Perspectives, issues and solutions in RNAi therapy: the expected and the less expected

摘要

The principle of RNAi & its advantages for therapeutic application: 20 years after the discovery of RNA interference in the late 1990s [1], it is obvious that RNAi has opened novel avenues toward innovative therapeutic strategies based on nucleic acids. Unlike most other oligonucleotide systems, RNAi relies on a catalytic mechanism and thus requires lower amounts of nucleic acid to be delivered to the cell, and siRNA-based cleavage is more efficient compared with ribozymes. In fact, highly potent siRNAs usually show activity even in picomolar concentrations and the delivery of less than 2000 siRNA molecules per cell was found to be sufficient to exert specific gene knockdown [2]. Also, with early encouraging results on the preclinical in vivo application of RNAi, such as the targeting of Fas to protect mice from fulminant hepatitis [3], it was not surprising that the discovery of the key mechanisms and siRNAs, led to a hype with regard to their translation into the clinics. This is particularly so since in the era of full human genomes and the availability of appropriate in silico siRNA prediction tools, it is relatively easy to find siRNAs against virtually any therapeutic target, despite the still remaining need for extensive experimental validation [4]. This also makes lead optimization a more rapid and straightforward process when compared with other pharmacological compounds. Additionally, siRNAs are relatively easy to synthesize, even on a large scale, which can pose a major issue in the case of biologicals. Based on the post-transcriptional rather than post-translational mechanism, in other words, siRNAs acting on the mRNA rather than on the protein level, it is also possible to target and eventually inhibit otherwise undruggable genes, in other words, those where no protein inhibitors exist or cannot be obtained. From a pharmacological viewpoint, this is particularly noteworthy since only a small fraction of the proteins in the human genome is potentially druggable [5]. This would leave the majority of proteins unavailable for inhibition by classical pharmacological approaches, for example due to the absence of active binding sites on the protein. Also, siRNAs can allow for distinguishing between wild-type genes and their pathological counterparts bearing rather subtle mutations like a single point mutation which could be barely or not at all addressed by an inhibitor on the protein level. Just two examples include the ADO II [6] or KRASG12D genes [7]. Taken together, this allows for the exploration of novel targets through the development of RNAi-based drugs and, provided that key problems are solved, substantial progress in therapy can be anticipated.