Aptamer-Mediated siRNA Targeting - RNA Interference from Biology to Therapeutics - page 210

half-life and markedly improved the bioavailability of the aptamer-siRNA chimera.
Compared with the swap-2¢-F chimera, the in vivo circulating half-life of the PEG-
conjugated chimera (the swap-2¢-F-PEG chimera) was substantially increased (from
<35 min to >30 h). Additionally, the therapeutic dose of the PEG-conjugated chi-
mera was dramatically reduced from 10×1 nmol to 5×0.25 nmol per injection,
minimizing both the cost of treatment and the risk for harmful side effects.
Consequently, the optimized version resulted in pronounced regression of PSMA-
expressing tumor growth after systemic administration in mice harboring human
prostate cancer cells, and this was accomplished with a reduced therapeutic dose.
More recently, Gliboa and coworkers also utilized the PSMA aptamer to specifically
deliver NMD (nonsense-mediated messenger RNA decay) factor targeting siRNAs
to tumor cells [ 72 ]. The systemic administration of PSMA aptamer-NMD siRNA
chimeras resulted in significant suppression of tumor growth in subcutaneous and
metastatic tumor animal models.
Using the same aptamer-siRNA fusion approach described by Giangrande, we
successfully developed a dual-functional anti-gp120 aptamer-siRNA chimeric
RNA [ 40, 73 ], in which both the aptamer and the siRNA portions have potent anti-
HIV activities. By blocking the interaction of HIV-1 envelop gp120 and CD4 recep-
tor, the aptamer portion can neutralize HIV-1 infectivity. The gp120 aptamer was
covalently linked to a 27-mer siRNA against the HIV-1 tat/rev common exon.
Treatment of HIV-1-infected T cells and PBMCs with these chimeras resulted in the
selective gp120-mediated cellular internalization and the specific gene silencing.
Furthermore, in an HIV-1-infected humanized Rag-hu mouse model, systemic
administration of the gp120 aptamer-siRNA chimera provided several logs of inhi-
bition of HIV-1 replication and complete inhibition of CD4+ T cell depletion nor-
mally mediated by viral infection [ 74 ]. Our results also demonstrated that the
specific gene silencing effect and the siRNAs were only detectable in gp120
aptamer-siRNA chimera-treated mice, but not in animals treated with naked siRNA
or a mutant aptamer-siRNA chimera, validating the aptamer-mediated cell-specific
siRNA delivery and RNAi activity.
Similarly, an alternative aptamer that binds to the human CD4 receptor was also
covalently fused with an siRNA to specifically induce gene silencing in CD4+ T cells
and macrophages and in cervicovaginal tissue explants [ 75 ] . When the CD4 aptamer-
siRNA chimeras (CD4-AsiCs) bearing siRNAs targeting HIV gag and vif or host
CCR5 were administrated by the vaginal route to humanized mice, HIV vaginal
transmission to cervicovaginal explants and to the mice was significantly prevented.
It has been reported that multivalent versions of aptamers can increase the
potency and antitumor response and promote receptor activation [ 76- 79 ] . In efforts
to promote the binding affinity and internalization of chimeras and enhance the
therapeutic potential, multivalent aptamer-siRNA conjugates have been generated.
For example, the siRNA itself has been used to serve as a linker to join the two
PSMA aptamers together or the siRNA was appended onto the 3¢-ends of each
aptamer to build bivalent aptamer-siRNA chimeras [ 80 ] . This bivalent design pro-
moted internalization of the chimeras and resulted in an almost complete triggering
of PSMA-positive cell death.