generator or reactor-produced
90 Y [56], and generator-produced
188 Re [57, 58]. The
former three radioisotopes are used for diagnostic and the latter three for therapeutic
purposes.
11.2.1 Diagnostic Radioisotopes
For the diagnostic purpose, usually radioisotope which emits gamma rays is used.
In the early days, gamma camera was used to detect these single photons of gamma
rays, however after the development of positron emission tomography (PET),
positron-emitting radioisotopes are now widely used. Low positron energy and high
branching ratio of b+ decay are the desired decay characteristics of positron
emitting radioisotopes for PET imaging. The physical half-life of radioisotopes is
also of great concern. Adequate radioisotopes are to be chosen among those with
short half-lives such as
13 N (t 1/2 = 9.97 min),
68 Ga (t 1/2 = 67.7 min),
18 F (t 1/2 =
109.8 min), and
99m
Tc (t 1/2 = 6 h), or those with longer half-lives such as
64 Cu (t 1/2
= 12.7 h),
72 As (t 1/2 = 26 h),
111 In (t 1/2 = 67 h), and
89 Zr (t 1/2 = 78.4 h) according to
the imaging purposes. The physical half-lives of the isotopes should match the
biological half-lives of the vectors being labeled with these isotopes to allow them
to reach the targets of interest. It is also important that the decay time would be as
short as possible in order to avoid unnecessary radiation exposure. In case of
tracking NPs, relatively long-lived isotopes such as
89 Zr are preferred for monitoring the clearance profiles [53].
11.2.2 Therapeutic Radionuclides
Radioisotopes which decay by emission of beta particles, alpha particles, or Auger
electron are potential therapeutic isotopes. The
131
I is a key example of radioisotope
used for therapeutic purpose in the medical field, and this isotope continues to be
used in many technologies focused on treatment including the field of radionanomedicine [59]. In 2004, Cao et al. had labeled
188 Re on the surface of
silica-coated magnetite NP immobilized with histidine with labeling yield of 91%
[44]. Liang et al. in 2007 reported
188
Re labeled superparamagnetic iron oxide NPs
(SPIONs) with a greater than 90% labeling efficiency and a good in vitro stability.
The NPs demonstrated the dose-dependent therapeutic effect in hepatocellular
carcinoma cells in vitro [45]. Interestingly, therapeutic radioisotopes,
131
I,
177 Lu
and
188 Re can be used for diagnostic imaging as well. This is because they emit
both beta and gamma rays and thus these radioisotopes are inherently theranostic.
NPs with proper type of intrinsic radioactivity can also be used for both imaging
and therapy.
198 Au/
199 Au is one prime example of this strategy. The radioactive
properties of
198 Au (beta ray = 0.96 meV, gamma ray = 411 keV) and
199 Au (beta
ray = 0.46 meV, gamma ray = 158 keV) make them ideal candidates for use in
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D. S. Lee et al.
90 Y [56], and generator-produced
188 Re [57, 58]. The
former three radioisotopes are used for diagnostic and the latter three for therapeutic
purposes.
11.2.1 Diagnostic Radioisotopes
For the diagnostic purpose, usually radioisotope which emits gamma rays is used.
In the early days, gamma camera was used to detect these single photons of gamma
rays, however after the development of positron emission tomography (PET),
positron-emitting radioisotopes are now widely used. Low positron energy and high
branching ratio of b+ decay are the desired decay characteristics of positron
emitting radioisotopes for PET imaging. The physical half-life of radioisotopes is
also of great concern. Adequate radioisotopes are to be chosen among those with
short half-lives such as
13 N (t 1/2 = 9.97 min),
68 Ga (t 1/2 = 67.7 min),
18 F (t 1/2 =
109.8 min), and
99m
Tc (t 1/2 = 6 h), or those with longer half-lives such as
64 Cu (t 1/2
= 12.7 h),
72 As (t 1/2 = 26 h),
111 In (t 1/2 = 67 h), and
89 Zr (t 1/2 = 78.4 h) according to
the imaging purposes. The physical half-lives of the isotopes should match the
biological half-lives of the vectors being labeled with these isotopes to allow them
to reach the targets of interest. It is also important that the decay time would be as
short as possible in order to avoid unnecessary radiation exposure. In case of
tracking NPs, relatively long-lived isotopes such as
89 Zr are preferred for monitoring the clearance profiles [53].
11.2.2 Therapeutic Radionuclides
Radioisotopes which decay by emission of beta particles, alpha particles, or Auger
electron are potential therapeutic isotopes. The
131
I is a key example of radioisotope
used for therapeutic purpose in the medical field, and this isotope continues to be
used in many technologies focused on treatment including the field of radionanomedicine [59]. In 2004, Cao et al. had labeled
188 Re on the surface of
silica-coated magnetite NP immobilized with histidine with labeling yield of 91%
[44]. Liang et al. in 2007 reported
188
Re labeled superparamagnetic iron oxide NPs
(SPIONs) with a greater than 90% labeling efficiency and a good in vitro stability.
The NPs demonstrated the dose-dependent therapeutic effect in hepatocellular
carcinoma cells in vitro [45]. Interestingly, therapeutic radioisotopes,
131
I,
177 Lu
and
188 Re can be used for diagnostic imaging as well. This is because they emit
both beta and gamma rays and thus these radioisotopes are inherently theranostic.
NPs with proper type of intrinsic radioactivity can also be used for both imaging
and therapy.
198 Au/
199 Au is one prime example of this strategy. The radioactive
properties of
198 Au (beta ray = 0.96 meV, gamma ray = 411 keV) and
199 Au (beta
ray = 0.46 meV, gamma ray = 158 keV) make them ideal candidates for use in
212
D. S. Lee et al.