Cell-type-specific, Aptamer-functionalized Agents for Targeted Disease Therapy

Abstract

One hundred years ago, Dr. Paul Ehrlich popularized the "magic bullet" concept for cancer therapy in which an ideal therapeutic agent would only kill the specific tumor cells it targeted. Since then, "targeted therapy" that specifically targets the molecular defects responsible for a patient's condition has become a long-standing goal for treating human disease. However, safe and efficient drug delivery during the treatment of cancer and infectious disease remains a major challenge for clinical translation and the development of new therapies. The advent of SELEX technology has inspired many groundbreaking studies that successfully adapted cell-specific aptamers for targeted delivery of active drug substances in both in vitro and in vivo models. By covalently linking or physically functionalizing the cell-specific aptamers with therapeutic agents, such as siRNA, microRNA, chemotherapeutics or toxins, or delivery vehicles, such as organic or inorganic nanocarriers, the targeted cells and tissues can be specifically recognized and the therapeutic compounds internalized, thereby improving the local concentration of the drug and its therapeutic efficacy. Currently, many cell-type-specific aptamers have been developed that can target distinct diseases or tissues in a cell-type-specific manner. In this review, we discuss recent advances in the use of cell-specific aptamers for targeted disease therapy, as well as conjugation strategies and challenges.

Figure 1

Covalent aptamer-therapeutic oligonucleotide chimeras. (a) Schematic of the first generation aptamer (RNA or DNA)-siRNA/miRNA chimera. The chimera was synthesized as two pieces followed by an annealing step to make the chimera. (b) Schematic of the optimized second-generation chimera. The aptamer portion was truncated from 71 to 39 nt. A two nucleotide (UU) overhang and polyethylene glycol (PEG) tail were added to the 3′-end of the guide strand (red) and the 5′-end of passenger strand (blue), respectively. (c) Schematic of the aptamer-shRNA chimera, which was synthesized as one piece. (d) Schematic of the AS1411 DNA aptamer-SSO chimera. SSO: splice-switching oligonucleotide.

Figure 2

Noncovalent RNA aptamer- therapeutic oligonucleotide conjugates. (a) Schematic of the aptamer-streptavidin-siRNA conjugate. The siRNA and PSMA RNA aptamers were chemically conjugated with a biotin group. Then the two biotinylated siRNAs and two aptamers were non-covalently assembled via a streptavidin connector. (b) Schematic of the aptamer-sticky bridge-siRNA conjugate. The aptamer and siRNA were appended to complementary 17-nt 2′ OMe/2′ Fl GC-rich bridge sequences and were annealed by simple mixing that allowed Watson-Crick base pairing. (c) Schematic of a bispecific PSMA-4-1BB aptamer conjugate. The PSMA RNA aptamer and a bivalent 4-1BB RNA aptamer were tethered to complementary linker sequences and were hybridized together through Watson-Crick base pairing.

Figure 3

Covalent aptamer-chemotherapeutic agent or protein conjugation. (a) Schematic of an aptamer (DNA or RNA)-anticancer drug conjugate made using an acid-labile acylhydrazone linkage or formaldehyde linkage. (b) Schematic of an aptamer (DNA or RNA)-protein (rGel toxin or lysosomal enzyme) conjugate.

Figure 4

Noncovalent aptamer-chemotherapeutic agent conjugation. (a) Schematic of the physical conjugation between an aptamer (DNA or RNA) and anthracycline drug (Dox) through intercalation. (b) Schematic of the physical conjugation between a G-quadruplex structure aptamer (AS1411 DNA aptamer) and photodynamic agent (TMPyP4) interaction.

Figure 5

Aptamer-functionalized inorganic nanoparticles. (a) Schematic of the antiEGFR aptamer-AuNP conjugate created through a hybridization approach. AuNPs were first coated with short capture DNA sequences (pink) and were subsequently hybridized with corresponding complementary sequences that were appended to 5′-end of the antiEGFR RNA aptamer. (b) Schematic of sgs8c DNA aptamer/hpDNA-Dox/AuNPs. Using standard gold-thiol chemistry, the sgs8c DNA aptamer and hpDNA were assembled onto the surface of AuNPs. The double-stranded region within the hpDNA was used to load the chemotherapeutic drug Dox. (c) Schematic of aptamer-functionalized SPIONs. The PSMA RNA aptamer was covalently conjugated onto the surface of a SPION and then Dox was intercalated in the duplex region of the A10 aptamer and also complexed with the SPION through charge interactions. (d) Schematic of aptamer-functionalized single-walled carbon nanotubes (SWNTs). The sgc8c DNA aptamer and anthracycline agent (Dau) were loaded onto SWNTs through a π electron interaction. (e) Schematic of aptamer-functionalized QDs. The mucin 1 DNA aptamer was covalently linked to the surface of QDs through EDC/NHS chemistry. Dox was attached to QDs through a pH-sensitive hydrazone bond, which provided stability to the complex and facilitated the release of drugs in the endosome.

Figure 6

Aptamer-functionalized liposome and micelle nanoparticles. (a) Schematic of aptamer-functionalized liposome NPs for delivering antitumor agents. The sgc8 DNA aptamer was covalently conjugated to the surface of the liposome. A dextran molecule was then encapsulated into the core of the liposome. (b) Schematic of aptamer-functionalized liposome NPs for delivering siRNA. A TfR aptamer was used to functionalize stable, nucleic acid-lipid particles (SNALPs) containing siRNAs. (c) Schematic of aptamer-functionalized micelles for delivering drugs. A simple lipid tail phosphomidite with diacyl chains was attached to the end of a TDO5 DNA aptamer by using a PEG spacer. The amphiphilic aptamer-PEG-lipid tail molecule further assembled itself into a spherical micelle.

Figure 7

Aptamer-functionalized protein nanoparticles. (a) Schematic of aptamer-functionalized viral capsids. MS2 bacteriophage capsids were loaded with photosensitizer for targeted photodynamic therapy. Next, in the presence of the cross-linker reagent SPDP, the capsid exterior was covalently decorated with a DNA aptamer (AS1411) containing a 3′-thiol group. (b) Schematic of a protein complement for delivering aptamer-siRNA chimeras. The protein contains a dsRNA binding domain (dsRBD) for siRNA docking and a pH-dependent polyhistidine that will disrupt the endosomal membrane SPDP, succinimidyl 3-(2-pyridyldithio)propionate.

Figure 8

Aptamer-functionalized polymeric nanoparticles. (a) Schematic of aptamer-functionalized polymeric NPs for delivering antitumor drugs. The RNA or DNA aptamer was covalently functionalized onto the surface of the polymeric NPs. Antitumor drugs were either encapsulated into the core of the NPs or loaded onto the aptamers by intercalation. (b) Schematic of aptamer-functionalized polymeric NPs for delivering siRNA or miRNA. The siRNA or miRNA were complexed with cationic NPs through an electrostatic interaction. At the same time, antitumor drugs were intercalated into the aptamers for codelivery. (c) Schematic of a multifunctional PEI-coated QD system. First, thiol-modified siRNAs were adsorbed on PEI-coated QD nanoparticles through a noncovalent electrostatic interaction, which partially neutralized the nanoparticle surface's positive charge, thereby minimizing some of the nonspecific electrostatic interactions that occur between negatively charged aptamers and the nanoparticle surface. Next, the thiol-modified PSMA RNA aptamers were added to covalently conjugate to siRNAs through the thiol-disulfide exchange reaction to form aptamer-S-S-siRNA conjugates.