N-Acetylgalactosamine (GalNAc) conjugation is a groundbreaking advancement in oligonucleotide delivery, enabling highly efficient and selective uptake by liver cells through the asialoglycoprotein receptor (ASGPR). Originally inspired by research into hepatic glycoprotein clearance, GalNAc ligands now play a central role in oligonucleotide therapeutics targeting hepatic diseases.
Why GalNAc-conjugated oligonucleotides?
- Targeted Delivery: High affinity binding to ASGPR enables hepatocyte-specific uptake.
- Improved Pharmacokinetics: Longer therapeutic half-life allows for monthly to semi-annual dosing.
- Clinical Success: Over 10 GalNAc-conjugated oligonucleotides are already FDA-approved.
- Subcutaneous Delivery: Facilitates patient-friendly administration with robust liver-specific gene silencing.
Mechanism of Action for GalNAc Oligonucleotide Conjugated ligand
The GalNAc ligand binds to ASGPR on the surface of hepatocytes, triggering receptor-mediated endocytosis. Once internalized, the oligonucleotide is released into the cytoplasm where it can perform its gene-silencing function via RNA interference (RNAi) or antisense mechanisms.
Figure 1: Structure of a 3’-trianennary GalNAc-based ASGPR ligand. This structure, when conjugated to an siRNA, allows specific delivery of the RNA drug to the liver since it targets the asialoglycoprotein receptor (ASGPR).
GalNAc Modified Oligonucleotide Chemistry & Compatibility
GalNAc ligands are fully compatible with automated solid-phase oligonucleotide synthesis. Building blocks are available in formats that integrate smoothly into standard protocols:
| Product | Description | Application | Format |
| 5'-GalNAc-C3 Phosphoramidite | 5' end-compatible GalNAc phosphoramidite with C3 linker | Solid-phase synthesis for siRNA/ASO delivery | Phosphoramidite |
| GalNAc-C3 CPG | 3' end-modified GalNAc with controlled pore glass support | 3' terminal GalNAc modification | CPG Solid Support |
| Triantennary GalNAc Ligand (5' and 3') | Pre-synthesized trivalent GalNAc cluster | Post-synthesis conjugation via click or amide chemistry | Solution-based Ligad |
Scientific Impact
GalNAc conjugation, pioneered by studies such as Nair et al. (2014), represents a revolution in siRNA and ASO delivery. The ability to achieve potent liver-specific gene knockdown from subcutaneous administration opens doors to safer, more accessible therapies for metabolic and genetic liver disorders.
Challenges of Oligonucleotide Therapeutics
Low oral bioavailability Rapid clearance by kidneys and reticuloendothelial system Hydrophilic nature limits passive diffusion across membranes Susceptibility to nuclease degradation Despite these challenges, GalNAc technology addresses many of the barriers by enhancing delivery, increasing potency, and reducing dose frequency.
Looking Ahead
While GalNAc focuses on liver-targeting, future developments aim to adapt similar receptor-targeting ligands for delivery to other tissues. This platform paves the way for expanding oligonucleotide therapeutics beyond hepatocytes to treat a wide range of diseases.
Reference
- Erik A.L. Biessen and Theo J.C. Van Berkel; N-Acetyl Galactosamine Targeting: Paving the Way for Clinical Application of Nucleotide Medicines in Cardiovascular Diseases. Arteriosclerosis, Thrombosis, and Vascular Biology. 2021;41:2855–2865. [Link]
- Nair JK, Willoughby JLS, Chan A, Charisse K, Alam MR, Wang Q, Hoekstra M, Kandasamy P, Kel’in AV, Milstein S, et al. (2014) Multivalent N-Acetylgalactosamine-Conjugated SiRNA Localizes in Hepatocytes and Elicits Robust RNAi-Mediated Gene Silencing. J. Am. Chem. Soc 136 (49), 16958–16961. [ACS]
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