Cell-Specific Aptamer-Functionalized RNAi: A New Prospect for Targeted siRNA Delivery - Nanotechnology for the Delivery of Therapeutic Nucleic Acids - page 114

lower expression of the desired targets in cell-based SELEX generally
requires a greater number of selection rounds (14 ~ 15 or up to 25)
in order to evolve the desired specific sequences. Consequently,
some nonspecific sequences might dominate the selection due to the
bias of enzymatic amplification reactions.
To overcome these disappointments with the whole cell-based
SELEX technology, some optimizations have been made to increase
its general applicability. For example, a fluorescence-activated cell
sorting (FACS) technology was combined with cell-based SELEX,
thus allowing the separation of living cells from dead cells within a
cultured cell mixture [64]. Through this novel FACS-SELEX, aptamers
specific to vital Burkitt lymphoma B cells were enriched after 10
selection rounds. Alternatively, a gentler method was performed
to significantly reduce the proportion of dead cells by dead cell
removal microbeads [63]. In addition, high-affinity cell-specific RNA
aptamers have been effectively identified with a crossover SELEX
strategy [65], in which the targets were crossing from the Tenasin-
C(TN-C)-expressing U251 glioblastoma cells (in the initial nine
rounds) to the purified TN-C protein (in the last two rounds).

5.3 Cell-Specific Aptamer-Functionalized RNAi

With their favorable properties (such as high affinity and specificity,
small physical size, facile evolution, low toxicity, and

accessibility for backbone modifications), cell-specific aptamers
offer great promise for cell type- or tissue-specific delivery of various
therapeutics. For example, RNAi therapeutics (siRNAs), therapeutic
nanopaticles, molecular carriers, chemotherapeutics, and molecular
imaging probes have been precisely functionalized with cell-specific
aptamers. As an actively recognized moiety, aptamers direct specific
cell recognition events and consequently facilitate cellular uptake
of payloads

in vitro

receptor-mediated endocytosis. Despite displaying
“escort” functionality, some cell-specific aptamers possess the ability
of directly blocking the function of target proteins or interrupting
the disease process, thereby making them attractive platforms to
develop multifunctional nanomedicines. The following sections
highlight current efforts using cell-specific aptamer to enable
targeted delivery of siRNAs (Fig. 5.1C) and their use as therapeutic
nanocarriers (Fig. 5.1D).

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