GalNAc-siRNA is an advanced gene therapy approach that combines N-acetylgalactosamine (GalNAc), a targeting molecule, with small interfering RNA (siRNA), a gene-silencing agent. This conjugate delivers siRNA specifically to certain cells, enhancing its therapeutic impact while minimizing effects on other tissues. This method addresses diseases at their genetic root by preventing the production of problematic proteins.
Understanding RNA Interference
RNA interference (RNAi) is a natural biological mechanism within cells that regulates gene expression by “silencing” specific genes. This process involves small RNA molecules, such as small interfering RNA (siRNA), which prevent the production of unwanted proteins. When double-stranded RNA (dsRNA) is introduced into a cell, an enzyme called Dicer cleaves it into shorter fragments, forming siRNA.
These siRNA molecules then assemble with protein components to form the RNA-induced silencing complex (RISC). Within this complex, the siRNA unwinds, and one strand, the guide strand, directs RISC to a complementary messenger RNA (mRNA) molecule. Upon binding, RISC cleaves and degrades the target mRNA, stopping it from being translated into a protein. This effectively “silences” the gene.
The Role of GalNAc in Delivery
N-acetylgalactosamine (GalNAc) is a sugar molecule that plays a significant role in targeted drug delivery, particularly to the liver. It has a high affinity for the asialoglycoprotein receptor (ASGPR), a protein found predominantly on the surface of liver cells, known as hepatocytes. The ASGPR is a high-capacity receptor that rapidly internalizes molecules it binds.
When GalNAc is chemically attached to a therapeutic agent, it binds to the ASGPR on liver cells. This interaction initiates receptor-mediated endocytosis, where the cell actively takes in the GalNAc-bound therapeutic. The use of multiple GalNAc units significantly increases binding affinity to ASGPR, promoting efficient uptake into liver cells. This liver-specific targeting ensures the therapeutic agent is concentrated where needed, minimizing off-target effects.
How GalNAc-siRNA Works
GalNAc-siRNA conjugates are designed by chemically attaching GalNAc to siRNA molecules. This conjugation allows the combined molecule to be administered and specifically delivered to liver cells. Once administered, the GalNAc component binds with high specificity to the asialoglycoprotein receptor (ASGPR) on the surface of hepatocytes.
The binding of the GalNAc-siRNA conjugate to ASGPR triggers endocytosis, where the cell engulfs the complex into endosomes. As these endosomes mature, their internal environment becomes more acidic, causing the GalNAc-siRNA to dissociate from the ASGPR. The ASGPR is then recycled back to the cell surface, while the GalNAc-siRNA remains within the endosome. A small fraction of the siRNA then escapes from the endosome into the cytoplasm of the hepatocyte. Once in the cytoplasm, the siRNA uncouples from GalNAc and enters the RNA interference pathway, where it can effectively silence its target gene by guiding the RISC to degrade specific messenger RNA.
Current and Future Therapeutic Uses
GalNAc-siRNA technology has found significant practical applications, especially in the treatment of liver-related genetic disorders. Several GalNAc-siRNA drugs have received regulatory approval and are currently used to treat conditions such as acute hepatic porphyria, hereditary transthyretin-mediated amyloidosis, and primary hyperoxaluria type 1. Examples include givosiran for acute hepatic porphyria, and inclisiran, which targets the PCSK9 protein in the liver to help lower cholesterol levels.
The liver-targeting specificity of GalNAc-siRNA makes it a suitable therapeutic option for these conditions, as problematic genes or proteins are primarily expressed in hepatocytes. This targeted delivery enhances the efficacy of gene silencing and minimizes unwanted effects on other tissues. The success of GalNAc-siRNA conjugates has opened doors for expanding their use to a wider range of liver-centric diseases, including chronic hepatitis B and nonalcoholic steatohepatitis, with many candidates in clinical development. While the primary focus remains on liver diseases, this targeted delivery concept could be extended to other organs by identifying different cell-specific receptors.