCAD maintained higher concentrations in deep brain structures and showed superior pharmacodynamic durability.
This long-lasting CNS effect could allow for extended dosing intervals in chronic neurodegenerative conditions.

Targeting Neurons and Glial Cells Alike

Beyond neurons, SCAD effectively delivers RNA to astrocytes and microglia — key players in neuroinflammation and disease progression.

This multi-cell-type reach is particularly valuable for diseases like ALS, Huntington’s, and Alzheimer’s, where both neuronal and non-neuronal dysfunction contribute to pathology.

By silencing mutant SOD1 across diverse CNS cell types, SCAD has demonstrated disease-modifying potential in preclinical ALS models — and is now being evaluated in clinical trials (RAG-17, NCT06556394).

Beyond the Brain: Local Delivery to Other Tissues

The modularity of SCAD extends its impact well beyond the CNS.
Ractigen’s team tested local administration routes in multiple tissues:

These results underscore SCAD’s versatility as a single chemistry platform capable of delivering duplex RNA to numerous extrahepatic tissues.

Safety First: Clean Profile and Low Immunogenicity

SCAD has demonstrated excellent safety and tolerability across preclinical models:

21. No significant body weight loss or clinical abnormalities
21. Stable CNS immune markers (Iba1, Gfap) for up to 5 months post-dose
21. Normal hematology and serum chemistry
21. No cytokine induction or systemic toxicity

Even at high doses, only mild, reversible neuromuscular effects were noted in isolated cases — with no dose-dependent trends.
Unlike lipid or antibody-based conjugates, SCAD avoids hydrophobicity-associated toxicity, offering a cleaner safety profile for chronic applications.

Why It Matters: A New Frontier for RNA Therapeutics

By merging chemical simplicity with biological sophistication, SCAD bridges the gap between lab innovation and clinical translation.

It provides a platform for designing potent, durable, and safe RNA drugs across therapeutic areas — from neurodegeneration to ophthalmology and respiratory disease.

Looking Ahead: From Discovery to Translation

Mechanistic studies suggest that the ACO component may act as an aptamer-like interface, binding specific proteins that facilitate uptake.
Future proteomic analyses will help unravel this mechanism — and further refine delivery performance.

With clinical trials now underway in SOD1-ALS, the coming years could see SCAD evolve from a promising preclinical platform to a new standard in oligonucleotide delivery.

Final Takeaway

“SCAD represents a new generation of oligonucleotide delivery — chemically defined, modular, and clinically scalable. It opens the door to efficient duplex RNA delivery to the CNS and beyond.”
— Long-Cheng Li, Ph.D., Founder & CEO, Ractigen Therapeutics

Key Highlights

• Published in Molecular Therapy – Nucleic Acids (Cell Press)
• Enables efficient duplex RNA delivery to CNS and extrahepatic tissues
• Achieves sustained knockdown >5 months in vivo
• Demonstrates broad cell-type accessibility and excellent safety
• Currently in ALS clinical evaluation (RAG-17)