12.3.4 Consideration of Targeting Molecules Geometry
on Nanomaterials
For ligation of targeting molecules to the surface of nanoparticle, there are several
factors we have to concern. Despite the targeting molecules play the critical role in
targeting of nanomaterials to disease site, relatively few studies have been reported
to maximize in vivo targeting efficiency through systematic variation of the
geometry or number of targeting molecules on nanomaterials.
The number of targeting molecules on nanomaterials has to be concerned
because that affects the affinity or avidity of nanoparticles for the target proteins, the
rate of nanoparticle-target interaction, and of course, the shape or surface charge of
nanomaterials. There are several reports on number of targeting molecule issue.
Garg et al. and Gindy et al. reported higher concentrations of the targeting molecules on the nanoparticles often increase cellular uptake [48, 49]. Olivier et al.
reported that targeting molecules must be present on the nanoparticle surfaces at
concentrations that exceed a minimum threshold for binding [50]. Gao et al. synthesize 0, 5, or 16% RGD ligand-immobilized Dox-loaded polymeric micelles to
compare their in vitro targeting and therapeutic properties, and the 16% RGD
ligand-bearing nanoparticles displayed the highest cellular uptake and cytotoxicity
in a v b 3 integrin-overexpressing SLK cells. However, several studies also insisted
that high targeting molecule densities did not improve binding to the target cancer
cells and could even promote nonspecific interactions with endothelial and other
non-cancerous cells, which increases immunogenicity, thereby causing
opsonization-mediated clearance of the nanoparticles [51]. Gu et al. prepared with
different compositions of the self-assembled block-copolymers and aptamers, and
the optimal aptamer density on the nanoparticle surface was initially determined
in vitro [52]. Increasing the density of targeting ligand molecule to 5% significantly
increased the nanoparticles uptake by the target cells (LNCaP), whereas further
increase in aptamer density did not increase the uptake. These results indicated that
the optimal density of targeting molecules for PSMA-specific endocytosis in vitro
was 10–80 nmol aptamer per lmol nanoparticle. They injected the targeting
nanoparticles to LNCaP xenograft mouse models and showed that increasing the
aptamer density from 0 to 5% significantly increased nanoparticle retention in
tumors, but the retention decreased for aptamer densities beyond 10%. The authors
suggested that higher aptamer densities may have reduced the nanoparticle stealth
properties, resulting in rapid clearance by the liver. Another report from Shmeeda
et al. optimized the ligand density in the Her2-targeted PEGylated liposomal Dox
system (HT-PLD) in vivo for the ligand ratios of 7.5, 15, or 30 per liposome [53].
The best in vivo performance resulted from 15 ligands per liposome in the HT-PLD
formulation. A 30 ligands per liposome showed the accelerated plasma clearance in
the tumor-bearing mice, and the 7.5 ligand per liposome reduced cytotoxicity after
in vivo passage.
12 Click Chemistry for Radionanomedicine Platform
239
on Nanomaterials
For ligation of targeting molecules to the surface of nanoparticle, there are several
factors we have to concern. Despite the targeting molecules play the critical role in
targeting of nanomaterials to disease site, relatively few studies have been reported
to maximize in vivo targeting efficiency through systematic variation of the
geometry or number of targeting molecules on nanomaterials.
The number of targeting molecules on nanomaterials has to be concerned
because that affects the affinity or avidity of nanoparticles for the target proteins, the
rate of nanoparticle-target interaction, and of course, the shape or surface charge of
nanomaterials. There are several reports on number of targeting molecule issue.
Garg et al. and Gindy et al. reported higher concentrations of the targeting molecules on the nanoparticles often increase cellular uptake [48, 49]. Olivier et al.
reported that targeting molecules must be present on the nanoparticle surfaces at
concentrations that exceed a minimum threshold for binding [50]. Gao et al. synthesize 0, 5, or 16% RGD ligand-immobilized Dox-loaded polymeric micelles to
compare their in vitro targeting and therapeutic properties, and the 16% RGD
ligand-bearing nanoparticles displayed the highest cellular uptake and cytotoxicity
in a v b 3 integrin-overexpressing SLK cells. However, several studies also insisted
that high targeting molecule densities did not improve binding to the target cancer
cells and could even promote nonspecific interactions with endothelial and other
non-cancerous cells, which increases immunogenicity, thereby causing
opsonization-mediated clearance of the nanoparticles [51]. Gu et al. prepared with
different compositions of the self-assembled block-copolymers and aptamers, and
the optimal aptamer density on the nanoparticle surface was initially determined
in vitro [52]. Increasing the density of targeting ligand molecule to 5% significantly
increased the nanoparticles uptake by the target cells (LNCaP), whereas further
increase in aptamer density did not increase the uptake. These results indicated that
the optimal density of targeting molecules for PSMA-specific endocytosis in vitro
was 10–80 nmol aptamer per lmol nanoparticle. They injected the targeting
nanoparticles to LNCaP xenograft mouse models and showed that increasing the
aptamer density from 0 to 5% significantly increased nanoparticle retention in
tumors, but the retention decreased for aptamer densities beyond 10%. The authors
suggested that higher aptamer densities may have reduced the nanoparticle stealth
properties, resulting in rapid clearance by the liver. Another report from Shmeeda
et al. optimized the ligand density in the Her2-targeted PEGylated liposomal Dox
system (HT-PLD) in vivo for the ligand ratios of 7.5, 15, or 30 per liposome [53].
The best in vivo performance resulted from 15 ligands per liposome in the HT-PLD
formulation. A 30 ligands per liposome showed the accelerated plasma clearance in
the tumor-bearing mice, and the 7.5 ligand per liposome reduced cytotoxicity after
in vivo passage.
12 Click Chemistry for Radionanomedicine Platform
239