Achim Aigner, Rudolf-Boehm-Institute for Pharmacology and Toxicology, Leipzig University, Germany:
In the past years, increasing insight has been gained into the physiological and pathophysiological roles of many important classes of ncRNAs. This also included the discovery of new gene silencing mechanisms, such as RNA interference (RNAi) or miRNA-mediated inhibition of specific protein synthesis. These discoveries substantially enhance the understanding of intra- and extracellular communication beyond proteins and provide important information on the basis of various diseases involving aberrant ncRNA expression. Importantly, they also allow for the exploration of these mechanisms for therapeutic purposes. Major players of RNA-based therapies include antisense oligonucleotides and siRNAs (that target all forms of RNA), locked nucleic acid anti-miRs, tiny locked nucleic acid anti-miRs, miRNA sponges and antagomiRs (i.e. chemically modified oligonucleotides that target specific miRNAs), miRNA mimics and ribozymes.
As a rule, RNA-based therapeutic agents are too large as molecules, negatively charged and instable in biological fluids. This constitutes technical disadvantages over many other drugs mainly with regard to poor pharmacokinetics. Therefore, chemical modifications and/or formulations have to be implemented to allow for therapeutic use. Despite many years of research, however, the issue of delivery is still a major obstacle in therapeutic RNA use. On the other hand, RNA-based therapeutics allow for broader target selection and offer new therapeutic strategies. Dependent on the RNA, they can address different MoAs that may be based on physiological processes. Generally, RNA-based therapeutic agents do not efficiently elicit antibody responses, even when they are bound to proteins (Ling et al, 2013). NcRNA-based therapies either aim at restoring normal ncRNA functionalities or at using small RNA molecules to trigger desired therapeutic effects. For example, RNAi allows for the inhibition of any target gene of choice. This provides the opportunity to develop novel concepts in therapy also with regard to otherwise ‘undruggable’ genes.
Consequently, since its discovery in the late 1990s, RNAi-based therapeutic drugs have been brought into more than 50 clinical trials involving 26 different siRNAs (Ling et al, 2013; Lundin et al, 2015; Wittrup et al, 2015). Specifically, two phase III trials are in progress to treat familial neurodegenerative and cardiac syndromes caused by transthyretin mutations (Singh and Peer, 2016). More recently, explorations on the potential of miRNA replacements or miRNA inhibitions have also been initiated, e.g. to reduce hypertrophic dermal scarring by targeting connective tissue growth factor or to treat pancreatic cancer (Singh and Peer, 2016). Outstanding major issues in ncRNA therapy, however, are targeted RNA-drug delivery to specific organs, specificity, efficacy and absence of side effects (Wittrup et al, 2015). Various RNA-delivery strategies, including the use of nanocarriers, and chemical RNA modifications have been developed and are currently under pre-clinical and clinical investigation (Dai and Tan, 2015; Grünweller and Hartmann, 2016; Singh and Peer, 2016).
How are doses selected for phase I and II trials? – This will depend on in vitro and in vivo studies that provide an indication on relevant concentrations. As a rule, concentrations will be very low, and only few molecules will end up reaching the target.
How specific are the effects elicited by RNA-based therapeutic agents, and are side effects to be expected? – Traditional chemotherapy is a good example for non-specificity, whereas RNA-based therapeutic agents allow for sequence specificity as has been explored in the field of miRNA inhibition or siRNA-mediated gene knockdown. Sequence specificity increases with decreasing length of the miRNAs. Currently, the major problem is to ensure the delivery of the RNA-based therapeutic agents, and strategies also include approaches for targeted delivery to a specific organ. Independent of delivery issues, the selection of a ncRNA without off-target effects is crucial.
How can one validate that a specific miRNA was successfully inhibited in the test animals? – This is performed by evaluating the miRNA itself or by selecting and assessing relevant target genes. This also allows for in vitro – in vivo comparisons.
Which animal species is best suited for preclinical studies? – An important determinant is efficacy, which will depend on the pathology in the best matching species. To investigate tumour therapies, rats or mice are frequently well suited. For other pathologies, the selection of the best suited animal species may be less straight forward.