Small interfering RNAs (siRNAs) are a promising class of drugs for treating genetically-defined diseases. Therapeutic siRNAs enable specific modulation of gene expression, but require chemical architecture that facilitates efficient in vivodelivery. siRNAs are informational drugs, therefore specificity for a target gene is defined by nucleotide sequence. Thus, developing a chemical scaffold that efficiently delivers siRNA to a particular tissue provides an opportunity to target any disease-associated gene in that tissue. The goal of this project was to develop a chemical scaffold that supports efficient siRNA delivery to the brain for the treatment of neurodegenerative diseases, specifically Huntington’s disease (HD).

HD is an autosomal dominant neurodegenerative disorder that affects 3 out of every 100,000 people worldwide. This disorder is caused by an expansion of CAG repeats in the huntingtin gene that results in significant atrophy in the striatum and cortex of the brain. Silencing of the huntingtin gene is considered a viable treatment option for HD. This project: 1) identified a hyper-functional sequence for siRNA targeting the huntingtin gene, 2) developed a fully chemically modified architecture for the siRNA sequence, and 3) identified a new structure for siRNA central nervous system (CNS) delivery—Divalent-siRNA (Di-siRNA). Di-siRNAs, which are composed of two fully chemically-stabilized, phosphorothioate-containing siRNAs connected by a linker, support potent and sustained gene modulation in the CNS of mice and non-human primates. In mice, Di-siRNAs induced potent silencing of huntingtin mRNA and protein throughout the brain one month after a single intracerebroventricular injection. Silencing persisted for at least six months, with the degree of gene silencing correlating to guide strand tissue accumulation levels. In Cynomolgus macaques, a bolus injection exhibited significant distribution and robust silencing throughout the brain and spinal cord without detectable toxicity. This new siRNA scaffold opens the CNS for RNAi-based gene modulation, creating a path towards developing treatments for genetically-defined neurological disorders.