A status report on RNAi therapeutics

Since the original reports of RNA interference (RNAi) in cells from a range of species [1-3], there has been increasing interest in harnessing this endogenous mechanism, which enables degradation of a specific mRNA, as a novel pharmacological approach to human disease. Indeed, from a drug discovery perspective, small interfering (si)RNAs have some distinct advantages over conventional drug therapies such as small molecules or antibodies (Table 1). However several major obstacles have had to be overcome before the entry of RNAi therapeutics to clinical trials. These include steps required for lead selection, the use of chemical modifications to confer appropriate biopharmaceutic properties, the design of formulations that enable delivery to a target tissue, and screening of these products for safety, including assessments for potential off-target effects. These aspects are addressed below and followed by a critical analysis of the 14 programs that have entered clinical development in the past decade. This review does not cover the related and rapidly expanding field of RNA therapeutics, which addresses microRNAs (miRNAs) rather than messenger mRNAs, as targets.We and others have developed high-throughput algorithms to support screening and selection of a lead siRNA. De Fougerolles et al. [4] reviewed the various steps involved, which include a bioinformatic screen to identify duplexes 19-23 bp in length with minimal off-target complementarity, small-scale synthesis of a panel of siRNAs, in vitro assays for potency and nonspecific cytotoxicity, and assessment of in vivo pharmacology. Embedded in this primary screen are subscreens for stability in a biological matrix (for example, serum, cerebrospinal or bronchoalveolar lavage fluid) relevant to the target tissue, and additional screens for specificity (addressed below). The ubiquitous nature of RNases requires that in most cases, a lead siRNA needs to be stabilized. Increased stability can be achieved by chemically mo