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D. Li et al.
compounds, which have been widely used in clinicals, pharmacy, biology, molecular
diagnostics, and other fields. They are initially derived from natural molecules, such
as 2-[
18 F] fluoro-2-deoxy-D-Glucose (FDG-PET), a radiolabeled glucose analog,
serves as a radio contrast agent in cancer metabolism imaging [5]. It is now a growing
requirement to develop novel molecular imaging probes that can interact with the
potential molecular target. Many new probes are fabricated from macromolecules,
like protein, which has three-dimensional structure to generate high affinity and specificity. However, protein-based imaging probes have some disadvantages: (1) structures are unstable and difficult to maintain bioactivity during in vivo/vitro labeling
process; (2) production processes are intricacy procedures with a high expenditure;
(3) immuno-competent animal models will reject the humanized proteins [6–8].
Aptamers, often called “chemical antibodies”, are oligomers composed of
deoxyribonucleotides (DNA) or ribonucleotides (RNA). These nucleic acid
sequences can form different three-dimensional structures by four forces of interaction (ionic bonds, hydrogen bonds, hydrophobic effects, and Van der Waals interactions) [9–12]. Specific secondary and tertiary structures allow aptamers to combine
with the target binding domain (aptatope). Because of their characteristics of high
reproducible, non-toxic, and non-immunogenic, quite a few aptamers have been
tested as molecular imaging probes in most of the imaging modalities, including
nuclear imaging (PET and SPECT), magnetic resonance imaging (MRI), echography and fluorescence imaging. Most aptamer-based probes are usually designed
by three components (Fig. 2.1). Two components provide a detectable signal and
a site-specific targeting ligand with a spacer link between them. Once injected
intravenously, the probe is distributed throughout the body and can be monitored by imaging instrument. Then the site-specific targeting ligand binds to the
targeted molecule instead of being cleaned from the tissues. Finally, high target-tobackground contrast will be detected by the imaging system. In this chapter, we will
introduce the studies on aptamer-based probes in five imaging modalities.
2.2 Nuclear Imaging
Nuclear imaging is a noninvasive imaging modality that employs radioactive isotopes
to obtain the imaging of molecular and cellular events under normal and disease
conditions in living subjects [13, 14]. It is a powerful tool to detect, localize, classify,
and quantify a disease precisely, which can assist to make an accurate diagnosis and
choose curative or palliative therapeutic scheme. Depending on the radioisotope,
there are several different techniques of nuclear imaging. Once the radioactively
labeled molecular imaging probes, usually called “radiotracers” or “radiopharmaceuticals,” are injected into the body, these probes emit radiation which will be detected,
visualized, and quantitatively monitored by either SPECT or PET [15]. SPECT uses
β emitters (such as
99m Tc,
123 I, or
111 In) emitting only one photon upon decay and
can be directly detected by gamma camera. Detection of a single photon involves
physical collimators, which facilitates the rejection of highly scattered photons and
D. Li et al.
compounds, which have been widely used in clinicals, pharmacy, biology, molecular
diagnostics, and other fields. They are initially derived from natural molecules, such
as 2-[
18 F] fluoro-2-deoxy-D-Glucose (FDG-PET), a radiolabeled glucose analog,
serves as a radio contrast agent in cancer metabolism imaging [5]. It is now a growing
requirement to develop novel molecular imaging probes that can interact with the
potential molecular target. Many new probes are fabricated from macromolecules,
like protein, which has three-dimensional structure to generate high affinity and specificity. However, protein-based imaging probes have some disadvantages: (1) structures are unstable and difficult to maintain bioactivity during in vivo/vitro labeling
process; (2) production processes are intricacy procedures with a high expenditure;
(3) immuno-competent animal models will reject the humanized proteins [6–8].
Aptamers, often called “chemical antibodies”, are oligomers composed of
deoxyribonucleotides (DNA) or ribonucleotides (RNA). These nucleic acid
sequences can form different three-dimensional structures by four forces of interaction (ionic bonds, hydrogen bonds, hydrophobic effects, and Van der Waals interactions) [9–12]. Specific secondary and tertiary structures allow aptamers to combine
with the target binding domain (aptatope). Because of their characteristics of high
reproducible, non-toxic, and non-immunogenic, quite a few aptamers have been
tested as molecular imaging probes in most of the imaging modalities, including
nuclear imaging (PET and SPECT), magnetic resonance imaging (MRI), echography and fluorescence imaging. Most aptamer-based probes are usually designed
by three components (Fig. 2.1). Two components provide a detectable signal and
a site-specific targeting ligand with a spacer link between them. Once injected
intravenously, the probe is distributed throughout the body and can be monitored by imaging instrument. Then the site-specific targeting ligand binds to the
targeted molecule instead of being cleaned from the tissues. Finally, high target-tobackground contrast will be detected by the imaging system. In this chapter, we will
introduce the studies on aptamer-based probes in five imaging modalities.
2.2 Nuclear Imaging
Nuclear imaging is a noninvasive imaging modality that employs radioactive isotopes
to obtain the imaging of molecular and cellular events under normal and disease
conditions in living subjects [13, 14]. It is a powerful tool to detect, localize, classify,
and quantify a disease precisely, which can assist to make an accurate diagnosis and
choose curative or palliative therapeutic scheme. Depending on the radioisotope,
there are several different techniques of nuclear imaging. Once the radioactively
labeled molecular imaging probes, usually called “radiotracers” or “radiopharmaceuticals,” are injected into the body, these probes emit radiation which will be detected,
visualized, and quantitatively monitored by either SPECT or PET [15]. SPECT uses
β emitters (such as
99m Tc,
123 I, or
111 In) emitting only one photon upon decay and
can be directly detected by gamma camera. Detection of a single photon involves
physical collimators, which facilitates the rejection of highly scattered photons and