Ian Maclachlan

RNAi-mediated gene silencing in non-human primates.

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 medicines. Although there are numerous examples of in vivo silencing of target genes after local delivery of small interfering RNAs (siRNAs), there remain only a few reports of RNAi-mediated silencing in response to systemic delivery of siRNA, 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.

Potent and persistent in vivo anti-HBV activity of chemically modified siRNAs

The efficacy of lipid-encapsulated, chemically modified short interfering RNA (siRNA) targeted to hepatitis B virus (HBV) was examined in an in vivo mouse model of HBV replication. Stabilized siRNA targeted to the HBV RNA was incorporated into a specialized liposome to form a stable nucleic-acid-lipid particle (SNALP) and administered by intravenous injection into mice carrying replicating HBV. The improved efficacy of siRNA-SNALP compared to unformulated siRNA correlates with a longer half-life in plasma and liver. Three daily intravenous injections of 3 mg/kg/day reduced serum HBV DNA >1.0 log10. The reduction in HBV DNA was specific, dose-dependent and lasted for up to 7 d after dosing. Furthermore, reductions were seen in serum HBV DNA for up to 6 weeks with weekly dosing. The advances demonstrated here, including persistence of in vivo activity, use of lower doses and reduced dosing frequency are important steps in making siRNA a clinically viable therapeutic approach.

Sequence-dependent stimulation of the mammalian innate immune response by synthetic siRNA

Short interfering RNAs (siRNAs) that mediate specific gene silencing through RNA interference (RNAi) are widely used to study gene function and are also being developed for therapeutic applications. Many nucleic acids, including double- (dsRNA) and single-stranded RNA (ssRNA), can stimulate innate cytokine responses in mammals. Despite this, few studies have questioned whether siRNA may have a similar effect on the immune system. This could significantly influence the in vivo application of siRNA owing to off-target effects and toxicities associated with immune stimulation. Here we report that synthetic siRNAs formulated in nonviral delivery vehicles can be potent inducers of interferons and inflammatory cytokines both in vivo in mice and in vitro in human blood. The immunostimulatory activity of formulated siRNAs and the associated toxicities are dependent on the nucleotide sequence. We have identified putative immunostimulatory motifs that have allowed the design of siRNAs that can mediate RNAi but induce minimal immune activation.

Rational design of cationic lipids for siRNA delivery

We adopted a rational approach to design cationic lipids for use in formulations to deliver small interfering RNA (siRNA). Starting with the ionizable cationic lipid 1,2-dilinoleyloxy-3-dimethylaminopropane (DLinDMA), a key lipid component of stable nucleic acid lipid particles (SNALP) as a benchmark, we used the proposed in vivo mechanism of action of ionizable cationic lipids to guide the design of DLinDMA-based lipids with superior delivery capacity. The best-performing lipid recovered after screening (DLin-KC2-DMA) was formulated and characterized in SNALP and demonstrated to have in vivo activity at siRNA doses as low as 0.01 mg/kg in rodents and 0.1 mg/kg in nonhuman primates. To our knowledge, this represents a substantial improvement over previous reports of in vivo endogenous hepatic gene silencing.

Postexposure protection of non-human primates against a lethal Ebola virus challenge with RNA interference: a proof-of-concept study

Summary Background We previously showed that small interfering RNAs (siRNAs) targeting the Zaire Ebola virus (ZEBOV) RNA polymerase L protein formulated in stable nucleic acid-lipid particles (SNALPs) completely protected guineapigs when administered shortly after a lethal ZEBOV challenge. Although rodent models of ZEBOV infection are useful for screening prospective countermeasures, they are frequently not useful for prediction of efficacy in the more stringent non-human primate models. We therefore assessed the efficacy of modified non-immunostimulatory siRNAs in a uniformly lethal non-human primate model of ZEBOV haemorrhagic fever. Methods A combination of modified siRNAs targeting the ZEBOV L polymerase (EK-1 mod), viral protein (VP) 24 (VP24-1160 mod), and VP35 (VP35-855 mod) were formulated in SNALPs. A group of macaques (n=3) was given these pooled anti-ZEBOV siRNAs (2 mg/kg per dose, bolus intravenous infusion) after 30 min, and on days 1, 3, and 5 after challenge with ZEBOV. A second group of macaques (n=4) was given the pooled anti-ZEBOV siRNAs after 30 min, and on days 1, 2, 3, 4, 5, and 6 after challenge with ZEBOV. Findings Two (66%) of three rhesus monkeys given four postexposure treatments of the pooled anti-ZEBOV siRNAs were protected from lethal ZEBOV infection, whereas all macaques given seven postexposure treatments were protected. The treatment regimen in the second study was well tolerated with minor changes in liver enzymes that might have been related to viral infection. Interpretation This complete postexposure protection against ZEBOV in non-human primates provides a model for the treatment of ZEBOV-induced haemorrhagic fever. These data show the potential of RNA interference as an effective postexposure treatment strategy for people infected with Ebola virus, and suggest that this strategy might also be useful for treatment of other emerging viral infections. Funding Defense Threat Reduction Agency.

siRNA and Innate Immunity

Canonical small interfering RNA (siRNA) duplexes are potent activators of the mammalian innate immune system. The induction of innate immunity by siRNA is dependent on siRNA structure and sequence, method of delivery, and cell type. Synthetic siRNA in delivery vehicles that facilitate cellular uptake can induce high levels of inflammatory cytokines and interferons after systemic administration in mammals and in primary human blood cell cultures. This activation is predominantly mediated by immune cells, normally via a Toll-like receptor (TLR) pathway. The siRNA sequence dependency of these pathways varies with the type and location of the TLR involved. Alternatively nonimmune cell activation may also occur, typically resulting from siRNA interaction with cytoplasmic RNA sensors such as RIG1. As immune activation by siRNA-based drugs represents an undesirable side effect due to the considerable toxicities associated with excessive cytokine release in humans, understanding and abrogating this activity will be a critical component in the development of safe and effective therapeutics. This review describes the intracellular mechanisms of innate immune activation by siRNA, the design of appropriate sequences and chemical modification approaches, and suitable experimental methods for studying their effects, with a view toward reducing siRNA-mediated off-target effects.

Stabilized plasmid-lipid particles: construction and characterization

A detergent dialysis procedure is described which allows encapsulation of plasmid DNA within a lipid envelope, where the resulting particle is stabilized in aqueous media by the presence of a poly(ethyleneglycol) (PEG) coating. These ‘stabilized plasmid-lipid particles’ (SPLP) exhibit an average size of 70 nm in diameter, contain one plasmid per particle and fully protect the encapsulated plasmid from digestion by serum nucleases and E. coli DNase I. Encapsulation is a sensitive function of cationic lipid content, with maximum entrapment observed at dioleoyldimethylammonium chloride (DODAC) contents of 5 to 10 mol%. The formulation process results in plasmid-trapping efficiencies of up to 70% and permits inclusion of ‘fusigenic’ lipids such as dioleoylphosphatidylethanolamine (DOPE). The in vitro transfection capabilities of SPLP are demonstrated to be strongly dependent on the length of the acyl chain contained in the ceramide group used to anchor the PEG polymer to the surface of the SPLP. Shorter acyl chain lengths result in a PEG coating which can dissociate from the SPLP surface, transforming the SPLP from a stable particle to a transfection-competent entity. It is suggested that SPLP may have utility as systemic gene delivery systems for gene therapy protocols.

Cationic lipid-mediated transfection of cells in culture requires mitotic activity

Cationic lipid-based delivery systems such as lipoplexes or stabilized plasmid–lipid particles (SPLP) represent a safer alternative to viral systems for gene therapy applications. We studied the impact of cell cycle status on the efficiency of transfection of human ovarian carcinoma tumor cells using two cationic-lipid based delivery systems. Cells arrested in the G1 phase of the cell cycle by treatment with aphidicolin were compared with an asynchronous dividing population of cells. Treatment of the cells with aphidicolin had no effect on the rate of internalization of the lipid formulated DNA or on the level of gene expression observable in stably transfected cells. However, cells treated with aphidicolin exhibited 20-fold lower reporter gene activity than asynchronous control cells upon incubation with lipoplexes. When cells arrested in the G1 phase were allowed to proceed though the cell cycle in the presence of the lipoplex or SPLP, transgene expression was found to coincide with the transition of cells from the G2/M phase into the G1 phase of the subsequent cell cycle. In addition, higher levels of reporter gene expression were observed when the cells were incubated with lipoplexes or SPLP during, or just before, mitosis. These results suggest that it may be possible to augment cationic lipid-mediated transfection by manipulating the cell cycle status of the target cells.

Postexposure Protection of Guinea Pigs against a Lethal Ebola Virus Challenge Is Conferred by RNA Interference

Abstract BackgroundEbola virus (EBOV) infection causes a frequently fatal hemorrhagic fever (HF) that is refractory to treatment with currently available antiviral therapeutics. RNA interference represents a powerful, naturally occurring biological strategy for the inhibition of gene expression and has demonstrated utility in the inhibition of viral replication. Here, we describe the development of a potential therapy for EBOV infection that is based on small interfering RNAs (siRNAs) MethodsFour siRNAs targeting the polymerase (L) gene of the Zaire species of EBOV (ZEBOV) were either complexed with polyethylenimine (PEI) or formulated in stable nucleic acid–lipid particles (SNALPs). Guinea pigs were treated with these siRNAs either before or after lethal ZEBOV challenge ResultsTreatment of guinea pigs with a pool of the L gene–specific siRNAs delivered by PEI polyplexes reduced plasma viremia levels and partially protected the animals from death when administered shortly before the ZEBOV challenge. Evaluation of the same pool of siRNAs delivered using SNALPs proved that this system was more efficacious, as it completely protected guinea pigs against viremia and death when administered shortly after the ZEBOV challenge. Additional experiments showed that 1 of the 4 siRNAs alone could completely protect guinea pigs from a lethal ZEBOV challenge ConclusionsFurther development of this technology has the potential to yield effective treatments for EBOV HF as well as for diseases caused by other agents that are considered to be biological threats

Overcoming the Innate Immune Response to Small Interfering RNA

Many types of nucleic acid, including canonical small interfering RNA (siRNA) duplexes, are potent activators of the mammalian innate immune system. Synthetic siRNA duplexes can induce high levels of inflammatory cytokines and type I interferons, in particular interferon-alpha, after systemic administration in mammals and in primary human blood cell cultures. These responses are greatly potentiated by the use of delivery vehicles that facilitate cellular uptake of the siRNA. Although the immunomodulatory effects of nucleic acids may be harnessed therapeutically, for example, in oncology and allergy applications, in many cases immune activation represents a significant undesirable side effect due to the toxicities associated with excessive cytokine release and associated inflammatory syndromes. The potential for siRNA-based drugs to be rendered immunogenic is also a cause for concern because the establishment of an antibody response may severely compromise both safety and efficacy. Clearly, there are significant implications both for the development of siRNA-based drugs and in the interpretation of gene-silencing effects elicited by siRNA. This review provides the background information required to anticipate, manage, and abrogate the immunological effects of siRNA and will assist the reader in the successful in vivo application of siRNA-based drugs.