Biomedical Engineering Reference
In-Depth Information
table 7.1
commonly used spect Radionuclides for Nanoparticles
a
Nuclide
T
1/2
Decay
main photon, keV (%)
production
67
ga
3.26 d
eC
93 (39.2), 185 (21.2), 300
(16.8)
68
Zn (p, 2n)
67
ga
99m
Tc
6.01 h
IT
140 (89.1)
99
mo/
99m
Tc generator
111
In
2.8 d
eC
171 (90.7), 245 (94.1)
111
Cd (p, n)
111
In
112
Cd(p, 2n)
111
In
123
I
13.27 h
eC
159 (83.3)
124
Te (p, 2n)
123
I
124
Xe (p, 2n)
123
I
125
I
59.41 d
eC
35.5 (6.68%)
124
Xe (n, γ)
125m/125g
Xe →
125
I
201
Tl
72.91 h
eC
167 (10.0)
203
Tl (p, 3n)
201
pb:
201
Tl
133
Xe
5.24 d
β
-
81 (38.0)
235
U fission
eC, electron capture (capture of an atomic electron converting a proton to a neutron and emission of a
neutrino); IT, isomeric transition (gamma decay from a long-lived metastable state); β
−
, electron emission
(conversion of a neutron to a proton and emission of an electron and an antineutrino).
a
based on Ref. [41].
7.3.2
Radiolabeled liposomes for spect imaging
Liposomes, a member of the lipid nanoparticle construct, are frequently used for
imaging with SpeCT radioisotopes. Liposomes are vesicles made of spherical lipid
bilayers. The lipid bilayer contains natural phospholipids and/or mixed lipid chains.
encapsulated within the bilayers is often an aqueous center that can deliver hydrophilic
molecules. Hydrophobic molecules are transported within the bilayer itself. Liposomal
anatomy allows fusing with similarly hydrophobic structures, such as cell membranes,
thus allowing both hydrophobic and hydrophilic molecules to be delivered simulta-
neously. This unique property resulted in liposomes being among the first generation
of nanoparticles to be utilized clinically.
Liposomes can be used to deliver radiolabeled drugs or imaging probes to regions
of interest [42]. The surfaces of liposomes can be modified with targeting ligands
(active targeting) or ligands can be encapsulated within the liposome and released at
the point of interest by changes in physiological environment [43]. Different surface
or encapsulation modifications of liposomes have been developed and tuned, result-
ing in improved pharmacokinetics, reduced toxicity, and increased compatibility for
drug delivery with selective targeting to tumor site [44]. Non-pegylated, or “naked,”
liposomes have shown short blood circulation half-lives, rapidly clearing out through
the reticuloendothelial system (ReS) (liver and spleen). In order for diagnostics and
therapy to be effective, longer-circulating liposomes were developed by attaching
polyethylene glycol (peg) chains onto the lipid bilayer, which act to shield the lipo-
some from recognition by plasma proteins and cell surface receptors [45]. These
long-circulating liposomes (LCLs) are sterically stabilized by the incorporated
pegs and thus are able to circumvent recognition by the ReS. The longer blood
circulation time provides enhanced entrapment of LCLs in tumors with improved
pharmacokinetics [46-48].
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