The Application of Plant Viral Nanoparticles in Tissue-Specific Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 396

Several peGylated VNps, such as CpMV [44-46], pVX [40, 47], TMV [48, 49], and
bacteriophage MS2 [50], have been generated. Minimal surface coverage (<1%) with
peG chains is sufficient to shield CpMV nanoparticles from interacting with cells,
leaving a large surface area and numerous attachment sites available on the viral
capsid for further modifications with targeting, imaging, and therapeutic agents
[46]. Therapies comprised of peGylated proteins are increasingly being utilized in
the clinic, indicating that peGylation is an accepted technology to overcome immu-
nogenicity [51].
Besides toxicity and biodistribution, it is also essential to understand the pharma-
cokinetic properties of VNps, as it is important to achieve the appropriate balance
between nanoparticle accumulation at disease sites and system clearance. Although
longer circulation times allow for the accumulation of drugs or imaging reagents
into target tissues, the level of toxicity or background-to-noise ratio significantly
increases as well. it is likely that the pharmacokinetics of VNps tailored for drug
delivery are different from those designed as imaging tools [18]. The pharmacoki-
netic properties of VNps are dependent on their surface composition, such as surface
charge and surface chemistry [52]. VNps with a negative surface charge, such as
CpMV and CCMV, tend to have short plasma half-lives (<15 min) [39, 42], while
particles such as Qβ that have a positive surface charge have a much longer circulation
time (more than 3 h) [38].
Chemical and genetic tuning resulting in altering the surface characteristics of a
VNp will change its in vivo properties. For example, acetylating the surface lysine
residues on Qβ and M13 capsid proteins neutralizes their positive charge and reduces
the plasma half-lives of the particles [38, 41]. peGylation of VNps (as previously
described) adds a hydration shell and has been shown to drastically increase circulation
time [53, 54]. other factors naturally have to be taken into account, and recent data
show that circulation times cannot be generalized based on surface charge: positively
charged, filamentous pVX has a charge comparable to Qβ but a circulation time sim-
ilar to CpMV [40]. Model systems have been developed to rapidly determine the
efficacy of VNp-based imaging agents in vivo [55, 56].
14.2 Vnps lAbeled wiTh fluorescenT dyes
for opTicAl imAging
optical fluorescence imaging is one of the most widely applied imaging technologies
with many applications in medicine and biological sciences. Fluorescence imaging
relies on the property of certain molecules to absorb light (see chapter 9) at a specific
wavelength and then emit light of a longer wavelength (reviewed in ref. [57]).
Studies to reconstruct light through tissues were facilitated by the discovery that
near-infrared (Nir) photons penetrate tissue far more efficiently than those within
visible range [58, 59]. Many targeted fluorescent imaging probes have since then
been developed for in vivo imaging [60-62].
The incorporation of fluorescent dyes and proteins into modified VNps has facilitated
their characterization. For example, VNp drug formulations are typically colabeled with
dyes and studied by fluorescence microscopy to evaluate cellular internalization and