As outlined above, synthetic siRNAs are very useful tools to address basic research questions. However, biological effects of siRNAs are always transient. Nevertheless, siRNAs are of therapeutic interest since they are chemically synthesized and can theoretically be applied repeatedly like a conventional drug. A major advantage of siRNA therapeutics is the possibility of local administration, e.g. via injection directly into a target organ or tissue. Whereas siRNA injections might be very useful for clinical pilot and proof-of-principle studies or in select cases where repeated administration is possible, any long-term therapeutic applications of RNAi technology will require a gene therapy approach to enable permanent expression of a specific shRNA, miRNA, or antisense cDNA. For those applications, AAV-based vector systems might be most suitable as they combine safety, tropism for brain cells, and long-term efficacy (McCown, 2009
; Noe et al., 2009
; Riban et al., 2009
; Gray et al., 2010
A recent database search performed on www.clinicaltrials.gov
(March 2010) revealed 2056 studies with “gene therapy”. In contrast, a search with “RNA interference OR RNAi OR siRNA OR shRNA OR miRNA” yielded only few results. Most of the latter studies (35) assess endogenous miRNAs (miRs) as biomarkers for disease and for pharmacogenomics studies. Only a few safety and feasibility studies were found, all of them based on siRNA delivery for the following conditions: cancer (5), macular degeneration (5), kidney injury (2), autosomal dominant inherited diseases (1), hypercholesterolemia (1), and hepatitis (1).
This analysis reveals the following: (i) Whereas gene therapies appear to be fairly advanced and considered for a wide range of conditions, RNAi-based therapeutics are in its infancy. (ii) Clinical trials to date focus largely on siRNAs. Effects of siRNAs are transient and therefore safer than permanent (i.e. gene therapy–based) approaches. Further, siRNAs can be chemically synthesized, produced, and marketed like conventional drugs - thus providing an economic incentive to favor siRNA-based approaches. (iii) RNAi has not yet been considered for clinical application for any neurological condition.
What are the challenges that need to be met for translating RNAi-based approaches into clinical applications for the prevention of seizures in epilepsy? The clinical examples mentioned above rely on multiple injections of therapeutic siRNAs. In the case of macular degeneration, multiple intraocular injections of conventional therapeutics are the current standard of care. Therefore, intraocular injection of a clinical preparation of synthetically generated siRNA is a logical extension of current practice. For the treatment of neurological disease (including epilepsy), however, systemic application of siRNA is likely not a therapeutic option due to limited metabolic half life and poor penetration of the blood brain barrier. Repeated direct injections of siRNAs into the brain are not feasible and long-term infusion via pumps is associated with risks. Therefore, the combination of standard gene therapy methods with the stable and long-term expression of a suitable shRNA, miRNA or antisense construct appears to be a necessity.
Neurodegenerative diseases have well-defined targets (e.g. dominant negative mutations) that are thought to be the underlying pathogenetic cause for the disease. Therefore, experimental RNAi approaches are well advanced in the field of neurodegenerative disorders. In contrast, epilepsy is likely a multifactorial condition with multiple possible targets. Many of those targets (e.g. ion channels) can be modulated with conventional antiepileptic drugs; thus, the need to develop gene therapy-based RNAi strategies to knockdown those targets may not be necessary. However, RNAi might become a superior therapeutic tool for novel targets that cannot be modified with conventional AEDs, or for focal applications that are restricted to an epileptogenic focus.