emerging concepts in molecular mri david e sosnovik and ralph...

Emerging concepts in molecular MRIDavid E Sosnovik and Ralph Weissleder

Molecular magnetic resonance imaging (MRI) offers the

potential to image some events at the cellular and subcellular

level and many significant advances have recently been

witnessed in this field. The introduction of targeted MR contrast

agents has enabled the imaging of sparsely expressed

biological targets in vivo. Furthermore, high-throughput

screens of nanoparticle libraries have identified nanoparticles

that act as novel contrast agents and which can be targeted

with enhanced diagnostic specificity and range. Another class

of magnetic nanoparticles have also been designed to image

dynamic events; these act as ‘switches’ and could be used in

vitro, and potentially in vivo, as biosensors. Other specialized

MR probes have been developed to image enzyme activity in

vivo. Lastly, the use of chemical exchange and off-resonance

techniques have been developed, adding another dimension to

the broad capabilities of molecular MRI and offering the

potential of multispectral imaging. These and other advances in

molecular MRI offer great promise for the future and have

significant potential for clinical translation.

Addresses

Center for Molecular Imaging Research, Massachusetts General

Hospital, Harvard Medical School, Boston MA, USA

Corresponding author: Sosnovik, David E

([email protected])

Current Opinion in Biotechnology 2007, 18:4–10

This review comes from a themed issue on

Analytical biotechnology

Edited by Joseph Wu and Jagat Narula

Available online 28th November 2006

0958-1669/$ – see front matter

# 2006 Elsevier Ltd. All rights reserved.

DOI 10.1016/j.copbio.2006.11.001

IntroductionThe attributes of magnetic resonance imaging (MRI; see

Box 1) allow high-resolution anatomical and functional

images of numerous organ systems to be obtained and also

make it highly suited to the molecular imaging mission,

namely the imaging of events at the cellular and sub-

cellular level [1��,2,3]. The detection of events at this

level, however, often requires nanomolar sensitivity, thus

precluding the use of conventional gadolinium chelates as

molecular MR imaging agents, as they display only

micromolar sensitivity. The principal response to the

inadequate sensitivity of conventional gadolinium che-

lates has been to synthesize novel MR contrast agents

with significantly higher relaxivities [3]. These include


paramagnetic gadolinium-containing liposomes [4,5] or

micelles [6,7], and superparamagnetic iron oxide nano-

particles [8,9]. The use of iron oxide nanoparticles for

liver imaging [10], lymph node characterization [11],

diagnosis of tissue inflammation [12,13�,14], and stem-

cell labeling [15,16] is becoming fairly well established,

and will thus not be further discussed in this review.

Several novel techniques and applications in molecular

MRI have emerged in the past few years and will be

highlighted here. These include the in vivo imaging of

sparsely expressed biological targets using targeted MR

contrast agents, the surface modification of nanoparticles to

yield nanoparticle libraries with enhanced target specifi-

city, the use of iron oxide containing magnetic relaxation

switches as in vitro and in vivo biosensors, in vivo imaging of

enzyme activity with activatable MR contrast agents, and

the development of CEST and PARACEST techniques

that allow multispectral proton MRI to be performed.

Cardiovascular disease remains the leading cause of mor-

tality in industrialized societies and applications in this

area will thus be highlighted, although the techniques

listed above are applicable to a broad range of disease

states. Particular, but not exclusive, attention will also be

paid to applications involving iron oxide nanoparticles.

The interested reader is referred to other articles in this

volume for a more detailed discussion on other nanopar-

ticles and nanostructures. Finally, given the scope of this

review, the discussion will be limited to proton MRI.

Targeted MR contrast agentsFibrin-targeted MR contrast agents were amongst the

first molecular MR probes to be developed, and exploited

the high levels of fibrin expressed within thrombi. These

agents were successfully used to detect both arterial and

venous thrombi in a variety of models [5,17,18]. Recently,

however, more sparsely expressed targets such as surface

integrins [19], surface phospholipids on apoptotic cells

[20��], vascular adhesion molecules [21��], and macro-

phage receptors [7], amongst others, have been imaged

non-invasively in vivo. The detection of a sparsely

expressed surface target, particularly one within an organ

parenchyma or an atherosclerotic plaque, ideally requires

the MR contrast agent to have a long half-life in the blood,

the ability to penetrate the tissue of interest, and the

ability to detect and amplify the biological signal of

interest. The superparamagnetic iron oxides, in particu-

lar, fulfill many of these criteria [22].

Cross-linked iron oxide (CLIO) is a highly stabilized

form of monodisperse iron oxide or MION [9,22]. The

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Molecular MRI Sosnovik and Weissleder 5

Box 1 MRI technology and its attributes.

Magnetic resonance imaging (MRI) is a non-invasive diagnostic

technique that has the potential to image some events at the cellular

or subcellular level. This imaging technology is based on the

interaction of protons with each other and with surrounding

molecules in a tissue of interest. When placed in a strong magnetic

field protons precess or rotate at a given frequency and are able to

accept energy from a radiofrequency wave applied at this rotational

or resonance frequency. The behavior of the energy inserted in to the

system is described by two relaxation constants: the T2 or

transverse relaxation time and the T1 or longitudinal relaxation time.

Different tissues have different relaxation times, and this can be used

to produce endogenous contrast between different tissues. Exo-

genous contrast agents can further enhance tissue contrast by

selectively shortening either the T1 or T2 in a tissue of interest. The

MR image can be weighted to detect differences in either T1 or T2 by

adjusting parameters in the acquisition.

MRI offers several advantages over other imaging modalities. Firstly,

it is non-ionizing as it detects the magnetic signals generated by

protons and other molecules. The technique is also tomographic,

enabling any tomographic plane through a three-dimensional volume

to be imaged. High-resolution images with excellent soft tissue

contrast between different tissues can be obtained. Lastly, multiple

contrast mechanisms are possible using MRI and the technique can

be used to provide anatomical as well as physiological readouts.

cross-links on CLIO are aminated, allowing a large variety

of ligands to be conjugated to the nanoparticle with a high

degree of stability and relative ease. Near-infrared fluor-

ochromes can be attached to the amine groups to form a

dual modality magnetofluorescent nanoparticle [23].

Thereafter, many copies of the targeting ligand can be

attached to the nanoparticle to form a multivalent (i.e.

more than one copy of the targeting ligand) magneto-

fluorescent nanoparticle. Recently reported examples of

two such ligands include annexin for apoptosis imaging

[24] and a phage-derived peptide specific for the adhesion

molecule VCAM-1 [21��,25�].

The properties of the magnetofluorescent annexin

probe, AnxCLIO-Cy5.5, have been previously described

[24]. Cardiomyocyte apoptosis in a mouse model of

ischemia-reperfusion was successfully imaged with this

probe by MRI in vivo [20��]. Preliminary results with the

probe also suggest that it can successfully detect even the

low levels of cardiomyocyte apoptosis seen in heart fail-

ure. The adhesion molecule VCAM-1 is expressed by

damaged endothelium early in the atherosclerotic pro-

cess and was successfully imaged in the aortic roots of

apoE�/� mice in vivo (Figure 1) [21��]. In addition, the

beneficial effect of statin therapy on VCAM-1 expression

could be documented with the probe in vivo by

demonstrating decreased probe accumulation in the

statin-treated mice [21��]. Thus, this VCAM-1-sensing

iron oxide probe not only demonstrated adequate

sensitivity to detect a sparsely expressed molecular

marker, but also exhibited an adequate dynamic range

to detect a treatment effect.


The therapeutic effect of the anti-angiogenic agent fuma-

gillin has also been demonstrated in a rabbit model of

vascular injury with a gadolinium-containing liposome

targeted to the aVB3 integrin. This integrin is a marker

of angiogenesis, inflammation and instability within an

atherosclerotic plaque [26��]. Although, larger than iron

oxide probes, the hyperpermeability of vessels involved

in plaque angiogenesis allows the gadolinium-containing

liposome to penetrate the plaque to enable imaging of

new vessel formation. Gadolinium-containing immuno-

micelles have also recently been developed and, when

decorated with an antibody to the macrophage scavenger

receptor, have been shown by ex vivo MRI to detect

plaque macrophages [7].

In the near future it is likely that superparamagnetic iron

oxides with even higher relaxivities will be generated

[27]. Further advances are also likely to be seen in the

synthesis of novel gadolinium chelates and constructs.

The imaging of a wide variety of sparsely expressed

molecular targets by MRI in small animal models will

thus become an established technique in molecular biol-

ogy and pharmaceutical development. Targeted molecu-

lar MRI in large animal models will require ligand

synthesis to be scaled up to support this, and will thus

require careful selection. Further translation into the

clinical realm will require even more careful considera-

tion, as each new construct will require a completely

separate profile of pharmacokinetic and toxicity screen-

ing. Nevertheless, the translation of a few well-selected

targeted iron oxide probes into the clinical realm remains

highly likely.

Iron oxide nanoparticle librariesAlthough the clinical translation of targeted MR contrast

agents will require careful selection of the most appro-

priate agents, this restriction will not apply in the pre-

clinical arena. In this realm, high-throughput screening

and synthesis techniques are likely to be used to generate

a diverse array of nanoparticles with small surface mod-

ifications. Minor modification of the surface of the iron

oxide nanoparticle CLIO-Cy5.5 has already been shown

to drastically alter its cellular uptake [28��]. In addition,

these modifications can now be performed quickly and at

room temperature using convenient ‘click’ chemistry

techniques [29].

The generation of a library of nanoparticles for macro-

phage imaging has recently been described [28]. In its

baseline state, CLIO-Cy5.5 has a high affinity for both

resting and activated macrophages. Conjugation of the

chemical moiety succinimidyl iodo acetate (SIA) to the

aminated sidechains on CLIO-Cy5.5, however, almost

completely abolished its uptake by any macrophages

whatsoever. High-throughput screening also revealed

CLIO conjugates with specific affinity for either resting

or activated macrophages: the agent CLIO-Gly, for


6 Analytical biotechnology

Figure 1

In vivo MRI of VCAM-1 expression in apoE�/� mice. The dotted white line marks the short axis plane of the aortic root, used in the remainder of the

images. MR imaging at 9.4 Tesla (a) before and (b) after the injection of a VCAM-1-targeted magnetofluorescent probe reveals a significant change in

signal intensity in the aortic root plaque. The accumulation of the probe in the endothelium of the aortic root and its specificity for VCAM-1 are

confirmed by (c) fluorescence microscopy and (d) immunohistochemistry. (Figure adapted from [21��] with permission.)

instance, was found to be taken up only by activated

macrophages (Figure 2), whereas the agent CLIO-bentri

was found to be avid for only resting macrophages [28��].

Given the potential of stem-cell therapy in a wide variety

of diseases, it is likely that large nanoparticle libraries will

be developed to aid the preclinical understanding of these

cells. Probes specific for cell type, activation and differ-

entiation will be developed. The use of such nanoparticle

libraries could facilitate the development of effective

stem-cell therapies and a better understanding of their

regenerative potential.

MR-detectable nanoswitches and biosensorsThe targeted molecular MR contrast agents, described

above, are generally designed to image targets with a

fairly constant level of expression during the image

acquisition. However, another class of magnetic nanopar-

ticles, designed to image dynamic events, has recently

been described [30]. These magnetic relaxation switches

consist of iron oxide nanoparticles that undergo reversible

assembly and disassembly in the presence of a specific


enzyme or chemical compound, producing a change in

transverse magnetic relaxivity. Magnetic relaxation

switches have previously been shown to be capable of

detecting proteases [30], oligonucleotides [31], viral par-

ticles [32], and entantiomeric impurities [33] in solution.

The synthesis of magnetic relaxation switches specific for

a wide variety of diagnostic markers used in clinical

medicine is certainly feasible, and could prove a useful

addition to currently used in vitro diagnostic techniques.

Magnetic relaxation switches have also recently been

used to determine the concentration of analytes, such

as glucose [34��] and calcium [35], in solution. A construct

consisting of CLIO-glucose and concavalin-A was able to

accurately determine glucose concentration over the

clinically meaningful range [34��]. In the presence of

lower glucose concentrations, the CLIO-glucose conju-

gates bind to concavalin-A and form aggregates with

higher relaxivities. However, as the glucose concentration

increases, the CLIO-glucose conjugates are displaced

from concavalin-A and the degree of nanoparticle aggre-

gation decreases [34��]. This concept is demonstrated in



Figure 2

Generation of an iron oxide nanoparticle library for macrophage sensing.

The parent compound CLIO-NH2 is taken up by both resting and

activated macrophages. Activation of the macrophages by granulocyte

macrophage colony stimulating factor (GMCSF), oxidized LDL (OxLDL)

or lipopolysaccharide (LPS) produces a significant increase in avidity for

the compound CLIO-Gly, which is otherwise minimally taken up by

resting macrophages. Conversely, the agent CLIO-bentri is taken up by

resting macrophages, but not by activated macrophages. (a) Bar chart

showing the degree of uptake of the different iron oxide nanoparticles.

The y axis shows the fluorescence on uptake in arbitrary units (au). (b)

Near-infrared fluorescence microscopy of probe uptake (color scale)

fused with phase contrast images of the macrophage populations.

(Figure adapted from [28��] with permission.)

Figure 3

Magnetic nanosensing of glucose concentrations. (a) In the presence of

low glucose concentrations the CLIO-glucose nanoparticles (red) bind to

concavalin-A (green) and form clusters. (b) In the presence of high

glucose (brown) concentrations, CLIO-glucose is displaced from

concavalin-A causing the disassembly of the nanoswitch. (c,d) Size

distribution of nanoparticles by light scattering. At baseline (red) the

majority of nanoparticles are between 20–40 nm in size, increase to 200–

260 nm after the addition of concavalin-A (brown) and then return to their

baseline size distribution (yellow) after the addition of glucose to the

solution. (e,f) Changes in glucose concentration can be sensed by

detecting changes in the transverse relaxation time (T2), induced by

assembly or disassembly of the nanoswitch. (Figure adapted from [34��]

with permission.)

Figure 3. The ability of this construct to accurately

measure glucose concentration across a semipermeable

membrane suggests it could find use as an implantable

biosensor in the future [34��]. It should also be possible

for a library of biosensors to be included within the

semipermeable membrane, allowing a panel of metabo-

lites to be sensed continuously, simultaneously and non-

invasively by MRI in vivo [36�].

In vivo MRI of enzyme activityNumerous situations exist where an enzyme of interest is

expressed only locally or within a solid tissue and, in

such cases, systemic detection within a semipermeable


membrane is inadequate. The use of magnetic relaxation

switches to image local enzyme activity, however, is com-

plicated by aggregation of the nanoparticles into lysosomes

when they are not protected by a semipermeable mem-

brane [37]. The concept of enzyme-mediated aggregation,

however, has recently been extended to novel gadolinium

conjugates, which are significantly less sensitive to non-

specific local aggregation [38,39��].

Myeloperoxidase is an enzyme involved in free radical

generation and is thought to have a central role in ather-

osclerotic plaque instability. The peptide serotonin,

when oxidized by this enzyme, tends to aggregate into

dimers and oligomers. A gadolinium-serotonin chelate,

capable of sensing myeloperoxidase activity in vivo, has

thus been developed [38]. Once the serotonin moiety in

the chelate is activated by myeloperoxidase, the probe



Figure 4

In vivo MRI of myeloperoxidase activity in mice. (a) Only a transient

increase in signal intensity is seen when the conventional gadolinium

chelate, gadolinium-diethylenetriaminepentaacetic acid (Gd-DPTA), is

given to mice with myositis. (b) A sustained increase in signal intensity is

seen when the gadolinium-serotonin chelate is used. (c) Human

myeloperoxidase embedded into a matrix gel and implanted into the

right flank of a mouse was also able to activate the gadolinium-serotonin

probe. Implanted matrix gel without myeloperoxidase (left flank) did not

activate the probe. (Figure adapted from [39��] with permission.)

assembles into dimers and quatromers that tend to be

retained locally and which have a significantly higher

longitudinal relaxivity than the parent compound

[38,39��]. The presence of myeloperoxidase in a mouse

model of myositis has been successfully imaged in vivowith this probe [39��], as shown in Figure 4.

Multispectral and off-resonance MRIOne of the traditional limitations of molecular imaging by

MRI has been its ability to provide only a single compo-

site readout of proton behavior. This is in contrast to

single-photon emission computed tomography (SPECT)

and fluorescence techniques, which in addition to being

highly sensitive are also multispectral and thus capable of

providing several simultaneous readouts of individual

probes. One potential solution to this limitation is to

combine proton MRI with, for instance, fluorine MRI

[40�]. Because the Larmor (or precession) frequencies of

protons and fluorine are reasonably close, the imaging of

both signatures can usually be performed on the same

scanner. An alternative approach, however, is to exploit

the magnetization transfer between two pools of protons


with an offset in their Larmor frequencies, a concept

which forms the basis of CEST, PARACEST and LIPO-

CEST imaging [41,42�,43��].

The protons in a targeted liposome, for instance, contain-

ing a particular paramagnetic lanthanide will have a

resonance frequency that is offset from the surrounding

bulk protons [42�]. In the presence of magnetization

transfer across the liposome, selective excitation at this

offset frequency results in a detectable change in the bulk

water signal. If two or more probes, directed against

distinct molecular targets, have sufficiently different off-

set frequencies this can allow multispectral proton MRI

of more than one target to be performed by sequential

selective excitation [44��].

The concept of selectively imaging protons shifted off the

central resonance frequency has now also been extended

to the imaging of iron oxide probes. A variety of off-

resonance techniques have been developed, all of which

result in positive contrast in the vicinity of the iron oxide

[45��,46�]. In vivo experience with these off-resonance

and CEST-based techniques, however, is quite limited,

although an initial CEST study in humans with brain

tumors has recently been reported [47]. While highly

promising, further experience will be needed with these

techniques to fully judge their true potential.

ConclusionsConventional MR contrast agents currently rely on

changes in probe biodistribution or pharmacokinetics in

disease states. Future MR contrast agents, however, will

probe anatomy at the cellular and subcellular level and

will either be directed against specific targets or activated

by specific enzymes. In addition, the same probes could

form the basis of MR-detectable biosensors. Thus, mole-

cular MRI is likely to continue to grow in importance in

the future and will strongly complement the central role

of MRI in conventional anatomical imaging.

References and recommended readingPapers of particular interest, published within the annual period ofreview, have been highlighted as:

� of special interest�� of outstanding interest

1.��

Weissleder R: Molecular imaging in cancer. Science 2006,312:1168-1171.

This is an excellent review of molecular imaging in cancer but the issuesdiscussed (early diagnosis of disease, drug development, personalizedmanagement of disease and the challenges involved in obtaining clinicalapproval for a novel agent) apply broadly.

2. Weissleder R: Molecular imaging: exploring the next frontier.Radiology 1999, 212:609-614.

3. Sosnovik D, Weissleder R: Magnetic resonance andfluorescence based molecular imaging technologies. ProgDrug Res 2005, 62:83-115.

4. Lanza GM, Abendschein DR, Yu X, Winter PM, Karukstis KK,Scott MJ, Fuhrhop RW, Scherrer DE, Wickline SA:Molecular imaging and targeted drug delivery with a novel,



ligand-directed paramagnetic nanoparticle technology.Acad Radiol 2002, 9(Suppl 2):S330-S331.

5. Winter PM, Caruthers SD, Yu X, Song SK, Chen J, Miller B,Bulte JW, Robertson JD, Gaffney PJ, Wickline SA et al.: Improvedmolecular imaging contrast agent for detection of humanthrombus. Magn Reson Med 2003, 50:411-416.

6. Frias JC, Williams KJ, Fisher EA, Fayad ZA: RecombinantHDL-like nanoparticles: a specific contrast agent forMRI of atherosclerotic plaques. J Am Chem Soc 2004,126:16316-16317.

7. Lipinski MJ, Amirbekian V, Frias JC, Aguinaldo JG, Mani V,Briley-Saebo KC, Fuster V, Fallon JT, Fisher EA, Fayad ZA:MRI to detect atherosclerosis with gadolinium-containingimmunomicelles targeting the macrophage scavengerreceptor. Magn Reson Med 2006, 56:601-610.

8. Shen T, Weissleder R, Papisov M, Bogdanov A Jr, Brady TJ:Monocrystalline iron oxide nanocompounds (MION):physicochemical properties. Magn Reson Med 1993,29:599-604.

9. Wunderbaldinger P, Josephson L, Weissleder R: Crosslinked ironoxides (CLIO): a new platform for the development of targetedMR contrast agents. Acad Radiol 2002, 9(Suppl 2):S304-S306.

10. Harisinghani MG, Jhaveri KS, Weissleder R, Schima W, Saini S,Hahn PF, Mueller PR: MRI contrast agents for evaluating focalhepatic lesions. Clin Radiol 2001, 56:714-725.

11. Harisinghani MG, Barentsz J, Hahn PF, Deserno WM,Tabatabaei S, van de Kaa CH, de la Rosette J, Weissleder R:Noninvasive detection of clinically occult lymph-nodemetastases in prostate cancer. N Engl J Med 2003,348:2491-2499.

12. Dunn EA, Weaver LC, Dekaban GA, Foster PJ: Cellularimaging of inflammation after experimental spinal cord injury.Mol Imaging 2005, 4:53-62.

13.�

Turvey SE, Swart E, Denis MC, Mahmood U, Benoist C,Weissleder R, Mathis D: Noninvasive imaging of pancreaticinflammation and its reversal in type 1 diabetes.J Clin Invest 2005, 115:2454-2461.

Very nice demonstration of the use of a long circulating magnetofluor-escent iron oxide probe to diagnose and monitor tissue inflammation.

14. Kraitchman DL, Heldman AW, Atalar E, Amado LC, Martin BJ,Pittenger MF, Hare JM, Bulte JW: In vivo magnetic resonanceimaging of mesenchymal stem cells in myocardial infarction.Circulation 2003, 107:2290-2293.

15. Lewin M, Carlesso N, Tung CH, Tang XW, Cory D, Scadden DT,Weissleder R: Tat peptide-derivatized magnetic nanoparticlesallow in vivo tracking and recovery of progenitor cells.Nat Biotechnol 2000, 18:410-414.

16. Hill JM, Dick AJ, Raman VK, Thompson RB, Yu ZX, Hinds KA,Pessanha BS, Guttman MA, Varney TR, Martin BJ et al.: Serialcardiac magnetic resonance imaging of injectedmesenchymal stem cells. Circulation 2003, 108:1009-1014.

17. Botnar RM, Perez AS, Witte S, Wiethoff AJ, Laredo J, Hamilton J,Quist W, Parsons EC Jr, Vaidya A, Kolodziej A et al.: In vivomolecular imaging of acute and subacute thrombosis using afibrin-binding magnetic resonance imaging contrast agent.Circulation 2004, 109:2023-2029.

18. Botnar RM, Buecker A, Wiethoff AJ, Parsons EC Jr, Katoh M,Katsimaglis G, Weisskoff RM, Lauffer RB, Graham PB,Gunther RW et al.: In vivo magnetic resonance imaging ofcoronary thrombosis using a fibrin-binding molecularmagnetic resonance contrast agent. Circulation 2004,110:1463-1466.

19. Winter PM, Morawski AM, Caruthers SD, Fuhrhop RW, Zhang H,Williams TA, Allen JS, Lacy EK, Robertson JD, Lanza GM et al.:Molecular imaging of angiogenesis in early-stageatherosclerosis with alpha(v)beta3-integrin-targetednanoparticles. Circulation 2003, 108:2270-2274.

20.��

Sosnovik DE, Schellenberger EA, Nahrendorf M, Novikov MS,Matsui T, Dai G, Reynolds F, Grazette L, Rosenzweig A,Weissleder R et al.: Magnetic resonance imaging of


cardiomyocyte apoptosis with a novel magneto-opticalnanoparticle. Magn Reson Med 2005, 54:718-724.

The testing and application of a MR-detectable apoptosis-sensing probeis described. Probe validation is performed in cell culture, in tissuephantoms and in a mouse model in vivo. The authors demonstrate thattargeted iron-oxide probes are able to penetrate the capillary endothe-lium and bind to a specific target within a tissue of interest. The ability toimage a targeted MR contrast agent in the myocardium in vivo with MRI isshown, and the value of a dual modality (MRI and fluorescence) imagingapproach is demonstrated.

21.��

Nahrendorf M, Jaffer FA, Kelly KA, Sosnovik DE, Aikawa E,Libby P, Weissleder R: Noninvasive vascular cell adhesionmolecule-1 imaging identifies inflammatory activation of cellsin atherosclerosis. Circulation 2006, 114:1504-1511.

Comprehensive molecular imaging paper ranging from the use of phagedisplay to identify a targeting ligand to the demonstration of a treatmenteffect by in vivo molecular imaging. The authors describe the synthesisand testing of a VCAM-1 sensing probe and its ability to image athero-sclerotic plaques in apoE knockout mice in vivo. The ability of the probe todetect early atheroma in the mouse in vivo, detect human plaques ex vivoand monitor a treatment effect in the mouse in vivo is shown.

22. Wunderbaldinger P, Josephson L, Weissleder R: Tat peptidedirects enhanced clearance and hepatic permeability ofmagnetic nanoparticles. Bioconjug Chem 2002, 13:264-268.

23. Kircher MF, Mahmood U, King RS, Weissleder R, Josephson L: Amultimodal nanoparticle for preoperative magnetic resonanceimaging and intraoperative optical brain tumor delineation.Cancer Res 2003, 63:8122-8125.

24. Schellenberger EA, Sosnovik D, Weissleder R, Josephson L:Magneto/optical annexin V, a multimodal protein. BioconjugChem 2004, 15:1062-1067.

25.�

Kelly KA, Nahrendorf M, Yu AM, Reynolds F, Weissleder R: In vivophage display selection yields atherosclerotic plaque targetedpeptides for imaging. Mol Imaging Biol 2006, 8:201-207.

Useful article on the use of phage display to detect ligands specific to atarget or cell of interest.

26.��

Winter PM, Neubauer AM, Caruthers SD, Harris TD, Robertson JD,Williams TA, Schmieder AH, Hu G, Allen JS, Lacy EK et al.:Endothelial alpha(v)beta3 integrin-targeted fumagillinnanoparticles inhibit angiogenesis in atherosclerosis.Arterioscler Thromb Vasc Biol 2006, 26:2103-2109.

Important article demonstrating the principle of a dual diagnostic andtherapeutic agent. A liposome targeted to the alphaV/Beta 3 integrin isused to image and treat pathological angiogenesis in a rabbit model ofaortic atheroma. The liposome contains both gadolinium for MR imagingand the anti-angiogenic agent fumagillin, which is released locally afterligand binding.

27. Moffat BA, Reddy GR, McConville P, Hall DE, Chenevert TL,Kopelman RR, Philbert M, Weissleder R, Rehemtulla A, Ross BD:A novel polyacrylamide magnetic nanoparticle contrastagent for molecular imaging using MRI. Mol Imaging 2003,2:324-332.

28.��

Weissleder R, Kelly K, Sun EY, Shtatland T, Josephson L: Cell-specific targeting of nanoparticles by multivalent attachmentof small molecules. Nat Biotechnol 2005, 23:1418-1423.

A library of nanoparticles is generated in this study by modifying thesurface of a nanoparticle with small molecules. The different compoundsin the library show markedly different affinities for resting and activatedmacrophages. The generation of these sorts of libraries is likely tobecome both more important and more common in the preclinical setting.

29. Sun EY, Josephson L, Weissleder R: ‘Clickable’ nanoparticlesfor targeted imaging. Mol Imaging 2006, 5:122-128.

30. Perez JM, Josephson L, O’Loughlin T, Hogemann D, Weissleder R:Magnetic relaxation switches capable of sensing molecularinteractions. Nat Biotechnol 2002, 20:816-820.

31. Perez JM, O’Loughin T, Simeone FJ, Weissleder R, Josephson L:DNA-based magnetic nanoparticle assembly acts as amagnetic relaxation nanoswitch allowing screening ofDNA-cleaving agents. J Am Chem Soc 2002, 124:2856-2857.

32. Perez JM, Simeone FJ, Saeki Y, Josephson L, Weissleder R:Viral-induced self-assembly of magnetic nanoparticles allowsthe detection of viral particles in biological media.J Am Chem Soc 2003, 125:10192-10193.



33. Tsourkas A, Hofstetter O, Hofstetter H, Weissleder R,Josephson L: Magnetic relaxation switch immunosensorsdetect enantiomeric impurities. Angew Chem Int Ed Engl 2004,43:2395-2399.

34.��

Sun EY, Weissleder R, Josephson L: Continuous analyte sensingwith magnetic nanoswitches. Small 2006, 2:1144-1177.

A highly visionary paper describing the use of magnetic nanoswitches tosense analyte concentrations across a semi-permeable membrane. Inparticular, the ability of this technology to accurately measure glucoseconcentrations is studied.

35. Atanasijevic T, Shusteff M, Fam P, Jasanoff A: Calcium-sensitiveMRI contrast agents based on superparamagnetic iron oxidenanoparticles and calmodulin. Proc Natl Acad Sci USA 2006,103:14707-14712.

36.�

Sun EY, Josephson L, Kelly KA, Weissleder R: Development ofnanoparticle libraries for biosensing. Bioconjug Chem 2006,17:109-113.

An extension of the prior work from this group. Surface modification ofiron oxide nanoparticles is used to generate a library of nanoparticles foranalyte sensing.

37. Hogemann-Savellano D, Bos E, Blondet C, Sato F, Abe T,Josephson L, Weissleder R, Gaudet J, Sgroi D, Peters PJ et al.:The transferrin receptor: a potential molecular imagingmarker for human cancer. Neoplasia 2003, 5:495-506.

38. Chen JW, Pham W, Weissleder R, Bogdanov A Jr: Humanmyeloperoxidase: a potential target for molecular MR imagingin atherosclerosis. Magn Reson Med 2004, 52:1021-1028.

39.��

Chen JW, Querol Sans M, Bogdanov A Jr, Weissleder R: Imagingof myeloperoxidase in mice by using novel amplifiableparamagnetic substrates. Radiology 2006, 240:473-481.

Excellent paper showing the ability to image enzyme activity in vivo withan activatable MRI probe. The activity of the enzyme myeloperoxidase issuccessfully imaged with a gadolinium-serotonin chelate. Exposure of thechelate to the enzyme causes it to undergo polymerization and increaseits longitudinal relaxivity.

40.�

Caruthers SD, Neubauer AM, Hockett FD, Lamerichs R,Winter PM, Scott MJ, Gaffney PJ, Wickline SA, Lanza GM:In vitro demonstration using 19F magnetic resonance to


augment molecular imaging with paramagneticperfluorocarbon nanoparticles at 1.5 Tesla. Invest Radiol 2006,41:305-312.

Paper dealing with the performance of both proton and fluorine MRI.Liposomes containing gadolinium and a perfluorocarbon are shown to bevisible at either frequency.

41. Zhou J, Payen JF, Wilson DA, Traystman RJ, van Zijl PC: Using theamide proton signals of intracellular proteins and peptides todetect pH effects in MRI. Nat Med 2003, 9:1085-1090.

42.�

Aime S, Delli Castelli D, Terreno E: Highly sensitive MRI chemicalexchange saturation transfer agents using liposomes.Angew Chem Int Ed Engl 2005, 44:5513-5515.

The concept of LIPOCEST is discussed in this paper.

43.��

Woods M, Woessner DE, Sherry AD: Paramagnetic lanthanidecomplexes as PARACEST agents for medical imaging.Chem Soc Rev 2006, 35:500-511.

Very good review article on PARACEST imaging.

44.��

Aime S, Carrera C, Delli Castelli D, Geninatti Crich S, Terreno E:Tunable imaging of cells labeled with MRI-PARACEST agents.Angew Chem Int Ed Engl 2005, 44:1813-1815.

The concept of multispectral or tunable proton MRI is demonstrated inthis paper. Rat hepatoma cells loaded with either Europium or Terbiumcan be separately and selectively visualized.

45.��

Cunningham CH, Arai T, Yang PC, McConnell MV, Pauly JM,Conolly SM: Positive contrast magnetic resonance imaging ofcells labeled with magnetic nanoparticles. Magn Reson Med2005, 53:999-1005.

Off resonance imaging of superparamagnetic iron oxides is described inthis paper.

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Mani V, Briley-Saebo KC, Itskovich VV, Samber DD, Fayad ZA:Gradient echo acquisition for superparamagnetic particleswith positive contrast (GRASP): sequence characterization inmembrane and glass superparamagnetic iron oxide phantomsat 1.5T and 3T. Magn Reson Med 2006, 55:126-135.

An alternative technique for off resonance imaging is described.

47. Jones CK, Schlosser MJ, van Zijl PC, Pomper MG, Golay X,Zhou J: Amide proton transfer imaging of human brain tumorsat 3T. Magn Reson Med 2006, 56:585-592.


emerging concepts in molecular mri david e sosnovik and ralph...

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