INSIGHTS | PERSPECTIVES
1070 6 MARCH 2015 • VOL 347 ISSUE 6226 sciencemag.org SCIENCE
Minimizing off-target effects at the RNAi
trigger design phase relies on error-prone
bioinformatic filters rather than on the iterative chemical optimization and monitoring of off-target activities in preclinical
Another long-term safety issue is whether
hijacking the endogenous microRNA pathway is safe given the functions of microRNAs throughout biology. Studies involving
the use of RNAi triggers generated from
strong transcriptional promoters revealed
that competition between therapeutic RNAi
triggers and endogenous microRNAs for
the same machinery can have adverse consequences given the pervasive importance
of microRNAs in gene regulation (10). Nevertheless, improvements in RNAi trigger
design to facilitate more efficient processing should lessen the burden by avoiding
congestion of the RNAi pathway (11).
The history of investment in RNAi ther-
apeutics has been something of a roller
coaster ride. Initial interest in the therapeu-
tic application of RNAi was limited to small
biotechnology companies specialized in
oligonucleotide therapeutics development
(2002 to 2005). There followed a sudden
stampede into the the technology by the
(~6 to 12 months) with the leading TTR
amyloidosis candidates will hold up in
the long run. So far, safety issues for the
three most advanced delivery technolo-
gies (SNALP liposomal nanoparticles by
Tekmira, GalNAc conjugates by Alnylam,
and DPC polyconjugates by Arrowhead
Research) have largely been related to
the route of administration (infusion and
injection site reactions) and the delivery
chemistry, and not the RNAi mechanism
of action per se (1, 2). In particular, the
propensity of the larger nanoparticulate
formulations such as liposomes to interact
with the innate immune system has been
a major challenge (7) and necessitated the
adoption of transient immune suppression
around the time of dosing. By contrast,
smaller systems such as the DPCs and Gal-
NAc conjugates rely on the use of heavily
modified RNAi triggers that may accumu-
late in compartments such as lysosomes,
the long-term safety of which remains to
Off-target activities, a challenge shared
with other drug modalities, may also influence the long-term safety experience, especially because RNAi triggers may modulate
expression of dozens of off-target genes (8).
of nanoparticle or vector
Vector RNA transcript
Target destruction and no protein made
Endogenous mRNA transcripts
made from host DNA contained
within our genome are transported
to the cytoplasm where they are
translated into protein
RNAi is the drug. RNAi delivery approaches include conjugates, liposomal nanoparticles (LNPs), and viral vectors.
Nanoparticles deliver synthetic dsRNA, whereas viral vectors deliver a transcriptional template to the nucleus. Cellular
uptake occurs nonspecifically or via receptor-mediated endocytosis. Nanoparticles deliver the RNAi trigger to the
cytoplasm. The trigger enters the RNAi pathway at the RISC or Dicer processing stage. Transcriptional templates
produce hairpin RNAs that enter the pathway at an earlier, nuclear stage. Ultimately, an active RISC complex is formed
that cleaves the mRNA target. RISC, RNA-induced silencing complex; shRNA, short hairpin-mediated RNA.
pharmaceutical industry (12). This period
of irrational exuberance (2005 to 2008) was
preceded by initially measured investments
by some large companies (Medtronic,
Merck, Novartis) and the award of the Nobel Prize for the discovery of dsRNA as the
efficient trigger of the RNAi gene silencing process. This exuberance culminated
in bidding wars, unrealistic expectations,
and misplaced investments, precipitating a
considerable backlash (2008 to 2011). It was
uncertain whether the industry could ever
recover (12). The realization that many of
the early most prominent claims for therapeutic gene silencing in animal models of
human disease were in retrospect best explained by innate immunostimulatory, an-tiproliferative, and antiviral artifacts was
a further cause of uncertainty. Important
scientific breakthroughs at this time, such
as the demonstration of on-target RNAi in
nonhuman primates, advances in conjugate
delivery, and strategies to abrogating innate
immune stimulation, struggled to be appropriately recognized (12). It was only after a
string of demonstrations of target gene silencing in humans that the industry started
to recover (13).
There has also been competition
from other nucleic acid therapeutics
platforms, in particular ribonuclease
H (RNase H) antisense technology
and genome editing. Especially with
recent advances in ligand-targeted
delivery, the long-held potency advantage of RNAi has diminished relative to
RNase H antisense technology (14). But
competition, both among RNAi technologies and with antisense oligonucleotides,
has also driven much progress in the field.
With more than $1 billion invested in RNAi-based therapeutics in the first 6 months of
2014 alone (13), together with the increased
acceptance of gene therapy in general, there
should be a rapidly expanding clinical pipeline of RNAi-based drugs and a maturing of
delivery technologies for targeting cells and
tissues outside the liver. ■
1. T. Coelho etal .,N.Engl.J.Med. 369, 819 (2013).
3. C.I.Wooddell etal., Mol. Ther. 21,973(2013).
4. D. Haussecker, J. Control. Release 195, 49 (2014).
5. S. Samakoglu etal ., Nat.Biotechnol. 24, 89 (2006).
6. C. Mueller etal ., Mol. Ther. 20, 590 (2012).
8. A. L. Jackson et al., RNA 12, 1179 (2006).
9. N. Vaish et al ., Nucleic Acids Res. 39, 1823 (2011).
10. D. Grimm et al ., Nature 441, 537 (2006).
11. S. Gu et al ., Cell 151, 900 (2012).
12. D. Haussecker, Mol. Ther. Nucleic Acids 1, e8 (2012).
13. J. Maraganore, Nature 510, 35 (2014).
14. T. P. Prakash et al., Nucleic Acids Res. 42, 8796 (2014).