What is RNAi (Ribonucleic Acid interference) technology used for?

RNAi Technology: Therapeutic Applications

RNAi technology is used for gene therapy through post-transcriptional gene silencing, specifically to treat gene-related diseases by inhibiting or suppressing the expression of disease-causing genes. 1

Primary Therapeutic Mechanism

RNAi functions as a gene intervention modality within the broader gene therapy framework, distinct from gene addition (replacing defective genes) and gene editing (directly modifying DNA). 1 The technology works by:

• Silencing target genes through sequence-specific Watson-Crick base pairing interactions between double-stranded RNA (dsRNA) and the target messenger RNA (mRNA) 1
• Mediating post-transcriptional gene regulation via two main pathways: short interfering RNAs (siRNAs) that cleave target mRNA, or microRNAs (miRNAs) that repress translation or cleave mRNA targets 1

Clinical Applications

Current FDA-Approved Therapeutics

Four siRNA medications have received FDA approval, demonstrating RNAi's transition from bench to bedside 2:

• Patisiran
• Givosiran
• Lumasiran
• Inclisiran

The approval of Onpattro (patisiran) proved that lipid nanoparticle technology is clinically relevant for nucleic acid delivery 1

Disease Targets Under Investigation

Cancer remains the predominant focus of RNAi-based therapeutics, though applications have expanded significantly 1:

• Cardiovascular diseases 3
• Viral infections including hepatitis B virus (HBV) 1, 3
• Neurodegenerative diseases where reduction of mutant or toxic gene expression provides therapeutic benefit 4
• Genetic diseases for which effective therapies are currently lacking 4

Emerging Applications in Immune Modulation

Recent trends show increasing focus on manipulating immune responses, with RNAi targeting:

• Macrophages (in brain, heart, liver, lung, and tumor microenvironments) 1
• T cells (CD4/8+ populations) 1
• Neutrophils and circulating monocytes 1
• Tumor-associated macrophages 1

This immune-targeting approach leverages the natural accumulation of immune cells at lesional tissue sites, potentially improving targeting efficiency for traditionally difficult-to-reach organs like brain and heart 1

Advantages Over Alternative Gene Therapies

RNAi offers broader applicability compared to gene addition or gene editing because:

• The translation repression strategy is theoretically suitable for all types of gene-related diseases 1
• Targeting is straightforward, governed by specific base-pairing rather than complex DNA integration 1
• Non-viral delivery vehicles provide favorable characteristics including lack of mutagenic risk, feasibility for modification, enhanced targeting capability, and lower costs compared to viral vectors 1

Critical Limitations and Safety Concerns

Off-Target Effects

Off-target gene silencing represents a major obstacle to clinical translation 1:

• Occurs through partial binding to non-target mRNA with incomplete homology 1
• Can result from improper loading of sense strand (rather than antisense strand) into the RNA-induced silencing complex (RISC) 1
• May cause unintended gene knockdown with consequences ranging from toxicity to cell death 1

Immune Stimulation

Exogenous RNAi can activate innate immunity through 1:

• Binding to cytosolic receptors (protein kinase R and 2'-5'-oligoadenylate synthetase) that recognize dsRNA
• Activation of toll-like receptors (TLR3, TLR7, TLR8) on cell surfaces or in endosomes
• Subsequent release of type I interferons and inflammatory cytokines

Clinical trial failures have occurred due to these adverse effects 1:

• Ocular neovascularization trials terminated at Phase III due to non-specific TLR3-mediated immune stimulation rather than intended gene silencing
• ARC-520 (anti-HBV siRNA) discontinued after deaths in non-human primates at highest doses
• MRX34 (liposomal miR-34a for solid tumors) stopped due to severe cytokine release syndrome

Research and Diagnostic Applications

Beyond therapeutics, RNAi serves as a powerful research tool for:

• Validating drug targets in preclinical development 4
• Performing reverse genetics to study gene function 4
• In vivo imaging to monitor RNAi delivery and silencing effects in clinical trials 5

Inducible RNAi systems provide temporal and spatial control through inducer-mediated small hairpin RNA expression, addressing concerns about non-specific gene silencing 6