Biomedical Engineering Reference
In-Depth Information
5.3.1
Cell-Specific Aptamer-Functionalized siRNAs
RNA interference is extensively being harnessed to silence mRNAs
encoding pathogenic proteins (Fig. 5.1B,C). The direct application
of RNAi-based therapeutic agents, such as 21-22 nt siRNAs or
corresponding Dicer substrates (25-27 nt duplexes or small hairpin
RNAs), is currently the most common method to harness the RNAi
pathway for targeted gene silencing [3]. It has been reported
that the silencing potency of an optimal Dicer substrate siRNAs
(dsiRNAs) can be greater than for traditional 21-22 nt siRNAs
[66]. By using appropriate formulations, targeted delivery of these
synthetic RNAi triggers to specific cell populations or tissues can be
accomplished with high efficiency. Several representative examples
are discussed below.
5.3.1.1
PSMA RNA aptamer-functionalized siRNAs
The first example of a cell-specific apatamer used for targeted siRNA
delivery is the RNA aptamer that binds PSMA, a trans-membrane
protein highly expressed in human prostate cancer and the vascular
endothelium [67, 68]. Through either covalent conjugation or
physical assembly, different siRNA molecules have been successfully
functionalized with anti-PSMA aptamers to achieve targeted RNAi
efficacy. In 2006, Chu
[69] showed a proof of concept study,
in which two-biotinylated anti-PSMA aptamers (A-9) and two-
biotinylated 27-mer dsiRNAs targeting lamin A/C or GAPDH were
noncovalently assembled on a streptavidin connector
et al.
biotin-
streptavidin interaction. The resulting multivalent construct
displayed selective cellular uptake into the cultured PSMA-positive
cells and mediated specific RNAi activity.
In the same year, a different approach [70], in which a 2
via
-F-
modified anti-PSMA aptamer (A-10) was covalently appended to
the sense strand of a 21-mer siRNA portion, which subsequently
was annealed with its complementary antisense strand. This simple
covalent aptamer-siRNA chimeric RNA allowed effective PSMA-
mediated internalization along with siRNA-triggered gene silencing
in athymic mice following intratumoral injection. In a recent
effort [71], the same group has further optimized the previous
design through rational modifications of the aptamer and siRNA
portions. Such optimized second-generation chimeras had increased
circulation and bio-availability
′
in vivo
, resulting in pronounced
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