Aptamer-Mediated siRNA Targeting - RNA Interference from Biology to Therapeutics - page 204

complementary Watson-Crick base pairing of a target messenger RNA (mRNA)
with the guide strand of the siRNA. Since double-stranded RNA-mediated RNAi
was discovered in 1998 [ 3 ], the development of siRNA-based therapeutics has been
an area of research [ 4, 5 ]. After more than a decade of development, there are over 20
siRNA-based preclinical and phase I/II clinical trials for a variety of human diseases
which have continued to fuel the interest in RNAi clinical translation [ 4, 6 ] . For
example, in 2004, OPKO Health (previous Acuity Pharmaceuticals) announced the
first siRNA-related clinical trial in which bevasiranib, an unmodified siRNA target-
ing vascular endothelial growth factor (VEGF), was given to patients with wet age-
related macular degeneration (AMD). The phase III trial of bevasiranib is the first and
most advanced clinical study [ 7 ]. Additionally, RNAi therapy for respiratory syncy-
tial virus (RSV) also has rapidly progressed from laboratory investigations to clinical
trials [ 8- 11 ]. In 2007, Alnylam Pharmaceuticals started phase II clinical studies for
ALN-RSV01, which is an siRNA targeting a highly conserved region of the mRNA
encoding the nucleocapsid (N) protein of RSV. The siRNA was shown to prophylacti-
cally reduce the incidence of RSV lung infections in adult patients [ 11 ] . The fi rst in-
human phase I clinical trial using a targeted nanoparticle-siRNA delivery system
showed the first direct evidence for siRNA-mediated gene silencing in humans [ 12 ] .
However, in similarity with the development of other drug types, some challenges/
hurdles have resulted in setbacks in clinical translation of RNAi [ 5, 13 ] . Due to the
negatively charged nature of nucleic acids, siRNAs cannot directly pass through the
cell membrane and are vulnerable to RNase degradation in the absence of backbone
modifications or protective delivery vehicles [ 14 ]. Although siRNA can be directly
administered to a target (such as intratumoral delivery), for many diseases, systemic
administration is required, which generally requires greater therapeutic doses, thus
leading to higher costs and harmful side effects. On the other hand, when siRNAs are
administered by systemic administration without some protective covering and/or
appropriate chemical modifications, they are rapidly cleared through renal filtration,
ultimately resulting in poor pharmacokinetics and marginal or no gene silencing.
Before reaching the targeted cells, it is difficult for a nonformulated siRNA to pass
through the blood vessel endothelial wall and multiple tissue barriers including liver,
kidney, and lymphoid organs [ 14, 15 ]. Therefore, a nontargeting systemic adminis-
tration of siRNA probably severely reduces the therapeutic efficacy and can even
lead to harmful toxicities [ 15, 16 ]. In the case of bevasiranib, it was as administered
as an unmodified siRNA without a delivery formulation that was given by intravitreal
injection. In 2009, OPKO terminated the phase III clinical trial of bevasiranib because
its primary end point of reducing vision loss was not achieved. Recently, another
clinical trial for AMD using a chemically modified siRNA—Sirna-027 (also known
as AGN 211745)—against a conserved region of the VEGF receptor-1 mRNA [ 17 ] ,
eventually failed to meet the efficacy end points in its phase II trial. The specificity
and mechanism of the aforementioned anti-VEGF siRNA drugs for treating AMD
was called into question by a recent study [ 18 ], which showed that the siRNA-mediated
inhibitory activity of neovascularization may be attributed to nonspecific immune
response associated with activation of the cell-surface toll-like receptor 3 (TLR3),
rather than to a target sequence-specific interaction.