An eye-opener for RNAi therapeutics.

The discovery of the mechanism of RNA interference (RNAi) for knockdown of gene expression has spurred a variety of therapeutic applications. An asset of using small interfering RNA (siRNA) as therapeutics is their exquisite sequence-specific mechanism of action. It seems possible that the first RNAi therapy that could reach patients would aim at a debilitating eye disease called age-related macular degeneration, which afflicts 30–50 million people globally; the reason being that siRNAs can be delivered directly to the diseased tissue— that is, literally injected into the eye. To minimize systemic exposure and to make it less likely that the drug will have unanticipated, harmful effects elsewhere in the body, the therapeutic focus has been on local delivery. The disease is triggered by a protein called vascular endothelial growth factor (VEGF) that promotes blood vessel growth. Too much of this protein leads to the sprouting of excess blood vessels behind the retina. Macular degeneration is induced by leakage of the blood vessels, which causes clouding and frequently destroys vision. The new RNAi drugs were supposed to shut down VEGF expression or its receptor (VEGFR1). However, recent evidence indicates that the preclinical efficacy observed in mouse models of macular degeneration probably results from nonspecific side effects rather than RNAi-mediated sequence-specific gene knockdown. Such nonspecific effects may trouble other RNAi therapeutic applications. The cellular mechanism of RNAi can be instructed to cleave specific mRNAs by addition of manmade siRNAs of which the antisense/ guide strand has perfect sequence complementarity with the targeted mRNA. In our research to develop gene therapy against human immunodeficiency virus type 1 (HIV-1), we observed impressive and specific inhibition via the RNAi mechanism. In fact, we documented that mutant escape viruses are selected under RNAi pressure with a single point mutation in the 19-nucleotide target sequence, thus demonstrating the exquisite sequence-specificity of inhibition.1 Therapeutic application of “naked” siRNA molecules remains a challenge because mammalian cells do not spontaneously take up such molecules without cell-permeating entities. Nevertheless, siRNAs targeting the VEGF system that were injected directly into an affected eye did reduce angiogenesis in preclinical studies with mice.2,3 Based on these promising preclinical results, clinical trials have been approved that exploit RNAi to shut down the VEGF signaling pathway that promotes angiogenesis. It is important to realize that although naked, unmodified siRNAs can be injected directly into the confined space of ocular tissue, there was little knowledge of how the siRNAs entered the target cells. The assumption has been that the siRNA enters an affected cell in some unexplained way, where it has a highly specific impact in An Eye-opener for RNAi Therapeutics