, 2007). These RNAi strategies lay a solid foundation for Htt-lowering therapies for HD. However, several lingering questions remain to be addressed. First, can such a therapy maintain its efficacy and safety profiles in situations requiring chronic administration, such as in the more slowly progressive

full-length mHtt mouse models? Second, are there alternative ways to deliver Htt-lowering therapy to broader brain regions and cell types beyond the striatum that may also contribute to symptoms check details of HD? A study in this issue of Neuron by Kordasiewicz et al. (2012) provides strong preclinical evidence to support the use of antisense oligonucleotides (ASOs) as an Htt-lowering therapeutic for HD. ASOs are single-stranded DNA oligonucleotides (usually 8–50 nucleotides) that target cellular mRNA transcripts via complementary base pairing. The resulting DNA/RNA duplex undergoes catalytic degradation of the RNA component by RNase H, an enzyme present in most mammalian cells. Importantly, the single-stranded ASO can be recycled to mediate

multiple rounds of selective mRNA degradation ( Figure 1A). The stability selleck screening library and potency of ASOs are due to the phosphorothioate backbone and 2′-O-methoxyethyl (MOE) deoxynucleotide (DNA) sugar modifications, with specificity conferred by bioinformatic analysis and cell-based screening to optimize target engagement while minimizing off-target toxicity (

Bennett and Swayze, 2010). A strength for ASOs as candidate therapeutic over agents is the safety profiles in human studies so far, with one approved drug in clinical use and another 35 in clinical development ( Bennett and Swayze, 2010). Indeed, one such clinical study is a phase I clinical trial of ASO-mediated lowering of mutant SOD1 in familial amyotrophic lateral sclerosis, based on the original preclinical study by Cleveland and colleagues ( Smith et al., 2006). To test ASO therapy in HD models, Kordasiewicz et al. (2012) first established drug-like properties for the Htt ASOs. In the BACHD model that expresses full-length human mHtt (Gray et al., 2008), a 2 week infusion of two separate ASOs into the right ventricle, one selectively targeting human and the other targeting both human and murine Htt, is sufficient to induce dose-dependent and selective reduction of Htt for up to 12 weeks, with Htt levels returning to baseline at 16 weeks. The stability and high potency of chemically modified ASOs probably contribute to the lengthy period of Htt lowering after transient ASO infusion. The second surprising finding from the pharmacokinetics study is the broad distribution of ASOs in many brain regions (e.g., cortex, striatum, thalamus, midbrain, and brainstem) from intraventricular ASO delivery.