TECHNOLOGIES

DRUG DISCOVERY

TODAY

Drug Discovery Today: Technologies Vol. 8, No. 2–4 2011

Editors-in-Chief

Kelvin Lam – Blue Sky Biotech Inc., USA

Henk Timmerman – Vrije Universiteit, The Netherlands

Imaging techniques

Contrast agents and mechanismsWalter Dastru, Dario Longo, Aime Silvio*Universita degli Studi di Torino, Dipartimento di Chimica Inorganica, Fisica e dei Materiali, Centro di Imaging Molecolare, via Nizza, 52-10126 Torino, Italy

MRI contrast agents are routinely used in clinical set-

tings. Important advances in their design have been

attained in the past few years to overcome sensitivity

issues and to make possible molecular imaging applica-

tions by means of this modality. Besides the sensitivity

enhancement of paramagnetic relaxation probes, out-

standing results have been obtained in the develop-

ment of novel classes of frequency-encoding agents

such as chemical exchange saturation transfer and

hyperpolarized 13C-enriched molecules.

*Corresponding author.: S. Aime (silvio.aime@unito.it)

1740-6749/$ � 2011 Elsevier Ltd. All rights reserved. DOI: 10.1016/j.ddtec.2011.11.013

Guest Editors:Nicolau Beckmann – Novartis Institutes for BioMedicalResearch, Basel, Switzerland.Bert Windhorst – VU Medical Center, Amsterdam,The Netherlands.

tion transfer (CEST) agents, and 19F-containing molecules or

Introduction

Currently, at least one-third of the MRI scans acquired at

clinical settings make use of contrast agents (CAs). Their use

led to remarkable improvements in medical diagnosis in

terms of higher specificity, better tissue characterization,

reduction of imaging artifacts and improved functional infor-

mation. The agents that have entered the clinical practice are

chemicals containing paramagnetic metal ions that markedly

affect the relaxation rates of tissues water protons in the

regions where they distribute. They belong to the class of

paramagnetic metal complexes (mainly Gd3+ chelates) [1]

and to the class of iron oxide particles [2]. The former systems

act predominantly on T1 yielding hyperintensity in the cor-

responding T1-weighted images and are often called T1 or

positive agents. The latter class deals with systems which

mainly cause a shortening of T2 or T2* that result in a

darkening effect on the corresponding T2 or T2* weighted

images and are therefore called T2 or negative agents.

In the past decade, the research activities in the field of MRI

CAs have undertaken several new directions. On one hand

much work has been done in chemical laboratories to

improve the characteristics of T1 and T2 agents, also at the

light of affecting their biodistribution (targeted and nano-

sized systems) and, on the other hand, new achievements

based on alternative modalities to alter the contrast in the 1H-

MR images, such as the case of the chemical exchange satura-

hyperpolarized probes, respectively. Therefore the current

picture of the possibilities of enhancing the contrast in MR

images is now very rich (Box 1) and well suitable to make MRI

more competitive with other imaging modalities in the emer-

ging field of molecular imaging [3].

T1 agents

The efficiency of a paramagnetic metal complex to act as T1

agent is first of all represented by its relaxivity (r1) that, for

commercial CAs such as Magnevist1, Dotarem1, ProHance1

and Omniscan1 is around 3.4–3.5 mM�1 s�1 (at 20 MHz and

398C). r1 is the relaxation enhancement of water protons

brought about by the presence of the paramagnetic agent

at 1 mM concentration [4]. Besides acting as catalyst for the

relaxation of water protons, a paramagnetic MRI CA has to

possess several additional properties to guarantee the safety

issues required for in vivo applications at the administered

doses, namely high thermodynamic (and possibly kinetic)

stability, good solubility and low osmolarity.

Chemists have tackled the task of attaining higher relax-

ivities first by designing highly stable complexes character-

ized by an higher hydration of the paramagnetic center

e109

Drug Discovery Today: Technologies | Imaging techniques Vol. 8, No. 2–4 2011

Box 1.

19F Containing

Probes

MRI

Contrast

Agents

CEST

Agents

Hyperpolarized

Probes

T1

Iron Oxide

Particles

T1

Paramagnetic

Metal Complexes

Drug Discovery Today: Technologies

(in principle as r1 scales up with hydration, increasing the

number of water molecules in the inner coordination sphere

of the paramagnetic metal ion from 1 to 2 means doubling

the relaxivity).

In this context remarkable is the work done at Raymond’s

lab [5] in the field of the development of hydroxypyridone-

based ligands (HOPO) designed to fit at best the coordination

preferences of Gd3+ especially as far as it concerns its oxophi-

licity (Fig. 1). The HOPO ligands provide a hexadentate coor-

dination for Gd3+, in which all the donor atoms are oxygens.

Because Gd3+ favors eight or nine coordination geometry, the

structural design offered by HOPO complexes provides two to

three open sites for inner-sphere water molecules coordina-

tion. These complexes also show a very high thermodynamic

stability. The relaxivity improvement provided by Gd(III)

complexes of HOPO-based ligands is two to three times higher

than that of commercial CAs. Other interesting solutions have

been sought in the family of heptadentate ligands that give rise

to Gd-complexes containing two coordinated water mole-

cules. In this context much attention has been given to AAZTA

(6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid)

and derived ligands for their easy synthesis (Fig. 1) [6].

Gd-containing targeting probes

Human serum albumin (HSA) has been the first deeply inves-

tigated target for Gd-complexes with the aim of developing

blood pool agents. Several systems have been investigated over

the past two decades for the set-up of supramolecular adducts

inwhichsuitably functionalizedGd-complexes reversiblybind

to albumin [7]. In addition to a higher vascular retention time,

e110 www.drugdiscoverytoday.com

these macromolecular adducts are often characterized by a

longer relaxivity (at 0.5–1.5 T) thanks to the elongation of the

molecular reorientational time which is a major determinant

of the relaxivity at these fields.

MultiHance1 is a representative example of complexes

that weakly bind to HSA [8] (with a consequent smaller

increase of the observed relaxivity in blood) whereas Vaso-

vist1 represents a system [9] with a high binding affinity with

the serum protein (at the recommended administration

doses, the amount of the bound form approaches 80%).

Much has been done (and is being currently done) in the

field of Gd-complexes designed to target specific epitopes on

the outer cell membrane. Because the MRI visualization of a

cell requires 107–109 Gd-containing units [10], this task has

been often addressed by using nanosized carriers able to

deliver a high payload of CAs. The investigated nanocarriers

include apoferritin [11], low-density lipoproteins (LDL) [12],

viral capsides [13,14] and yeast [15] as naturally occurring

systems as well as several ‘artificial’ supramolecular systems

such as micelles, liposomes, solid lipid nanoparticles, and

dendrimers, among others [16].

The use of the latter systems has opened the new direction

of MRI-guided drug delivery that is currently under intense

scrutiny. Alternatively, with the use of nano-/micro-sized

carriers, the threshold for the MRI visualization may be

reached by exploiting cellular internalization through high

capacity transporters [11]. Good results have been obtained

with the use of suitably functionalized Gd-complexes that

target the organic anion transport protein (Fig. 2) or amino

acids [17] and folate transporters [18,19], respectively.

Vol. 8, No. 2–4 2011 Drug Discovery Today: Technologies | Imaging techniques

H3C CH3

CH3

CH3

N

N

N

N

N

N

N

HOPO - based ligands

HN

O

O O

O

O

O

O

O

OAAZTA

HO

HO

HO

ONH NH

OH OH

OH

OH

Drug Discovery Today: Technologies

Figure 1. Chemical structures of the HOPO-based (top) and

AAZTA (bottom) ligands.

Gd-based responsive agents

The in-depth understanding of the structural and dynamic

determinants of the observed relaxivities has prompted che-

mists to design systems able to modify their ability to

enhance water proton relaxation rates in response to changes

of physicochemical parameters of the microenvironment in

which they distribute [20]. Typical parameters of primary

diagnostic relevance include pH, temperature, pO2, enzy-

matic activity, redox potential. Unfortunately, the in vivo

translation is still uncertain because the transformation of

T1 into r1 maps (the parameter that owns the responsiveness

toward a given physicochemical or biological parameter)

requires to know the actual concentration of the paramag-

netic agent that it is not easily accessible. It has been shown

that, in some instances, this issue can be circumvented by

setting up a proper ratiometric method based on the measure

of T1 and T2 provided that the two relaxation times have

different dependence from the change in the parameters of

interest [21].

Mn(II)-based complexes

Paramagnetic chelates of Mn(II) (five unpaired electrons)

have also been considered. The main drawback appears to

be related to the stability of these complexes. Mn(II) is

an essential metal; therefore, the evolution has selected

biological structures for sequestering Mn(II) ions with high

efficiency. Combined with the fact that Mn(II) forms highly

labile coordination complexes, it has been difficult to design

Mn(II) chelates that maintain their integrity when adminis-

tered to living organisms.

The only approved Mn(II) agent is Mangafodipir (manga-

nese-dipyridoxal diphosphate, Mn-DPDP or Teslascan1) [7].

It is a CA proposed for liver-associated diseases. Mn-DPDP is a

manganese chelate derived from vitamin B6 (pyridoxal 5-

phosphate) and is specifically taken up by the hepatocytes.

In particular, the agent is taken up by normal hepatocytes

resulting in increased signal on T1-weighted images, and is

excreted in the biliary system. It is the only agent that releases

metal ions to endogenous macromolecules. The huge proton

relaxation enhancement brought about by the resulting

Mn(II) protein adducts is responsible for the MRI visualiza-

tion of hepatocytes also at low administered doses of Mn-

DPDP.

T2-susceptibility agents

The term susceptibility refers to the tendency of a certain

substance to become magnetized. The intensity of magneti-

zation (M) is related to the strength of the inducing magnetic

field (B) through a constant of proportionality (k) known as

the magnetic susceptibility (M = kB). The magnetic suscept-

ibility is a unitless constant that is determined by the physical

properties of the magnetic material. It can take on either

positive or negative values. Positive values imply that the

induced magnetic field is in the same direction as the indu-

cing field. Negative values imply that the induced magnetic

field is in the opposite direction as the inducing field. Thus T2

susceptibility agents work upon distorting the applied mag-

netic field leading to a loss of signal in T2 weighted images.

Iron oxide particles

Superparamagnetic iron oxide particles (SPIO) have the gen-

eral formula Fe(III)2O3Fe(II)O [2]. The change in the magnetic

susceptibility caused by the superparamagnetic core induces

a large distortion of the externally applied magnetic field

which, in turn, leads to hypo-intensities in T2 weighted

images. Thus the areas containing particles display fast trans-

verse relaxation rates and low signal intensity (negative

contrast). Because of the large magnetic susceptibility of an

iron oxide particle, the signal void is much larger than the

particle size, enhancing detectability at the expense of reso-

lution. Iron oxide particles for MRI applications are divided

into two classes, namely SPIO and USPIO (ultrasmall super-

paramagnetic iron oxide), on the basis of the overall size of

the protective cover on the surface of the magnetic particle.

In fact, in both types of particles the iron oxide colloidal

particles are encapsulated by organic material such as dextran

and carboxydextran to improve their compatibility with the

biological systems. The difference in the overall size of the

particles cause marked changes in their use as MRI CAs.

www.drugdiscoverytoday.com e111

Drug Discovery Today: Technologies | Imaging techniques Vol. 8, No. 2–4 2011

pre-contrast post-contrast

1.5x10-8

1.0x10-8

O

O

OH

NN

NH

N

O

O

O-

O

O

O

O-

O-O-

O- O-Gd3+

5.0x10-9

0.0

0 1 2 3 4 5 6

[Gd-L] mM

Gd(

III)

mol

es /

mg

prot

.

2.0x10-8

2.5x10-8

3.0x10-8

HTC

HEPATOCYTES

37 ºC, 6h

Drug Discovery Today: Technologies

Figure 2. The lipophilic Gd(III) complex enters hepatocytes through organic anion transport protein (OATP). Therefore the liver parenchyma appears

hyperintense and the occurrence of colon cancer metastasis is well detectable as a black spot (arrow) in the corresponding post-contrast MR image

(courtesy of Bracco Imaging). HTC, hepatoma tissue culture cells.

USPIO have longer half-life in the blood vessels than SPIO

because their smaller size (<50 nm) makes their uptake from

macrophages more difficult. On the contrary, SPIO (with

diameters of the order of 100–200 nm) are very rapidly

removed from the circulation by the reticuloendothelial

system. In terms of relaxation enhancement properties, the

larger magnetic susceptibility of SPIO yields to larger R2/R1

relaxation rates and higher T2-shortening effects than those

brought about by the smaller USPIO particles. These particles

have been used to track different cell types ‘in vivo’, including

T lymphocytes, macrophages and stem cells [22–24].

CEST agents

The new landscape of molecular imaging applications

prompts the search for new paradigms in the design of

MR-imaging reporters. A possibility relies on the exploita-

tion of the frequency, the key parameter of the NMR phe-

nomenon. The roots for this class of agents go back to the

well established magnetization transfer (MT) procedure,

which makes use of the transfer of saturated magnetization

from tissue mobile protons (primarily from proteins) to bulk

water upon radiofrequency (RF) irradiation of their semi-

solid-like, broad NMR absorption. The use of exogenous,

e112 www.drugdiscoverytoday.com

mobile molecules, dubbed CEST agents, introduces the

possibility of a selective RF irradiation of the sharper

NMR signal of the exchangeable protons of the probe

[25,26]. Thus, one may design protocols in which the con-

trast in the MR image is generated ‘at will’ only if the

appropriate frequency corresponding to the labile protons

of the exogenous agent is irradiated. Importantly, the new

approach offers the possibility of detecting more than one

agent in the same region.

The CEST contrast arises from the decrease in the intensity

of the bulk water signal following the saturation of the

exchanging protons of the CEST agent by means of a selective

RF pulse (Fig. 3).

Hence, the basic requisite for a CEST agent is that the

exchange rate between its mobile protons and the bulk water

protons (kCEST) has to be smaller than the frequency differ-

ence between the absorption frequencies of the exchanging

spins (i.e. kCEST < Dv).

In the CEST experiment, the RF pulse of angular frequency

v2 = g � B2, where B2 is the intensity of the applied RF pulse, is

applied at the Dv frequency offset corresponding to the

resonance of the mobile protons of the CA, to saturate their

longitudinal magnetization (MzCEST).

Vol. 8, No. 2–4 2011 Drug Discovery Today: Technologies | Imaging techniques

H H

H

ωCEST

rf irradiation

at ωCEST

ωbulk water

If Δω ≥ kex

H

H

HCESTagent

O

kex

rf

1.0

0.8

0.6

0.4

0.2

0.00

10 8 6 4 2

ppm

irradiation time (s)

no irradiation Irradiation at ωCEST

0 -2 -4 10 8 6 4 2

ppm0 -2 -4

1 2 3 4 5

Drug Discovery Today: Technologies

Figure 3. Schematic representation of the origin of the CEST contrast in a MR image. Upon irradiating the NMR resonance of a pool of exchangeable

protons, saturated magnetization is transferred to the ‘bulk’ water signal yielding a hypointensity in the corresponding MR image.

One of the main limiting factors of CEST agents for in vivo

application is represented by their lower sensitivity compared

with the conventional Gd and Fe based agents. Among the

different parameters governing the CEST effect, the exchange

rate of the mobile protons of the agent, kCEST, has received so

far much attention, because this parameter can be easily

modulated by changing the chemical characteristics of the

exchanging group. In addition, kCEST is usually dependent on

physicochemical variables like temperature or pH, thus

allowing CEST agents to be used as responsive reporters.

CEST agents are usually classified into two main groups:

diamagnetic and paramagnetic systems [27]. The members of

each class can be further divided into subgroups depending

on other criteria such as their size or particular chemical

characteristics. All the PARACEST (PARAmagnetic chemical

exchange saturation transfer) agents proposed so far are

lanthanide(III)-complexes endowed with two kinds of mobile

protons: (i) protons of water molecules coordinated to the

Ln(III) ion and (ii) mobile protons belonging to the ligand

structure.

The unique feature of CEST agents, that is, the possibility to

visualize more than one agent in the same image, is extremely

advantageous for designing MRI cell-tracking experiments in

which two (or more) cell lines, each one labeled with a

specific CEST probe, are simultaneously injected and visua-

lized over time by MRI. Analogously, it is possible to visualize

two CEST agents in vivo (Fig. 4) [28].

Hyperpolarized molecules

Both MRI and in vivo magnetic resonance spectroscopy (MRS)

suffer from low sensitivity, which is intrinsic in the NMR

experiment. For this reason, the applications of MRI have

been essentially restricted to 1H (100% natural abundance,

high g value). Low g heteronuclei have been used only for

MRS [29,30], but high concentrations and very long acquisi-

tion times are in this case necessary.

Because the NMR signal intensity is proportional to polar-

ization, much attention has been devoted to procedures that

lead to a modification of the spin population distribution to

enhance the response. The non-equilibrium state obtained is

called hyperpolarized state and can be exploited to directly

detect the heteronuclei with fast acquisition of MR images

and spectra which are characterized by high signal to noise

ratios and are free from any background signal [3,31–34].

Hyperpolarized molecules are themselves the source of the

NMR signal, and signal intensity linearly depends upon their

concentration and polarization level. For this reason, a fun-

damental requisite for a hyperpolarized molecule to be used

www.drugdiscoverytoday.com e113

Drug Discovery Today: Technologies | Imaging techniques Vol. 8, No. 2–4 2011

@-17 ppm

@ 7 ppm

Drug Discovery Today: Technologies

Figure 4. In vivo visualization of two CEST agents characterized by

absorptions of the exchangeable pools of protons at 7 (green) and

�17 (red) ppm, respectively. The saturation transfer maps are

superimposed over the anatomic image.

as MRI/MRS CA is the long relaxation time of the hetero-

nucleus of interest. In fact, there must be a time long enough

to acquire the image/spectrum before the relaxation pro-

cesses re-establish the equilibrium spin populations, thus

causing loss of signal.

Four different methods have been proposed to produce

hyperpolarized molecules: (i) the brute force method (which

consists of subjecting the sample to a very strong magnetic

field at a temperature close to absolute zero), (ii) optical

pumping and spin exchange, (iii) para-hydrogen induced

polarization and (iv) dynamic nuclear polarization (DNP).

In spite of its technological requirements and expensive-

ness, DNP is the most versatile method, which allows, in

principle, to polarize every nucleus in every molecule. It

consists of the polarization transfer from electrons to nuclei

in solids by irradiation with a proper microwave (mw) fre-

quency, at or near to the electron resonance frequency

[35,36]. In practice, the material to be polarized is dissolved

in a glass-forming solvent, doped with a stable radical species

and placed into the magnetic field. The solution is brought to

very low temperature (1–2 K, liquid helium) to generate high

electron polarization, and irradiated to transfer it to nuclei.

After the polarization transfer has taken place, the mw is

switched off, the sample is raised above the liquid helium

e114 www.drugdiscoverytoday.com

level and is rapidly warmed (usually by dissolution in hot

water) still inside the magnetic field. It is then quickly trans-

ferred for the MRI acquisition.

Several 13C-labeled substrates, hyperpolarized by the DNP

method, have been tested for metabolic applications. Most

attention has been devoted to 13C-labeled pyruvate. Pyruvate

is a key-molecule in major metabolic and catabolic pathways

in the mammalian cells, as it is converted to alanine, lactate

or carbonate by alanine transaminase, lactate dehydrogenase

(LDH) and pyruvate dehydrogenase, respectively, to a differ-

ent extent depending on the status of the cells.

Several papers about metabolic imaging by 13C-enriched

hyperpolarized pyruvate have appeared in the literature since

2006. The most appealing application is its use for tumor

diagnosis, stadiation and monitoring of response to treat-

ment, by exploiting the increased lactate production through

LDH in tumor tissues. Murine lymphoma, P22 rat sarcoma,

adenocarcinoma of mice prostate, glioblastoma and breast

cancer in mice have been studied by this method. Further-

more, the reduction of the conversion of pyruvate to lactate

in tumors has been observed within 24 hours of chemother-

apy in the treatment of lymphoma-bearing mice, suggesting

that hyperpolarized 1-[13C]pyruvate may also find applica-

tion in the evaluation of the response to therapeutic treat-

ment. This result has been recently confirmed by comparing

the response to treatment as measured by hyperpolarized 1-

[13C]pyruvate MRI and by PET determination of 2-[18F]fluor-

odeoxyglucose (FDG). Uptake of [18F]FDG was shown to be a

good reporter of the response immediately after treatment

compared with 1-[13C]pyruvate, while at 24 hours after treat-

ment the results were comparable for both technologies.

Conclusions

Molecular imaging is a new science that will have a tremen-

dous impact in understanding biology and in the develop-

ment of innovative diagnostic tools for early diagnoses and

monitoring therapeutic treatments. In the first stage of its

enrollment it has relied massively on PET/SPECT and Optical

Imaging technologies because of the superior sensitivity of

their tracers. In the long term MRI/MRS approaches may

recover a central role provided that further sensitivity

improvement will be attained. As discussed in this survey,

there are several routes to tackle these issues and the promises

are very positive thanks to the multi-nuclear, multi-para-

metric characteristics of the NMR experiment. Along the

way to improve the competitive asset of MRI with respect

to the other imaging modalities there is the need to endow

the probes with higher sensitivity and targeting specificity.

The development of frequency-encoding CAs will be very

important to make the MRI approach the one that allows the

multi-detection of different reporting agents in the same

image. As molecular imaging is the evolution of biologist

in vitro work that has revolutionized the way living cells and

Vol. 8, No. 2–4 2011 Drug Discovery Today: Technologies | Imaging techniques

intact tissues were investigated, MRI multiplex-visualization

of biological processes appears to be a key task for the forth-

coming years for an efficient translation of such outstanding

achievements. Moreover, new avenues for MRI probes will

have to be undertaken to face the technologies advances, for

instance in the field of dual-modality scanners that will be

soon available to the clinical practice.

Acknowledgements

We gratefully acknowledge financial support from the EU

project ENCITE (European Network for Cell Imaging and

Tracking Expertise: HEALTH-2007-1.2-4), the MIUR (Minis-

tero della Istruzione, Universita e Ricerca) project PRIN

2009235JB7 and the regional project PIIMDMT (Regione

Piemonte).

References1 Caravan, P. et al. (1999) Gadolinium(III) chelates as MRI contrast agents:

structure, dynamics, and applications. Chem Rev 99, 2293–2352

2 Muller, R. et al. (2005) Relaxation by metal-containing nanosystems. In

Advances in Inorganic Chemistry (van Eldik, R. and Bertini, I., eds), pp. 239–

292, Elsevier

3 Terreno, E. et al. (2010) Challenges for molecular magnetic resonance

imaging. Chem Rev 110, 3019–3042

4 Laurent, S. et al. (2006) Comparative study of the physicochemical

properties of six clinical low molecular weight gadolinium contrast agents.

Contrast Media Mol Imaging 1, 128–137

5 Datta, A. and Raymond, K.N. (2009) Gd-hydroxypyridinone (HOPO)-

based high-relaxivity magnetic resonance imaging (MRI) contrast agents.

Acc Chem Res 42, 938–947

6 Aime, S. et al. (2004) [Gd-AAZTA]: a new structural entry for an improved

generation of MRI contrast agents. Inorg Chem 43, 7588–7590

7 Aime, S. et al. (1998) Lanthanide(in) chelates for NMR biomedical

applications. Chem Soc Rev 27, 19–29

8 Cavagna, F.M. et al. (1997) Gadolinium chelates with weak binding to

serum proteins. Invest Radiol 32, 780–796

9 Uppal, R. and Caravan, P. (2010) Targeted probes for cardiovascular MRI.

Future Med Chem 2, 451–470

10 Aime, S. et al. (2002) Insights into the use of paramagnetic Gd(III)

complexes in MR-molecular imaging investigations. J Magn Reson Imaging

16, 394–406

11 Aime, S. et al. (2002) Compartmentalization of a gadolinium complex in

the apoferritin cavity: a route to obtain high relaxivity contrast agents for

magnetic resonance imaging. Angew Chem Int Ed Engl 41, 1017–1019

12 Geninatti Crich, S. et al. (2007) Magnetic resonance imaging detection of

tumor cells by targeting low-density lipoprotein receptors with Gd-loaded

low-density lipoprotein particles. Neoplasia 9, 1046–1056

13 Datta, A. et al. (2008) High relaxivity gadolinium hydroxypyridonate –

viral capsid conjugates: nanosized MRI contrast agents. J Am Chem Soc 130,

2546–2552

14 Hooker, J.M. et al. (2007) Magnetic resonance contrast agents from viral

capsid shells: a comparison of exterior and interior cargo strategies. Nano

Lett 7, 2207–2210

15 Figueiredo, S. et al. (2011) Yeast cell wall particles: a promising class of

nature-inspired microcarriers for multimodal imaging. Chem Commun 47,

10635

16 Delli Castelli, D. et al. (2008) Metal containing nanosized systems for MR-

molecular imaging applications. Coord Chem Rev 252, 2424–2443

17 Stefania, K. et al. (2009) Tuning glutamine binding modes in Gd-DOTA-

based probes for an improved MRI visualization of tumor cells. Chem Eur J

15, 76–85

18 Wang, Z.J. et al. (2008) MR imaging of ovarian tumors using folate-

receptor-targeted contrast agents. Pediatr Radiol 38, 529–537

19 Corot, C. et al. (2008) Tumor imaging using P866, a high-relaxivity

gadolinium chelate designed for folate receptor targeting. Magn Reson Med

60, 1337–1346

20 Aime, S. et al. (2005) Gd(III)-based contrast agents for MRI. In Advances in

Inorganic Chemistry. Elsevier pp. 173–237

21 Aime, S. et al. (2006) A R2/R1 ratiometric procedure for a concentration-

independent, pH-responsive, Gd(III)-based MRI agent. J Am Chem Soc 128,

11326–11327

22 Cunningham, C.H. et al. (2005) Positive contrast magnetic resonance

imaging of cells labeled with magnetic nanoparticles. Magn Reson Med 53,

999–1005

23 Laurent, S. et al. (2009) Iron oxide based MR contrast agents: from

chemistry to cell labeling. Curr Med Chem 16, 4712–4727

24 Ho, C. and Hitchens, T.K. (2004) A non-invasive approach to detecting

organ rejection by MRI: monitoring the accumulation of immune cells at

the transplanted organ. Curr Pharm Biotechnol 5, 551–566

25 Aime, S. et al. (2006) High sensitivity lanthanide(III) based probes for MR-

medical imaging. Coord Chem Rev 250, 1562–1579

26 Sherry, A.D. and Woods, M. (2008) Chemical exchange saturation transfer

contrast agents for magnetic resonance imaging. Annu Rev Biomed Eng 10,

391–411

27 Woods, M. et al. (2006) Paramagnetic lanthanide complexes as PARACEST

agents for medical imaging. Chem Soc Rev 35, 500

28 Terreno, E. et al. (2010) Encoding the frequency dependence in MRI

contrast media: the emerging class of CEST agents. Contrast Media Mol

Imaging 5, 78–98

29 Gadian, D. (1995) NMR and its Applications to Living Systems (2nd edn),

30 Muller, S. and Beckmann, N. (1989) 13C spectroscopic imaging A simple

approach to in vivo 13C investigations. Magn Reson Med 12, 400–406

31 Viale, A. et al. (2009) Hyperpolarized agents for advanced MRI

investigations. Q J Nucl Med Mol Imaging 53, 604–617

32 Viale, A. and Aime, S. (2010) Current concepts on hyperpolarized

molecules in MRI. Curr Opin Chem Biol 14, 90–96

33 Kurhanewicz, J. et al. (2011) Analysis of cancer metabolism by imaging

hyperpolarized nuclei: prospects for translation to clinical research.

Neoplasia 13, 81–97

34 Brindle, K.M. et al. (2011) Tumor imaging using hyperpolarized 13C

magnetic resonance spectroscopy. Magn Reson Med 66, 505–519

35 Abragam, A. and Goldman, M. (1978) Principles of dynamic nuclear

polarisation. Rep Prog Phys 41, 395–467

36 Comment, A. et al. (2007) Design and performance of a DNP prepolarizer

coupled to a rodent MRI scanner. Concepts Magn Reson B 31B, 255–269

www.drugdiscoverytoday.com e115