696. Pre-Clinical Evaluation of Allele-Specific Mutant Huntingtin Gene Silencing Antisense Oligonucleotides

Huntington disease (HD) is a dominant, genetic neurodegenerative disease characterized by progressive loss of voluntary motor control, psychiatric disturbance, and cognitive decline, for which there is currently no disease-modifying therapy. HD is caused by the expansion of a CAG tract in the huntingtin (HTT) gene. The mutant HTT protein (muHTT) acquires toxic functions, and there is significant evidence that muHTT lowering would be therapeutically efficacious. However, the wild-type HTT protein (wtHTT) serves vital functions, making allele-specific muHTT lowering strategies potentially safer than non-selective strategies. CAG tract expansion is associated with single nucleotide polymorphisms (SNPs) that can be targeted by gene silencing reagents such as antisense oligonucleotides (ASOs) to accomplish allele-specific muHTT lowering. We have evaluated ASOs targeted to HD-associated SNPs in acute in vivo studies including screening, distribution, duration of action and dosing, using a humanized mouse model of HD, Hu97/18, that is heterozygous for the targeted SNPs. We have identified four well-tolerated lead ASOs that potently and selectively silence muHTT at a broad range of doses throughout the central nervous system for 36 weeks or more after a single intracerebroventricular injection.We next conducted a pre-clinical therapeutic efficacy trial of lead ASOs and evaluated them for effect on the HD-like phenotypes of Hu97/18 mice. Treated mice underwent longitudinal behavioral and biochemical assessment followed by terminal neuropathology. Thus far we have determined that pre-symptomatic allele-specific muHTT silencing prevents onset of behavioral HD-like phenotypes. Evaluation of neuropathology and post-symptomatic intervention is ongoing. Contingent on findings from these studies and using delivery and dosing information gained from ongoing CNS ASO clinical trials, a primary SNP-targeted ASO drug could be fairly rapidly translated for human applications.