Aptamer-targeted cell-specific RNA interference

Abstract

This potent ability of small interfering (si)RNAs to inhibit the expression of complementary RNA transcripts is being exploited as a new class of therapeutics for a variety of diseases. However, the efficient and safe delivery of siRNAs into specific cell populations is still the principal challenge in the clinical development of RNAi therapeutics. With the increasing enthusiasm for developing targeted delivery vehicles, nucleic acid-based aptamers targeting cell surface proteins are being explored as promising delivery vehicles to target a distinct disease or tissue in a cell-type-specific manner. The aptamer-based delivery of siRNAs can often enhance the therapeutic efficacy and reduce the unwanted off-target effects of siRNAs. In particular, for RNA interference-based therapeutics, aptamers represent an efficient agent for cell type-specific, systemic delivery of these oligonucleotides. In this review, we summarize recent attractive developments in creatively using cell-internalizing aptamers to deliver siRNAs to target cells. The optimization and improvement of aptamer-targeted siRNAs for clinical translation are further highlighted.

Figure 1

Anti-prostate-specific membrane antigen (PSMA) aptamer-mediated small interfering (si)RNA delivery. (a) Schematic of anti-PSMA aptamer-streptavidin-siRNA conjugates. The 27-mer Dicer substrate RNA duplex and RNA aptamers were chemically conjugated with a biotin group. Thus, two biotinylated siRNAs and two aptamers were non-covalently assembled via a streptavidin platform. (b) Schematic of the first generation anti-PSMA aptamer-siRNA chimeras. The 2'-Fluoro-modified aptamer and siRNA sense strand were co-transcribed, followed by annealing of the complementary siRNA antisense strand to complete the chimeric molecule. (c) Schematic of the optimized second generation chimeras. Compared with the first generation chimeras, the aptamer portion of the chimera was truncated from 71 to 39 nucleotides, and the sense and antisense strands of the siRNA portion were swapped. A 2 nucleotide (UU)-overhang and a polyethylene glycol tail were added to the 3'-end of the guide strand and the 5'-end of passenger strand, respectively.

Figure 2

Anti-CD4 aptamer mediated small interfering (si)RNA delivery. Schematic of a dimer of the chimeric phage (p)RNA-CD4 aptamer and chimeric pRNA-siRNA. The anti-CD4 aptamer or siRNAs were non-covalently joined via phi29 RNAs containing complementary loop domains. Through interactions of the interlocking left and right loops, chimeric phi29 RNAs could be fabricated into the dimers shown as an example.

Figure 3

Anti-HIV-1 gp120 aptamer-mediated small interfering (si)RNA delivery. (a) Schematic of the anti-HIV-1 gp120 aptamer-siRNA chimeras. The anti-gp120 aptamer binds to gp120 and the 27-mer Dicer substrate RNA duplex targets a common exon of the HIV-1 tat/rev transcript. Dicer processing results in 21-mer siRNAs that are incorporated into an RNA-induced silencing complex (RISC). (b) Schematic of the anti-HIV gp120 aptamer-'sticky bridge'-siRNA conjugates. Either the antisense or the sense strand of the 27-mer Dicer substrate RNA duplex and the aptamer were attached with to complementary 'sticky' sequences. After a simple annealing, they form stable base pairs.