RNAi-mediated gene silencing in non-human primates

Abstract

Two papers this week highlight the impact of RNAi (RNA interference) in clinical medicine. Ngo et al. have developed a novel ‘Achilles heel' screen to identify genes that, if silenced, cause cancer cells to stop dividing. The novelty lies in a successful ‘negative’ screen that can reveal potential therapeutic targets that do not necessarily contain mutations or other alterations. Use of the screen on 2,500 genes in B-cell lymphoma cells identified three genes that were essential for cancer cell survival and growth of one particular B-cell lymphoma subtype. In particular the protein CARD11 looks a prime target. Zimmermann et al. report a significant step towards harnessing RNAi as a new class of drug treatment. They used systemic administration of small interfering RNA (siRNA) to silence a disease-causing gene in a non-human primate: it had previously been demonstrated in mice. Specifically, siRNA targeted against the gene for apolipoprotein B (ApoB) in cynomolgus monkeys successfully reduced in ApoB protein, serum cholesterol and low-density lipoprotein levels. This has implications for diseases associated with high cholesterol levels, such as coronary heart disease, and more broadly demonstrates that potential therapies may be developed against historically ‘non-druggable’ targets. Systemically administered modified siRNAs can be used in monkeys to achieve long-lasting silencing of a gene — previously, this had been established only in mice. The opportunity to harness the RNA interference (RNAi) pathway to silence disease-causing genes holds great promise for the development of therapeutics directed against targets that are otherwise not addressable with current medicines1,2. Although there are numerous examples of in vivo silencing of target genes after local delivery of small interfering RNAs (siRNAs)3,4,5, there remain only a few reports of RNAi-mediated silencing in response to systemic delivery of siRNA6,7,8, and there are no reports of systemic efficacy in non-rodent species. Here we show that siRNAs, when delivered systemically in a liposomal formulation, can silence the disease target apolipoprotein B (ApoB) in non-human primates. APOB-specific siRNAs were encapsulated in stable nucleic acid lipid particles (SNALP) and administered by intravenous injection to cynomolgus monkeys at doses of 1 or 2.5 mg kg-1. A single siRNA injection resulted in dose-dependent silencing of APOB messenger RNA expression in the liver 48 h after administration, with maximal silencing of >90%. This silencing effect occurred as a result of APOB mRNA cleavage at precisely the site predicted for the RNAi mechanism. Significant reductions in ApoB protein, serum cholesterol and low-density lipoprotein levels were observed as early as 24 h after treatment and lasted for 11 days at the highest siRNA dose, thus demonstrating an immediate, potent and lasting biological effect of siRNA treatment. Our findings show clinically relevant RNAi-mediated gene silencing in non-human primates, supporting RNAi therapeutics as a potential new class of drugs.