siRNA Versus Antisense Locked Nucleic Acids: Stay Single!.

Much discussion has centred around the utility and benefits of siRNA in both target validation and as a therapeutic option. This has been driven by significant publications including that of Soutcheck et al (Nature432, 173–177 2004), which demonstrated liver targeting as well as in vivo efficacy when siRNA against ApoB was tethered to a cholesterol moiety. Santaris Pharma has developed a third generation nucleic acid chemistry referred to as locked nucleic acid (LNA) which delivers unmatched affinity and stabiliy benefits, largely overcoming the drawbacks associated with traditional antisense molecules. We therefore sought to compare this chemistry with targets which siRNA has been successfully used in in vivo/in vitro settings. The same motif used in the Soutcheck study was targeted with a LNA molecule, and the free siRNA activity was compared to the cholesterol linked and free LNA molecules in their ability ot down regulate ApoB expression. LNA (SPC3197) inhibited ApoB expression by 90% while at an equimolar concentration siRNA was ineffective in the liver and jejunum. Cholesterol linked siRNA was only effective in the jejunum (c50% reduction in mRNA) Fig1. Only the LNA mediated inhibition of ApoB expression was paralleled by a drop in serum cholesterol in the host animal. In a second model siRNA molecules targeting Hif-1a mRNA (Yu et al Lab Invest84, 553–561 2004) were compared to our lead LNA molecule targeting Hif-1a, SPC2968. Interestingly in in vitro analyses these 2 molecules were equally effective. However in a murine model the increased half life of the LNA molecules translated to a potent inhibition of Hif-1a as measured by QPCR. This effect was noted in jejunum and liver, and persisted for at least 4 days. Hif-1a inhibition mediated by siRNA was not seen in any tissue analysed (Fig 2). Finally a 3rd molecule targeting Bcl-2 has entered clinical Phase 1 trials, and data will be presented documenting its improved affinity and stabitily in relation to competitor molecules such as Genasense. Figure Figure