Over the past decade there has been an explosion in the number of nanotechnology-based
agents that have been applied to biological and medical applications. It is
generally believed that these agents will revolutionize how medicine is practiced.
One particularly promising direction that has garnered a great deal of interest
is molecular imaging.
The development of nanotechnology-based imaging probes offers to substantially
improve the specificity and sensitivity of diagnostic imaging by allowing for
the non-invasive and quantitative detection of specific biomolecules in living
In general, molecular imaging probes consist of a nanoparticle that has been
functionalized with a targeting agent. The targeting agent is typically selected
to recognize a disease biomarker located on the cell surface;1-4
however, probes have also been developed that strictly bind healthy tissue,
thus leaving malignancies within target tissues unlabeled.5-7
In either case, the nanoparticles serve to enhance the contrast between malignant
and benign tissue.
Interest in the use of nanoparticles stems from their ability to provide improved
contrast compared with more traditional contrast agents and the ability to control
their pharmacokinetics through variations of their size, surface properties,
and shape.8 The strong contrast enhancing capabilities
of nanoparticles can typically be attributed to atomic constraints that occur
at the nanometer size-scale and/or the cumulative effect that results from packing
many contrast agents into nanometer-sized particles.
For example, when iron oxide particles are synthesized at the nanometer-size
scale they exhibit "superparamagnetic" properties because they can
exist as single-domain crystals. In contrast, larger iron oxide particles generally
consist of multiple magnetic domains that are aligned in the short range, but
at longer distances the domains are anti-aligned and thus exhibit a reduced
net magnetic effect per iron ion.
As a result, on a per iron ion basis, nano-sized iron oxide particles are
generally able to generate more contrast on magnetic resonance images than larger
micron-sized nanoparticles. Of course, the total iron content cannot be ignored.
Since, micron-sized iron oxide particles are composed of significantly more
iron ions than nanoparticles, they exhibit much stronger MR contrast on a per
particle basis. This has allowed single micron-sized particles to be imaged
To date, most nanoparticles that have been developed for magnetic resonance
imaging applications have been characterized in terms of their relaxivity per
ion (e.g. Fe, Gd, etc). Although this is certainly of great value, it can be
argued that for molecular imaging applications it is even more important to
calculate relaxivity on a per particle basis. For example, if a tumor cell has
ten receptors on its surface, the binding of ten micron-sized particles of iron
oxide would certainly provide more contrast than ten nanoparticles, even though
the smaller nanoparticles would likely have a higher relaxivity per iron ion
than the micron-sized particle.
This argument is, of course, not limited to iron oxide particles. A recent
comparison that we made between Gd-labeled dendrimers and Gd-labeled dendrimer
nanoclusters (DNCs) also highlights the importance of calculating the relaxivity
per nanoparticle.1 In this study, Gd-labeled DNCs
were formed by simply cross-linking Gd-labeled dendrimers into a higher-order
structure, with a mean hydrodynamic diameter of ~150nm. While both the dendrimers
and DNCs exhibited a similar relaxivity per Gd ion, the DNCs possessed >1,000-times
more Gd per particle. As a result of this higher payload, the tumor-targeted
Gd-labeled DNCs provided a dramatic improvement in contrast compared with Gd-labeled
dendrimers, in tumor-bearing mice.
Aside from the contrast-enhancing capabilities of molecular imaging agents,
it is also of critical importance to characterize the pharmacokinetics of new
nanoparticle formulations. Particle size, shape, and charge are all known to
be major driving forces responsible for dictating the blood half-life and biodistribution.
In general, nanoparticles at the length scale of ~10-100nm have generally exhibited
longer circulations times and improved tissue penetration than micron-sized
particles. These pharmacokinetic properties can lead to improved targeting and
in many cases can be used to overcome the lower contrast-enhancing capabilities
of smaller particles - hence the growing interest in using nanoparticles
as opposed to micron-sized particle for molecular imaging applications.
In applications where long circulation times and additional contrast is not
necessary, there has even been a movement to make molecular imaging probes that
are <5.5nm in diameter to encourage renal filtration.10
This would allow for more rapid imaging, since unbound nanoparticles would be
cleared much faster, and reduced toxicity for the same reasons.
In addition to the physical-chemical properties of the nanoparticle itself,
the targeting agent also plays an instrumental role in the utility of nanoparticle-based
contrast agents. For the most part, targeting agents that have been evaluated
for molecular imaging applications have mirrored those used for targeted therapeutics
(e.g. folic acid, transferrin, anti-HER2/neu antibodies, etc.).
For cancer imaging, these agents have shown a great deal of promise when used
to assess the availability of therapeutic targets and monitor the efficacy of
treatment; however, tumor cell receptors are unlikely to be adopted for diagnostic
imaging due to the lack of any single receptor that is highly up-regulated across
For diagnostic imaging, a biomarker that is universally present would have
to be identified for clinical utility. Borrowing from FDG-PET imaging, one option
that is being explored involves taking advantage of the increased metabolic
rate of cancer cells and the resultant acidic microenvironment. Accordingly,
various agents are being developed that specifically bind to cells that exist
in subphysiologic pH.11 Since, an acidic microenvironment
is common to most tumors, it is expected that tumor pH could serve as a more
universal target. Ligands that target angiogeneisis or hypoxia could also potentially
be utilized to expand the versatility of targeted molecular imaging probes.
Of course, when biomarkers with increased universality are selected, it often
comes at the cost of reduced specificity - a criticism that has often
In conclusion, nanoparticles have shown great promise as molecular imaging
probes. However, as the number of nanoparticle formulations continues to expand
it will be increasingly important to establish proper indices by which they
can be compared. It will also be important to develop creative targeting strategies
that can be used to identify disease with high sensitivity and high predictive
1. Cheng Z, Thorek DL, Tsourkas A. Gadolinium-conjugated
dendrimer nanoclusters as a tumor-targeted T1 magnetic resonance imaging contrast
agent. Angew Chem Int Ed Engl. 2010;49(2):346-50.
2. Thorek DL, Chen AK, Czupryna J, Tsourkas
A. Superparamagnetic iron oxide nanoparticle probes for molecular imaging. Ann
Biomed Eng. 2006 Jan;34(1):23-38.
3. Tsourkas A, Shinde-Patil VR, Kelly KA,
Patel P, Wolley A, Allport JR, Weissleder R. In vivo imaging of activated endothelium
using an anti-VCAM-1 magnetooptical probe. Bioconjug Chem. 2005 May-Jun;16(3):576-81.
4. Zhang CY, Lu J, Tsourkas A. Iron chelator-based
amplification strategy for improved targeting of transferrin receptor with SPIO.
Cancer Biol Ther. 2008 Jun;7(6):889-95.
5. Montet X, Weissleder R, Josephson
L. Imaging pancreatic cancer with a peptide-nanoparticle conjugate targeted
to normal pancreas. Bioconjug Chem. 2006 Jul-Aug;17(4):905-11.
6. Reimer P, Weissleder R, Shen T,
Knoefel WT, Brady TJ. Pancreatic receptors: initial feasibility studies with
a targeted contrast agent for MR imaging. Radiology. 1994 Nov;193(2):527-31.
7. Tanimoto A, Kuribayashi S. Hepatocyte-targeted
MR contrast agents: contrast enhanced detection of liver cancer in diffusely
damaged liver. Magn Reson Med Sci. 2005;4(2):53-60.
8. Moghimi SM, Hamad I. Factors Controlling Pharmacokinetics
of Intravenously Injected Nanoparticulate Systems. In: de Villiers MM, Aramwit
P, Kwon GS, editors. Nanotechnology in Drug Delivery. New York: Springer; 2009.
9. Shapiro EM, Skrtic S, Sharer K, Hill JM, Dunbar CE, Koretsky
AP. MRI detection of single particles for cellular imaging. Proc Natl Acad Sci
U S A. 2004 Jul 27;101(30):10901-6.
10. Choi HS, Liu W, Misra P, Tanaka E, Zimmer JP, Itty Ipe
B, Bawendi MG, Frangioni JV. Renal clearance of quantum dots. Nat Biotechnol.
11. Reshetnyak YK, Andreev OA, Lehnert U, Engelman DM. Translocation
of molecules into cells by pH-dependent insertion of a transmembrane helix.
Proc Natl Acad Sci U S A. 2006 Apr 25;103(17):6460-5.
Copyright AZoNano.com, Professor Andrew Tsourkas (University
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