4 Aptamer-Based Drug Delivery Systems
83
conformational flexibility is a unique trait of aptamers during target recognition, it
also allows easy exposure of the aptamer’s single-stranded regions, making them
vulnerable to nuclease. Thus, it is essential to increase aptamers’ conformational
sustainability in complicated environment, such as human blood. Previous studies
revealed that circular RNAs, designed by covalently linking the 3
end of an exon
with the 5
end of either the same exon or a further upstream exon, can effectively
avoid the nuclease degradation, because of lacking free ends and enhanced conformational integrity. Inspired by this circular structure, Tan’s group engineered circular
bivalent aptamers (cb-aptamers) aiming to improve their conformational stability and
nuclease resistance [38]. Specifically, each aptamer (Sgc8, TD05 and XQ-2d) was
designed into two mono-aptamers with additional flanking complementary sequences
at 3
or 5
end. The additional complementary sequences could hybridize to form
a hybrid bivalent aptamer with two nicks (nc-aptamer). Then, cb-aptamers were
constructed by sealing the two nicks in the nc-aptamer with T4 DNA ligase. They
found that these prepared cb-aptamers showed vastly enhanced nuclease resistance
and binding affinity, comparing with single aptamers. Recently, the same group used
one of the cb-aptamers, cb-XQ-2d, to prepare circular bivalent aptamer-drug conjugates (cb-ApDCs), where drugs with different anticancer pharmacological activities
were linked via ester bonds [39]. Notably, drug ratios of the cb-ApDCs can be
precisely controlled. Systematic studies revealed that the cb-ApDCs demonstrated
high stability, specific recognition, esterase-triggered drug release, and combinatorial
cancer therapy.
Aptamers can also be linked with RNA through covalent linkage (Fig. 4.3a). The
common conjugation strategies include hybridization, direct coupling, and using
connecting molecules such as streptavidin-biotin complex. To increase the flexibility of aptamer and make it better realize the targeting function, a short DNA
strand or/and a PEG chain usually act as a linker between aptamer and RNA. In
2013, Zhou et al. hybridized the aptamer and siRNA via terminating these two
portions with “sticky bridges,” which in nature were GC-rich sequence complements
(Fig. 4.3b) [40]. Later, two RNA aptamers, targeting CD44 and EpCAM, respectively, were linked via a double-stranded RNA. Studies showed that this chimeric
aptamer displayed an enhanced inhibitory effect of cell growth and induced apoptosis, compared with aptamer only (Fig. 4.3c) [41]. In another study, Hong et al.
reported an RNA-based aptamer-siRNA chimera containing two siRNA and two
aptamers (Fig. 4.3d–e) [42]. The aptamer-siRNA conjugates completely suppressed
HIV-1 viral loads in a long time after injection. In 2015, the same group screened a G3 aptamer against C-C chemokine receptor type 5 (CCR5) to target HIV-1 susceptible
cell (Fig. 4.3f) [43]. Conjugating functional siRNAs with the aptamer via random
DNA sequences allowed the selective transferring of functional siRNAs to HIV1 susceptible cells, which together inhibited HIV-1 infectivity. Inspired from an
individual microRNA targeting multiple mRNAs, the designed chimera could also
target two mRNA (survivin mRNA and epidermal growth factor receptor (EGFR)
mRNA). In an intraperitoneal ovarian cancer xenograft model, the bispecific aptamers
induced obvious inhibitory of intraperitoneal tumor growth as well. Besides, using
streptavidin-biotin integration as the bridge, Li et al. conjugated biotinylated aptamer
83
conformational flexibility is a unique trait of aptamers during target recognition, it
also allows easy exposure of the aptamer’s single-stranded regions, making them
vulnerable to nuclease. Thus, it is essential to increase aptamers’ conformational
sustainability in complicated environment, such as human blood. Previous studies
revealed that circular RNAs, designed by covalently linking the 3
end of an exon
with the 5
end of either the same exon or a further upstream exon, can effectively
avoid the nuclease degradation, because of lacking free ends and enhanced conformational integrity. Inspired by this circular structure, Tan’s group engineered circular
bivalent aptamers (cb-aptamers) aiming to improve their conformational stability and
nuclease resistance [38]. Specifically, each aptamer (Sgc8, TD05 and XQ-2d) was
designed into two mono-aptamers with additional flanking complementary sequences
at 3
or 5
end. The additional complementary sequences could hybridize to form
a hybrid bivalent aptamer with two nicks (nc-aptamer). Then, cb-aptamers were
constructed by sealing the two nicks in the nc-aptamer with T4 DNA ligase. They
found that these prepared cb-aptamers showed vastly enhanced nuclease resistance
and binding affinity, comparing with single aptamers. Recently, the same group used
one of the cb-aptamers, cb-XQ-2d, to prepare circular bivalent aptamer-drug conjugates (cb-ApDCs), where drugs with different anticancer pharmacological activities
were linked via ester bonds [39]. Notably, drug ratios of the cb-ApDCs can be
precisely controlled. Systematic studies revealed that the cb-ApDCs demonstrated
high stability, specific recognition, esterase-triggered drug release, and combinatorial
cancer therapy.
Aptamers can also be linked with RNA through covalent linkage (Fig. 4.3a). The
common conjugation strategies include hybridization, direct coupling, and using
connecting molecules such as streptavidin-biotin complex. To increase the flexibility of aptamer and make it better realize the targeting function, a short DNA
strand or/and a PEG chain usually act as a linker between aptamer and RNA. In
2013, Zhou et al. hybridized the aptamer and siRNA via terminating these two
portions with “sticky bridges,” which in nature were GC-rich sequence complements
(Fig. 4.3b) [40]. Later, two RNA aptamers, targeting CD44 and EpCAM, respectively, were linked via a double-stranded RNA. Studies showed that this chimeric
aptamer displayed an enhanced inhibitory effect of cell growth and induced apoptosis, compared with aptamer only (Fig. 4.3c) [41]. In another study, Hong et al.
reported an RNA-based aptamer-siRNA chimera containing two siRNA and two
aptamers (Fig. 4.3d–e) [42]. The aptamer-siRNA conjugates completely suppressed
HIV-1 viral loads in a long time after injection. In 2015, the same group screened a G3 aptamer against C-C chemokine receptor type 5 (CCR5) to target HIV-1 susceptible
cell (Fig. 4.3f) [43]. Conjugating functional siRNAs with the aptamer via random
DNA sequences allowed the selective transferring of functional siRNAs to HIV1 susceptible cells, which together inhibited HIV-1 infectivity. Inspired from an
individual microRNA targeting multiple mRNAs, the designed chimera could also
target two mRNA (survivin mRNA and epidermal growth factor receptor (EGFR)
mRNA). In an intraperitoneal ovarian cancer xenograft model, the bispecific aptamers
induced obvious inhibitory of intraperitoneal tumor growth as well. Besides, using
streptavidin-biotin integration as the bridge, Li et al. conjugated biotinylated aptamer