The Application of Plant Viral Nanoparticles in Tissue-Specific Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 408

MS2-hopo- R , R of 29.6 mM −1 s −1 and MS2-hopo-Lin of 32.6 mM −1 s −1 per Gd 3+ .
The relaxivity was thus improved by using a rigid linker, with linker chirality also
playing a factor. The validation of the design concept guides the formulation of
contrast agents with even greater sensitivity for future applications in targeting
areas requiring greater enhancement [105].
Another contribution to the VNp contrast agent toolbox explored the use of
CpMV and Qβ protein cages. These VNps are similar to CCMV and MS2, also
icosahedrons approximately 28 nm in diameter. The particles were modified using
an azide linker conjugated to the lysines available on the exterior of both. Then,
attachment of an alkyne derivative of Gd-DoTA (tetraazacyclododecane tetraacetic
acid), a clinically approved gadolinium complex, was accomplished via Cu(i)-
mediated azide-alkyne cycloaddition (CuAAC). Around 220 and 150 Gd-DoTA
complexes were conjugated to CpMV and Qβ, respectively, using this method. in
addition, the affinity of lanthanides for rNA was investigated, and it was found that
approximately 80 Gd 3+ ions could be loaded per CpMV, with similar values observed
for Tb 3+ . Dual-labeled CpMV particles were then made by covalent conjugation of
Gd-DoTA followed by internal binding of Gd 3+ , without any decrease in the efficiency
Gd 3+ loading found. The T 1 relaxivities were measured for the various CpMV and
Qβ formulations. relaxivities of up to 15.5 mM −1 s −1 per Gd 3+ and 4150 mM −1 s −1 per
particle were obtained for the dual-labeled particles at 64 Mhz, values comparable
to what was reported for the MS2(Gd-DTpA) contrast agents and reflecting a three-
fold enhancement compared to Gd-DoTA alone [106]. The CpMV formulations
were further evaluated in mice for their fate in vivo . CpMV with Gd-DoTA on the
exterior and Gd 3+ loaded in the interior showed similar plasma clearance kinetics,
with very low levels of particles detected in the bloodstream after 30 min. At this time
point, the biodistribution was analyzed, with over 90% found in the liver and a small
amount in the spleen. histological examination was also conducted and showed no
evidence of pathology, indicating that the CpMV constructs are safely cleared from
the system with no toxic effects [39].
A new class of nanoparticles for Mri that was previously unexplored is com-
prised of rod-shaped particles. rods offer many advantages, such as greater tumor
penetration and a larger surface area that offers more sites for functionalization [40].
recently, TMV particles measuring 300 by 18 nm in size were investigated to test if
the advantages of rod-shaped particles could be extended to applications in Mri.
These particles were functionalized with Gd-DoTA on either their interior or exterior
surface by targeting their solvent-exposed carboxylic acid and tyrosine residues,
respectively. in addition, interior-modified TMV can undergo a thermal transition to
form rNA-free spherical nanoparticles (SNps) with a controlled size distribution.
TMV with around 3,500 Gd-DoTA loaded within, TMV with 2,000 gadolinium
chelates conjugated to its exterior surface, and 170nm SNps containing 25,000
Gd-DoTA were compared. T 1 relaxivities of 10.7, 18.4 mM −1 s −1 , and 15.2 mM −1 s −1
per Gd 3+ ion at 60Mhz, respectively, were obtained. This is a two- to three-fold
increase compared to 4.9 mM −1 s −1 measured for Gd-DoTA alone and similar to values
found earlier for VNp Gd-DoTA and Gd-DTpA conjugates [102, 106]. in addition,
there is a substantial increase in the relaxivity when the relaxivities per particle are