The Application of Plant Viral Nanoparticles in Tissue-Specific Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 406

payload of gadolinium in a small volume (high concentration) can be delivered in
a tissue-specific manner; and (2) the size of a large macromolecular carrier slows
down its tumbling rate, resulting in long correlation times and increased relaxivity.
early work on VNp Mri contrast agents was based on CCMV. interest in this
platform came from earlier demonstration that CCMV has 180 metal binding sites
in the interface between its subunits. These binding sites normally bind to calcium
in vivo but were shown to have two orders of magnitude greater affinity for binding
to terbium (Tb 3+ ) [27]. Since Gd 3+ is also a lanthanide with a similar size and charge
distribution as Tb 3+ , it was hypothesized that CCMV could be used to coordinate
Gd 3+ and the Gd 3+ -CCMV complex used as an Mri contrast agent. indeed, the
binding affinity to Gd 3+ was found to be similar to Tb 3+ , and the presence of 1.5 mM
Ca 2+ had very little effect, indicating that the metal binding is stable. The T 1 and T 2
relaxivities were measured to be around 250 and 350 mM −1 s −1 for Gd 3+ -CCMV at
a frequency of 20 Mhz, much higher than the 20 mM −1 s −1 measured for the GdCl 3
control, 5 times higher than what was reported for Gd 3+ -albumin [98], and 10 times
higher than for Gd 3+ -dendrimers [99, 100]. The properties of Gd 3+ -CCMV still
have to be explored in vivo to evaluate stability and possible immunogenicity.
however, these initial results are very encouraging, showing that the size of these
nanoparticles and large number of bound Gd 3+ are ideal for greatly enhancing the
relaxivity [101].
Since this initial study, there have been additional reports of paramagnetic VNp
formulations that would also be promising candidates for applications in Mri.
A popular platform that has been explored is the MS2 capsid. Although the direct
binding of Gd 3+ to CCMV demonstrated the power of VNp contrast agents, stronger
chelates of Gd 3+ is desired for reducing possible toxicity from free Gd 3+ ions for
in vivo applications. MS2 is a 27 nm icosahedral particle comprised of 180 identical
coat proteins, with three lysine residues per coat protein available for modification
on its exterior surface. The first account of MS2-based contrast agents utilized the
540 lysines to covalently attach an isothiocyanate (iTC) derivative of Magnevist; a
Gd-diethylenetriaminepentaacetic acid (DTpA) Mri contrast agent is widely used in
the clinic for the chelation of gadolinium. Using a 1.5 T clinical scanner (64 Mhz,
protons have a gyromagnetic ratio of 42.58 Mhz/T), T ), the resultant MS2(Gd-DTpA)
contrast agents showed a three-fold increase in the T 1 relaxivity, with a relaxivity of
14.0-16.9 mM −1 s −1 per Gd 3+ compared to 5.2 mM −1 s −1 for just Gd-DTpA (Fig. 14.6).
Although not as high as what was found for direct attachment of Gd 3+ to CCMV, it is
expected for T 1 relaxivities to be lower at higher field strengths. Nevertheless, the
relaxivity corresponds to up to 7200mM −1 s −1 per MS2 particle, over 1000 times
greater than any currently approved Mri contrast agent. in addition to demonstrating
the potential use of MS2 for Mri, these particles could additionally be modified with
fluorescein isothiocyanate (FiTC) for optical imaging [102].
Further evaluation of MS2 looked at comparing the difference in relaxivity
enhancement when Gd 3+ is internally versus externally conjugated to an empty
MS2 capsid. in addition to the three lysines on the exterior, MS2 also displays a
tyrosine per coat protein on its interior surface and contains 1.8 nm pores that allow
molecules to diffuse into the capsid. By means of these respective amino acid resi-
dues, orthogonal bioconjugation was used to functionalize MS2 with aldehyde