4 Aptamer-Based Drug Delivery Systems
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4.3 Aptamer-Drug Conjugates
Aptamer-drug conjugates (ApDCs) refer to drugs directly linked to aptamers,
including covalent conjugation and physical intercalation. In addition to excellent cell-specific targeting ability, ApDCs also feature with small size, fast tissue
penetration, precise design, and biocompatible ingredients. In this section, some
representative examples of ApDCs were introduced.
4.3.1 Covalent Conjugation
Covalent conjugation is the most commonly used strategy to conjugate aptamer
with therapeutic drugs. Covalent ApDCs require three elements, including targeting
aptamers, proper linkers and therapeutic agents (Fig. 4.2a) [7, 32, 33]. Different kinds
of drugs, such as photosensitizers, proteins and therapeutic genes, were reported to
be linked with aptamers to prepare ApDCs for targeted cancer therapy (Fig. 4.2b)
[32]. For example, Li et al. synthesized aptamer-paclitaxel conjugates (NucA-PTX)
through linking nucleolin aptamer with paclitaxel (PTX) via cathepsin B sensitive
dipeptide bond [33] (Fig. 4.2c). The conjugates maintained the binding affinity
of NucA aptamer, which together showed high water-solubility, tumor-targeting
ability, excellent stability, nonimmunogenicity and nontoxicity. Inserting dipeptide
bond linker endowed the system with cathepsin B responsive release of PTX for
chemotherapy.
The first ApDC system dated back to 10 years ago when Tan’s team utilized
aptamer Sgc8 to covalently link with doxorubicin (DOX) via an acid-sensitive linker.
The Sgc8 can specifically bind with PTK7 receptor highly expressed in CEM cells
[34]. Thus, the Sgc8-DOX conjugates were selectively internalized by target CEM
cells, and then the acid-sensitive linker was cleaved in endosome (pH 4.5-5.5) to
release free DOX. Tan et al. [35] also developed aptamer-mitomycin C conjugates
using cross-linking chemistry. The cross-linking strategy enabled the introduction of
a reductant-sensitive disulfide bond, which caused significantly enhanced cytotoxicity of mitomycin C against the target cancer cell lines. Moreover, three parameters
were critical for the enhanced cytotoxicity effects: the target recognition, specific
binding and receptor-mediated internalization of aptamer (Fig. 4.2d). In another
study, Liu’s group prepared aptamer-pyropheophorbide A (Pyro) conjugates (APCs)
by linking Pyro to hydrophilic nucleic acid aptamer Sgc8, which endowed it with
enhanced water solubility, specific targeting and penetration to PTK7 overexpressed
tumor spheroid for photodynamic therapy (Fig. 4.2e) [36]. The cytotoxicity assays
in vitro demonstrated that the ApDCs showed selective toxicity to Hela and MCF-7
cells, while alleviated the toxicity to control L02 and HepG2 cells.
However, the studies mentioned above possessed a relatively low ratio of drug
loading content. To address this issue, Wang et al. proposed a method to conjugate
multiple copies of drugs to each aptamer molecule at specific sites to prepare modular
81
4.3 Aptamer-Drug Conjugates
Aptamer-drug conjugates (ApDCs) refer to drugs directly linked to aptamers,
including covalent conjugation and physical intercalation. In addition to excellent cell-specific targeting ability, ApDCs also feature with small size, fast tissue
penetration, precise design, and biocompatible ingredients. In this section, some
representative examples of ApDCs were introduced.
4.3.1 Covalent Conjugation
Covalent conjugation is the most commonly used strategy to conjugate aptamer
with therapeutic drugs. Covalent ApDCs require three elements, including targeting
aptamers, proper linkers and therapeutic agents (Fig. 4.2a) [7, 32, 33]. Different kinds
of drugs, such as photosensitizers, proteins and therapeutic genes, were reported to
be linked with aptamers to prepare ApDCs for targeted cancer therapy (Fig. 4.2b)
[32]. For example, Li et al. synthesized aptamer-paclitaxel conjugates (NucA-PTX)
through linking nucleolin aptamer with paclitaxel (PTX) via cathepsin B sensitive
dipeptide bond [33] (Fig. 4.2c). The conjugates maintained the binding affinity
of NucA aptamer, which together showed high water-solubility, tumor-targeting
ability, excellent stability, nonimmunogenicity and nontoxicity. Inserting dipeptide
bond linker endowed the system with cathepsin B responsive release of PTX for
chemotherapy.
The first ApDC system dated back to 10 years ago when Tan’s team utilized
aptamer Sgc8 to covalently link with doxorubicin (DOX) via an acid-sensitive linker.
The Sgc8 can specifically bind with PTK7 receptor highly expressed in CEM cells
[34]. Thus, the Sgc8-DOX conjugates were selectively internalized by target CEM
cells, and then the acid-sensitive linker was cleaved in endosome (pH 4.5-5.5) to
release free DOX. Tan et al. [35] also developed aptamer-mitomycin C conjugates
using cross-linking chemistry. The cross-linking strategy enabled the introduction of
a reductant-sensitive disulfide bond, which caused significantly enhanced cytotoxicity of mitomycin C against the target cancer cell lines. Moreover, three parameters
were critical for the enhanced cytotoxicity effects: the target recognition, specific
binding and receptor-mediated internalization of aptamer (Fig. 4.2d). In another
study, Liu’s group prepared aptamer-pyropheophorbide A (Pyro) conjugates (APCs)
by linking Pyro to hydrophilic nucleic acid aptamer Sgc8, which endowed it with
enhanced water solubility, specific targeting and penetration to PTK7 overexpressed
tumor spheroid for photodynamic therapy (Fig. 4.2e) [36]. The cytotoxicity assays
in vitro demonstrated that the ApDCs showed selective toxicity to Hela and MCF-7
cells, while alleviated the toxicity to control L02 and HepG2 cells.
However, the studies mentioned above possessed a relatively low ratio of drug
loading content. To address this issue, Wang et al. proposed a method to conjugate
multiple copies of drugs to each aptamer molecule at specific sites to prepare modular