Aptamer-Mediated siRNA Targeting - RNA Interference from Biology to Therapeutics - page 205

Considering these setbacks, strategies to improve the therapeutic performance of
siRNAs including appropriate chemical modifications and effective delivery formu-
lations with targeting moieties, as well as combinatorial applications with other ther-
apeutic agents, have been explored [ 19 ]. In particular, it is highly desirable to develop
an intelligent delivery formulation for carrying siRNAs to the desired target tissues
at therapeutically effective doses, especially by systemic administration [ 4 ] . With the
intent of developing targeted delivery vehicles, nucleic acid-based aptamers which
bind cell-surface proteins have been explored as targeting ligands or delivery vehi-
cles for tissue and cell-type-specific delivery [ 20, 21 ] . Speci fi c cellular internaliza-
tion of the siRNAs and selective accumulation of the drug in the targeted tissues or
cells can be achieved by conjugating cell-specific aptamers to siRNAs. This chapter
focuses on cell-specific aptamer-mediated delivery of RNAi-based therapeutics.

10.2 Development of Cell-Speci fi c Aptamers

Aptamers are evolved in vitro through a random combinatorial library, single-
stranded nucleic acids that can specifically recognize and bind their cognate targets
by means of well-defined stable, three-dimensional structures [ 22 ] . Over the past
20 years, numerous nucleic acid aptamers have been raised against a wide variety
of targets by an in vitro procedure called systematic evolution of ligands by expo-
nential enrichment (SELEX). The SELEX technique was first reported in 1990
[ 23- 25 ] and has undergone several refinements and modifications to improve the
selection efficiency and speed [ 26 ]. Many aptamer properties are comparable to
those of protein monoclonal antibodies, but the nucleic acid aptamers with target
binding affinities in the low to mid-nanomolar range offer some advantages over
Mabs, including the potential for chemical synthesis and modification, stability, and
lack of immunogenicity [ 26, 27 ]. Aptamers, therefore, have been utilized for a vari-
ety of applications ranging from diagnostics to therapeutics [ 28, 29 ] .
Advances in the development of DNA or RNA aptamers that specifically target
membrane receptors to deliver and enhance the efficacy of other therapeutics agents
have created interest in exploiting cell-specific aptamers as targeted drug delivery
carriers. Aptamers can be directly assembled with other therapeutic nucleic acids
(like siRNAs) or be conjugated onto nanocarriers into versatile multifunctional
molecular devices [ 21 ]. In order to generate a cell-specific aptamer, the selection
procedure can vary from traditional purified membrane protein-based SELEX [ 25 ]
(Fig. 10.1a ) to live cell-based SELEX [ 30- 32 ] (Fig. 10.1b ). Alternatively, cell-
specific aptamers also can be identified through a combinatorial SELEX strategy via
switching between cells expressing the target protein with purified protein selection
[ 33 ]. Generally, in a typical SELEX procedure, the initial single-stranded DNA/RNA
pool contains a 20-60-nt random sequence, which largely guarantees the presence of
active structures with high binding affinity to the target protein. By repetitive rounds
of binding and selection, aptamers against virtually any given target can be routinely
isolated from an initial combinatorial oligonucleotide library.