2 Aptamer-Based Probes for Molecular Imaging
43
and monitor aptamer degradation in vivo by deoxyribonuclease (DNase)-activatable
fluorescence probes (DFProbes). The results showed that PEI protected the degradation of TD05 aptamers from DNase without affecting its specific recognition [67].
Another study reported by Calzadaet al. developed two protein tyrosine kinase-7
(PTK7)-targeting aptamer probes by coupling frameworks from the fluorescent dye
AlexaFluor 647 or the 6-hydrazinonicotinamide (HYNIC) chelator-labeled to
99m Tc
[68]. Melanoma (B16F1) and lymphoma (A20) tumor-bearing mice were used as
caner model in vivo imaging studies. Thus, fluorophore-based aptamer probes could
be used as a promising tool for biomedical imaging and application. Fluorescence
diffuse optical tomography (fDOT), also called fluorescence molecular tomography
(FMT), is an imaging technique that requires instruments with a transillumination
mode for excitation, generally by lasers. fDOT could reconstruct the original fluorescence emission information in three dimensions with algorithms and significantly
improves in vivo visualization and quantification of the fluorescence signal [66, 69].
Cibiel et al. identified the aptamer, named ACE4, and labeled it with AlexaFluor 680
dye. The aptamer targets CHO-K1 cells expressing human Endothelin type B receptor
(ETBR) and binds to the Annexin A2, a protein overexpressed in several cancers [70].
The tumor targeting of the aptamer was evaluated in MCF-7 tumor xenografted nude
mice by fDOT imaging compared with the scramble sequence. Mi et al. quantified
the tumor targeting of the aptamer targeting to DHX9, an RNA helicase protein and
evaluated in hepatic colorectal cancer metastases animal model [71]. This study also
solved the difficulty in evaluating the size of metastases tumor utilizing the property
of fluorescence imaging that it can measure several fluorophores at the same time by
another fluorescent probe.
Nonspecific binding of the probe is difficult to discriminate in molecular imaging.
It is a main drawback of these imaging modalities that causes high background noise
and limited contrast. Because fluorescence probes could produce a fluorescent signal
only when they are bound to their target by modulating fluorescence emission. It is
possible to evaluate the activatable probes instead of “always-on” aptamer probes.
Hence, Shi et al. developed an activatable aptamer probe (AAP) using a DNA aptamer
targeting PTK7, sgc8 [72]. AAP was produced by elongating the aptamer with a polythymidine sequence. Then the aptamer elongated partially by hybridization to a short
DNA sequence, which has a fluorophore and a quencher attached at each extremity.
In the absence of the target, the fluorophore was close to the quencher because of the
hairpin structure. When aptamer is bound to the targeted protein, the conformation of
construct changed, and fluorescence signal was generated. Compared with “alwayson” aptamer probes which needed to wait for clearance of the unbound probe, the
AAP displayed substantially enhanced contrast (shortened diagnosis time to 15 min)
and minimize the background signal originating from nontarget tissues. Li et al.
reported another activatable fluorescent probe, OMCN/P 0 -Cy3 aptasensor, against
MUC1. A dye (Cy3)-labeled ssDNA probe could be quenched fluorescence by noncovalent π–π stacking interactions with oxidized mesoporous carbon nanospheres
(OMCN) (Fig. 2.5). After exposure to targets, the aptamer was gradually released
from OMCN due to the much stronger interaction with the protein. Consequently, an
43
and monitor aptamer degradation in vivo by deoxyribonuclease (DNase)-activatable
fluorescence probes (DFProbes). The results showed that PEI protected the degradation of TD05 aptamers from DNase without affecting its specific recognition [67].
Another study reported by Calzadaet al. developed two protein tyrosine kinase-7
(PTK7)-targeting aptamer probes by coupling frameworks from the fluorescent dye
AlexaFluor 647 or the 6-hydrazinonicotinamide (HYNIC) chelator-labeled to
99m Tc
[68]. Melanoma (B16F1) and lymphoma (A20) tumor-bearing mice were used as
caner model in vivo imaging studies. Thus, fluorophore-based aptamer probes could
be used as a promising tool for biomedical imaging and application. Fluorescence
diffuse optical tomography (fDOT), also called fluorescence molecular tomography
(FMT), is an imaging technique that requires instruments with a transillumination
mode for excitation, generally by lasers. fDOT could reconstruct the original fluorescence emission information in three dimensions with algorithms and significantly
improves in vivo visualization and quantification of the fluorescence signal [66, 69].
Cibiel et al. identified the aptamer, named ACE4, and labeled it with AlexaFluor 680
dye. The aptamer targets CHO-K1 cells expressing human Endothelin type B receptor
(ETBR) and binds to the Annexin A2, a protein overexpressed in several cancers [70].
The tumor targeting of the aptamer was evaluated in MCF-7 tumor xenografted nude
mice by fDOT imaging compared with the scramble sequence. Mi et al. quantified
the tumor targeting of the aptamer targeting to DHX9, an RNA helicase protein and
evaluated in hepatic colorectal cancer metastases animal model [71]. This study also
solved the difficulty in evaluating the size of metastases tumor utilizing the property
of fluorescence imaging that it can measure several fluorophores at the same time by
another fluorescent probe.
Nonspecific binding of the probe is difficult to discriminate in molecular imaging.
It is a main drawback of these imaging modalities that causes high background noise
and limited contrast. Because fluorescence probes could produce a fluorescent signal
only when they are bound to their target by modulating fluorescence emission. It is
possible to evaluate the activatable probes instead of “always-on” aptamer probes.
Hence, Shi et al. developed an activatable aptamer probe (AAP) using a DNA aptamer
targeting PTK7, sgc8 [72]. AAP was produced by elongating the aptamer with a polythymidine sequence. Then the aptamer elongated partially by hybridization to a short
DNA sequence, which has a fluorophore and a quencher attached at each extremity.
In the absence of the target, the fluorophore was close to the quencher because of the
hairpin structure. When aptamer is bound to the targeted protein, the conformation of
construct changed, and fluorescence signal was generated. Compared with “alwayson” aptamer probes which needed to wait for clearance of the unbound probe, the
AAP displayed substantially enhanced contrast (shortened diagnosis time to 15 min)
and minimize the background signal originating from nontarget tissues. Li et al.
reported another activatable fluorescent probe, OMCN/P 0 -Cy3 aptasensor, against
MUC1. A dye (Cy3)-labeled ssDNA probe could be quenched fluorescence by noncovalent π–π stacking interactions with oxidized mesoporous carbon nanospheres
(OMCN) (Fig. 2.5). After exposure to targets, the aptamer was gradually released
from OMCN due to the much stronger interaction with the protein. Consequently, an