optical probes and the applications in multimodality imaging

Review

Received: 3 August 2010, Revised: 23 September 2010, Accepted: 8 October 2010, Published online in Wiley Online Library: 19 January 2011

(wileyonlinelibrary.com) DOI:10.1002/cmmi.428

Optical probes and the applications inmultimodality imagingYang Liua,b,c,d, Gang Yua,b,c,d, Mei Tiane** and Hong Zhanga,b,c,d*

Optical imaging essentially refers to in vivo fluore

Contrast M

scence imaging and bioluminescence imaging. These types ofimaging are widely used visualizationmethods in biomedical research and are important inmolecular imaging. A newgeneration of imaging agents called multimodal probes have emerged in the past few years. These probes can bedetected by two or more imaging modalities, which harnesses the strengths of the different modalities and enablesresearchers to obtain more information than can be achieved using only one modality. Owing to its low cost and thelarge number of probes available, the optical method plays an important role in multimodality imaging. In thismini-review, we describe the available multimodal imaging probes for in vivo imaging that combine optical imagingwith other modalities. Copyright # 2011 John Wiley & Sons, Ltd.

Keywords: optical probes; molecular imaging; fluorescence imaging; multimodal imaging; report gene; fluorophore

* Correspondence to: H. Zhang, Department of Nuclear Medicine, Second

Hospital of Zhejiang University School of Medicine, 88 Jiefang Road, Hang-

zhou, Zhejiang 310009, China.

E-mail: [email protected]

** Co-correspondence to: M. Tian, Department of Experimental Diagnostic

Imaging, The University of Texas, M.D. Anderson Cancer Center, Houston, TX, USA.

E-mail: [email protected]

a Y. Liu, G. Yu, H. Zhang

Department of Nuclear Medicine, Second Affiliated Hospital of Zhejiang

University School of Medicine, Hangzhou, China

b Y. Liu, G. Yu, H. Zhang

Zhejiang University Medical PET Center, Hangzhou, China

c Y. Liu, G. Yu, H. Zhang

Institute of Nuclear Medicine and Molecular Imaging of Zhejiang University,

Hangzhou, China

d Y. Liu, G. Yu, H. Zhang

Key Laboratory of Medical Molecular Imaging of Zhejiang Province,

Hangzhou, China

e M. Tian

Department of Experimental Diagnostic Imaging, The University of Texas

M.D. Anderson Cancer Center, Houston, TX, USA

1. Introduction

Molecular imaging is an exciting and booming research field.There are several imaging modalities available for in vivo imagingresearch, such as nuclear imaging (positron emission tomogra-phy, PET, and single photon emission computed tomography,SPECT), optical imaging (fluorescence and bioluminescence),ultrasound (US) imaging and magnetic resonance (MR) imaging,each of which has its own advantages and disadvantages. Forexample, PET has high sensitivity but poor spatial resolution,whereas MRI provides high anatomic resolution but has lowsensitivity.Among these imaging modalities, optical imaging is the most

widely used visualization method in biomedical research,especially translational medicine (1–5). The advantages of opticalimaging compared with other imaging modalities include (1) lowcost, (2) availability of a large number of optical probes, (3)facilitated toolbox for study of biological events from themolecular level to whole-animal levels and (4) high instrumentsensitivity. The disadvantages of optical imaging methodsinclude poor spatial resolution and low ability to obtainquantitative information in vivo caused by limitations ofpenetration and scattering of light in the tissue.In recent years, molecular imaging techniques have greatly

improved. A group of imaging agents called multimodal probeshave emerged. These probes can be detected by two or moreimaging modalities, which takes advantage of the strengths ofdifferent modalities and enables access to more information thancan be obtained by one imaging technique. This mini-review willgive a brief introduction to optical probes and then describemultimodal imaging probes that combine optical imaging withother modalities.

2. Optical probes for in vivo imaging

Optical imaging essentially refers to in vivo fluorescence imagingand bioluminescence imaging (BLI). In fluorescence imaging,

edia Mol. Imaging 2011, 6 169–177 Copy

fluorescent proteins or organic dyes are excited by an externallight source. Fluorescence imaging can be performed at differentresolutions and depth penetrations, ranging from micrometers(intravital microscopy) to centimeters (fluorescence moleculartomography). In BLI, a substrate that can be catalyzed byluciferases is needed rather than external light. The completion ofthis chemical reaction results in the emission of light at a specificwavelength.Optical imaging probes can be divided into two main

categories: fluorophore-based optical probes and reportergene-based optical probes. The imaging unit of a fluorophorebased–optical probe is a fluorophore, which is usually an organicdye or an inorganic nanoparticle, see Fig. 1. The reportergene-based optical probes are genes that are transferred to and

right # 2011 John Wiley & Sons, Ltd.

169

Biographies

Yang Liu, received his B.S. degree from Zhejiang University in biological sciences. He received his Ph.D. degreein chemical biology from Zhejiang University. Now he is a postdoctoral fellow at the Key Laboratory of MedicalMolecular Imaging of Zhejiang Province, Hangzhou, China. His research interest is the development of novelmolecular imaging agents.

Gang Yu, received his Ph.D. degree in materials science from Shizuoka University, Japan. As a postdoctoralfellow, he worked at the Innovative Joint Research Center of Shizuoka University and Photon Medical ResearchCenter of Hamamtsu University School of Medicine. He is working at the Center of Excellence of MedicalMolecular Imaging of Zhejiang Province, Hangzhou, China. His research focuses on the development of novelmultimodality imaging agents.

Mei Tian is an assistant professor in the Department of Experimental Diagnostic Imaging at the University ofTexas MD Anderson Cancer Center, USA. She received her doctoral degree of Medical Science in DiagnosticRadiology and Nuclear Medicine at Gunma University, Japan, and trained at Gunma University, Japanese Societyfor the Promotion of Science, Japan, and Dana-Farber Cancer Institute/Brigham and Women‘s Hospital, HarvardMedical School. Her research interests are clinical and translational molecular imaging. She serves as aneditorial board members of the Journal of Nuclear Medicine, European Journal of Nuclear Medicine and MolecularImaging, Nuclear Medicine Communications and Oncology. She is the recipient of several awards from the JSNM,RSNA and ASCO.

Hong Zhang is a Professor and Chairman of the Department of Nuclear Medicine at the Second AffiliatedHospital of Zhejiang University School of Medicine and Center of Excellence of Medical Molecular Imaging ofZhejiang Province, China. He received his doctoral degree of Medical Science in Diagnostic Radiology andNuclear Medicine at Gunma University, Japan, and trained and worked at Gunma University School of Medicine,St Bartholomew’s Hospital of the University of London, UK, and the National Institute of Radiological Sciences,Japan. His research focus is on functional diagnostic imaging in oncology and neurology, stem cell and geneimaging, imaging probe development for PET, MRI and optical imaging. He serves as an editorial board memberof the European Journal of Nuclear Medicine and Molecular Imaging, World Journal of Nuclear Medicine, NuclearMedicine Communications, Annuals of Nuclear Medicine, Nuclear Medicine and Molecular Imaging and Oncology.He is the recipient of five Awards from the JSNM, AMI, AACR and ASCO.

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expressed in target cells. Their gene products are used for theoptical imaging. These products include green fluorescentproteins (GFP), luciferase for BLI, and other enzymes or receptors.Fluorophore-based optical probes are injectable, so they have

the potential to be used in the clinic. On the other hand, reportergene-based optical probes are suitable for cell trafficking andgene expression studies. These probes also play a key role inpreclinical gene therapy research by allowing efficient, non-invasive and rapid assessment of gene expression.

2.1. Fluorophore-based optical probes

Thousands of organic fluorescent dyes have been developed (6),most of which were designed for in vitro imaging utilizingfluorescencemicroscopymethods. Fluorescent dyes are powerfulresearch tools for biochemical and cellular studies. These organicsmall molecules can be modified to visualize specific moleculartargets or events. However, in vivo imaging is different fromcellular visualization, and most dyes developed for cell imagingare not suitable for in vivo imaging. Fluorescent probes for in vivoimaging should have the following properties: (1) their

wileyonlinelibrary.com/journal/cmmi Copyright # 2011 Jo

fluorescence should have large penetration length in tissues;(2) they should have low toxicity; and (3) they should be stable inthe complicated in vivo environment. Organic dyes which emit inthe near-infrared region (NIR; 650–900 nm) are suitable for in vivoimaging. NIR fluorescent dyes that can avoid most of theautofluorescence of tissues and have high penetration ability aremost often used. Among the thousands of organic dyes, cyaninedyes are themost widely used for in vivo imaging because of theirlong emission wavelength. Their synthesis (7), labeling (8–11),physical properties, biodistribution and pharmacokinetics havebeen studied in detail, and some such dyes (e.g. Cy3, Cy5, Cy5.5,and Cy7) are commercially available. Rhodamines (12), fluor-esceins (13) and oxazine dyes (14) are other dyes that have beenused in fluorescence imaging. However, more organic dyes withlow toxicity and desirable properties are needed. The results ofrecent fine design and screening studies of organic dyes usingcombinatorial chemistry have been promising (15–18). Morework should also be done to examine the toxicity of these newdyes and their metabolic process in vivo.Another class of probes for in vivo fluorescence imaging is

called quantum dots (QDs) (19,20). QDs are semiconductor

hn Wiley & Sons, Ltd. Contrast Media Mol. Imaging 2011, 6 169–177

Figure 1. Some fluorophores used for in vivo imaging [reprinted with permission from Cai et al. (33)].

OPTICAL PROBES IN MULTIMODALITY IMAGING

1

nanocrystals that have a core/shell structure of 2–8 nm indiameter. These particles can be covered with amphiphilicpolymers and conjugated with different ligands for targetedimaging. QDs are excellent fluorophores with unique opticalproperties, such as high absorbency, high quantum yield, narrowemission bands, large Stokes shifts and high resistance tophoto-bleaching. The design and application of fluorescentprobes based on QDs are rapidly growing (21–24).Lanthanide complexes have gained more and more

attention in recent years. Trivalent lanthanide ions have uniqueluminescent properties both in lifetime and spectra. Lanthanideoptical probes have become useful analytic tools in many aspectsof biosciences, such as determination of enzyme activity,detection of simple analytes and immunoassays. Morethan that, their application for in vivo imaging is quitepromising (25).

Contrast Media Mol. Imaging 2011, 6 169–177 Copyright # 201

Nuclear imaging usually has high background signal becausethe signal of nuclear probes is always on. In contrast, opticalimaging using fine-designed fluorophore-based probes has theadvantage of target-specific imaging, which can greatly reducethe background signal. A fluorophore is usually linked to atargeting unit such as a target-specific small molecule, peptide orprotein. Some other probes are designed to be activated onlyafter interaction with the target molecules or target analyte (e.g.enzymes, pH and oxidants). Some fluorophore-based probesdesigned for in vivo imaging with desirable characteristics arelisted in Table 1.

2.2. Reporter gene-based optical probes

The common genetic reporters include luciferase (Luc) for BLI, FPsfor fluorescence imaging, transferrin receptor for MRI, and herpes

1 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/cmmi

71

Table

1.Exam

plesoffluorophore-based

probes

fortarget-specificin

vivo

imag

ing

Fluorophore

Targetingunit

Target

Application

Administration

Reference

Red

-emittingQDs

Antibody

Prostate-specificmem

brane

antigen

Human

prostatetumor-bearing

mouse

0.6nM/m

ouse,intraven

ous

injection(i.v.)

(20)

Cy5.5

Anti-EGFR

Mab

Erbitux

EGFR

Breasttumor-bearingmouse

i.v.

(26)

Cy3,Cy5,Cy5.5

Anti-SSEA-1;9.2.27

SSEA

;9.2.27an

tigen

Can

cerin

nudemouse

10–1

00mg/m

ouse

i.v.

(27)

Cy7

Antibodyfrag

men

ts(ED-B)offibronectin

Angiogen

esis,nudemouse

200ml/mouse

(0.5mg/m

l),i.v.

(28)

Indocyan

inegreen

Pan

itumumab

trastuzumab

HER

1an

dHER

2HER

1þ,HER

2þ

tumor-bearing

mouse

50mg/m

ouse,i.v.

(29)

Cy5.5

Endostatin

Angiogen

esis

(LLC

-LM)tumor-bearingmouse

10–1

00ml/mouse

(1mg/m

l),i.v.an

di.p.

(30)

Carbocyan

inedyes

Octreotate

Somatostatin

(SST

)receptor

RIN38/SSTR2tumor-bearingmouse

0.02mmol/kg

bodyweight,i.v.

(31)

Cy5

Cyclic

RGD

Integrinavb3

aVb3-positive

andaVb3-neg

ative

tumor-bearingmouse

10nmol/mouse,i.v.

(32)

Nan

oparticles

Cyclic

RGD

Integrinavb3

U87MGtumor-bearingmouse

200pmol/mouse,i.v.

(33)

Cy7

Pipetides

Activated

factorXIII

(FXIIIa)

Intravascularthrombi

7.5mg/kgbodyweight,i.v.

(34)

IDCC

Vasoactive

intestinal

pep

tide(VIP)

(VIP)receptorsubtype1[VPA

C(1)]

Can

cer

2mmol/kg

bodyweight,i.v.

(35)

Fluoresceinan

dcarbocyan

inedyes

Somatostatin

and

bombesin

Somatostatin

andbombesin

receptor

Pan

creatictumor-bearingrat

i.v.

(36)

Carbocyan

inedyes

Folate

Folate

receptor(FR)

FR-positive

tumors

i.v.

(37)

Cy5.5

Annexin

VExtracellular-facing

phosphatidylserine

Tumorap

optosis

3.1mg/kgbodyweight,i.v.

(38)

Cy5

Vitam

inB12

SLNiden

tificationin

pigs

Injectedin

adermal

location

(39)

IRDye

800CW

Bisphosphonates

Hyd

roxyap

atite

Calcificationin

large-an

imal

models

0.06mmol/kg

(40)

wileyonlinelibrary.com/journal/cmmi Copyright # 2011 John Wiley & Sons, Ltd. Contrast Media Mol. Imaging 2011, 6 169–177

Y. LIU ET AL.

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simplex virus-1 thymidine kinase (HSV-tk) for nuclear ima-ging(41,42). These gene reporters can be coupled together toconstruct multimodal gene reporters.GFP and related fluorescent protein families have contributed a

great deal to biological research (43,44). However, some GFPshave disadvantages, such as a short wavelength, and thus are notsuitable for in vivo imaging. A lot of work has been done tofine-tune the photophysical and photochemical properties of FPs,and there exists a collection of FPs that can be reliably imagedfrom the blue to the far-red spectral regions. Tsien and coworkershave worked to create new mutant GFPs by combining iterativesomatic hypermutation and fluorescence-activated cell cytome-try. The group recently reported novel infrared-fluorescentproteins (45) (IFPs) with excitation and emission maxima of684 and 708 nm, respectively. The newly IFPs were based on abacteriophytochrome from Deinococcus radiodurans, incorporat-ing biliverdin as the chromophore. They showed that IFPsexpressed well in both mammalian cells and mice. Thewavelength of IFPs makes them suitable for whole-body imaging,but further engineering is necessary to determine which IPFs canbe used in vivo (46).Firefly or Renilla luciferase is commonly used in biolumines-

cence report systems (47,48). Luciferases are a diverse group ofphotoproteins that have been isolated from a large variety ofinsects, marine organisms and prokaryotes. They work asoxidation enzymes that can catalyze the light-emitting reactionsbetween oxygen and the substrate luciferin. There are manyluciferase/substrate pairs available. BLI does not requireexcitation light, so the background autofluorescence is verylow and has a high signal-to-noise level. Compared withfluorescence imaging methods, optical imaging based onluciferase reporters provides a more sensitive means to imagefundamental biological processes in vivo. However, the emissionspectra range between 400 and 620 nm, making them lesssuitable for deep-tissue imaging. Development of mutantluciferases and synthesis new substrates may overcome thislimitation. Branchini (49) engineered two luciferases usingmutagenesis techniques, and finally got luciferase variants withred-shifted bioluminescence and high specific activity.b-Galactosidase (b-gal), expressed by the bacterial LacZ

gene, has been used as a genetic reporter for decades. Manysubstrates of b-gal have been developed that can be usedin both MRI and optical imaging. For example, Tung et al.(50) synthesized 9H-(1,3-dichloro-9,9-dimethylacridin- 2-one-7-yl)

Table 2. Non exhaustive list of multimodal probes containing flu

Imaging modality Structure and the imaging un

Opticalþ nuclear 111In-DTPA-Lys(IRDye800)-c(KRGDf)Opticalþ nuclear 111In-DTPA-K(IR-783)-c(CGRRAGGSC)Opticalþ nuclear Pamidronate-99Tc-IRDye800Opticalþ nuclear 64Cu-labeled DOTA–QD–RGDOpticalþ nuclear (111In-DTPA)n-trastuzumab-(IRDye800)mOpticalþ nuclear Paramagnetic and fluorescent RGD LNPsOpticalþMRI Cy5.5-Thermally cross-linked iron oxideOpticalþMRI Luminescent hybrid nanoparticles with a pOpticalþMRI Gd-DO3A-ethylthiourea-fluorescein


b-D-galactopyranoside for in vivo imaging, allowing detection ofbeta-gal activity by fluorescence imaging technology.

3. Fluorophore-based multimodal probes

In general, fluorescence imaging units can be organic dyes orinorganic nanoparticles such as QDs for multimodal imaging. MRimaging units can be gadolinium chelates, functionalized QDs oriron oxide nanoparticles. The nuclear imaging units can be PETtracers and radionuclide chelates. The combination of these unitsresult in an multimodal probes. Examples of optical-MRI andoptical-nuclear multimodal probes are listed in Table 2.As shown in Fig. 2, multimodal probes based on fluorophores

have at least two imaging units, which can also be imaged byMRI,nuclear imaging, or other imagingmethods (60–63). Many probesalso contain a targeting unit, usually a peptide, antibody ortargetable ligand. Optical imaging techniques are limited bydepth of light penetration and scatter of emitted light photons.Thus, it is difficult to obtain quantitative or tomographicinformation using optical imaging. By contrast, PET- or MR-basedimaging does not have these limitations. Multimodal probes takeadvantage of the strengths of both types of imaging; for example,Ogawa et al. labeled the monoclonal antibodies panitumumaband trastuzumab with In-111 and ICG (29). They found thatICG-labeled antibodies were highly target-specific and only gavea bright fluorescence signal in the target cells. In addition, nuclearimaging showed the biodistribution profile of the injectedantibody. Using these multimodal probes, we can characterizethe molecular targets by the activatable optical probes and studythe distributions and dynamics of the targeting moleculesquantitatively by nuclear probes.In the past few years, functionalized nanoparticles have

attracted a lot of attention as multimodal probes. New kinds ofnanoparticles have been constructed as imaging probes, such assilica particles (64) and liposomes (65), and new coating methodshave been developed (66). Some examples are presented below.Kumar et al. (64) constructed organically modified silica

(ORMOSIL) nanoparticles and demonstrated their use for in vivobioimaging, biodistribution, clearance and toxicity studies. Thesenanoparticles were conjugated with NIR fluorophores andradiolabeled with 124I for optical and PET imaging in vivo.Kamaly et al. (65) synthesized a new paramagnetic and

fluorescent bimodal probe that can be used for both MRI and

orophores

it (in bold) Targeting unit Reference

RGD (51)c(CGRRAGGSC) (52)Bisphosphonates (53)RGD (54)Trastuzumab (55)RGD (56)

(57)aramagnetic Gd2O3 core (58)

(59)


173

Table 3. Reporter genes commonly used in multimodalprobe based on optical report genes

Imaging modality Reporter Reference

Optical Fluorescent proteins (73)Optical Luciferase (74)Nuclear HSV1 TK (75)Optical/nuclear/MRI b-Galactosidase (76)MRI Transferrin receptor (77)

Figure 2. Schematic diagram of multimodal imaging probes.

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fluorescence microscopy utility. This probe was used to formcationic and neutral pegylated liposomes, and then theliposomes were used to label cells in vitro and image humanovarian xenografts in vivo. The results showed this new probe wasvery effective and sensitive for cellular labeling and tumor MRI.An aminosilane-coated superparamagnetic nanoparticle was

developed by Stelter et al. (67) for cell labeling and subsequentmultimodal MR, PET and fluorescent imaging in vivo. Thenanoparticles had multiple free amino groups, which allowedvarious modifications. The nanoparticles were covalently linkedwith the transfection agent HIV-1 tat, the fluorescent dyefluorescein isothiocyanate and the positron-emitting radio-nuclide gallium-68. These superparamagnetic nanoparticles weredemonstrated to be stable under in vitro and in vivo conditionsand therefore applicable for efficient cell labeling and sub-sequent multimodal molecular imaging.Xie et al. (66) modified the surface of iron oxide nanoparticles

(IONPs) with dopamine. These nano-conjugates were encapsu-lated into human serum albumin (HSA) matrices and duallylabeled with 64Cu-DOTA and Cy5.5. This tri-modality (PET,near-infrared fluorescence and MRI) imaging probe was testedin a subcutaneous U87MG xenograft mouse model.Ke et al. (68) reported a kind of poly(acrylic acid) iron oxide

(PAAIO) complexes. These new complexes have free carboxylicgroups which allow for covalent attachment of fluorescent dyes.Conjugation of rhodamine 123 and folic acid-linked poly(ethy-lene glycol) (FA-PEG) to PAAIO resulted in a dual-modalitymolecular probe that was shown to target cancer cells.Xu et al. (69) developed a dual-mode imaging probe based on

poly(lactic-co-glycolic acid) nanobubbles (NBs) for cancertargeting and simultaneous fluorescence/ultrasound imaging.The NBs were conjugated with cancer-targeting ligands and wereencapsulated with Texas Red dye. Simultaneous fluorescence andultrasound images could be acquired using tumor simulators ingelatin.Hwang et al. (70) reported amultimodal cancer-targeted probe

composed of a targeting unit (the AS1411 aptamer, which targetsnucleolin, a cellular membrane protein highly expressed incancer) and three detecting units. The cobalt–ferrite (MRI)nanoparticle was surrounded by rhodamine (fluorescenceimaging) and was also linked with 67Ga-NOTA (radionuclideimaging). The probe could be used to detect cellular distributionby fluorescence confocal microscopy. In vivo nuclear and MRimaging were performed using a g-camera and a 1.5 TMR imager,respectively. Results showed that this probe can be used as aversatile imaging tool for specific cancer diagnosis.Optical probes are powerful tools for studies from intracellular

level (in vitro imaging) to whole-body levels (in vivo imaging).


Therefore many optical probes have been successfully used inbiochemical and cellular studies. However, there is still a long wayto go for the clinic use. The toxicity and metabolism offluorophores and the conjugates in vivo should be systemicallyinvestigated. Assessment of the agent’s toxicity in multiplespecies is needed before clinical trials. Only a few organicfluorophores, such as ICG and fluorescein, have been investigatedfor their toxicity to tissues and in vivo (71). Toxicity is also a bigproblem for QDs (72). New structures and new coating methodsare employed in order to solve this problem. Biostability isanother issue which requires much effort, especially for thefluorophore-based multimodal probes. Integrality of the probe isessential for the accomplishment of multimodal imaging. Formost of the fluorophore-based multimodal probes, the detectingunit and targeting unit are usually linked by amido bonds whichcan be hydrolyzed by many enzymes. Therefore minimizinghydrolysis in vivo is quite important. Photo-bleaching of organicfluorophores also causes loss of the integrality of the multimodalprobes.

4. Reporter gene-based multimodalprobes

There are many imaging units other than optical units that can beused for multimodal probes based on reporter genes. Forexample, HSV1-tk, somatostatin type 2 receptor and sodiu-m-iodide symporter can be used for SPECT. HSV1-tk anddopamine type 2 receptor can be used for PET, and transferrinreceptor can be used for MRI. The reporter genes commonly usedin multimodal probes are listed in Table 3.These imaging genes, when coexpressed with fluorescence or

bioluminescent reporter genes, will result in multimodal probes(78–83). Combining reporter genes into a single fusion canprovide additional advantages. The coupling of a nuclear reportergene (e.g. HSV1-tk) with an optical reporter gene (e.g.enhanced-GFP or Luc) is also common.More recently, three different reporter proteins, red fluorescent

protein, firefly Luc and HSV1-sr39 truncated thymidine kinase,were linked through a caspase-3 recognizable polypeptide linker.This multimodality reporter vector was used successfully tomonitor caspase-3 activation indirectly in live cells and tumorsundergoing apoptosis of living animals (84).Reporter gene-based probes are valuable tools for cell tracking,

especially in the field of stem cell-based therapies. Waerzeggers(85) constructed a triple-fused report system (HSV1-tk, GFP andLuc) for the noninvasive detection of implanted neural progenitorcells (NPCs). They successfully monitored the fate and functionalstatus of the NPCs by sequential multimodal molecular imaging


Figure 3. Schematic diagram of multimodal imaging using multimodal optical probes [reprinted with permission of Wang et al. (52) and Ponomarev

et al. (78)].


technology. Daadi (86) used BLI and MRI for the tracking ofgrafted human embryonic stem cell-derived human neural stemcells in stroke-damaged rat brain. A double fusion reporter genethat stably expressed eGFP and f-Luc reporter proteins wasconstructed to provide reliable real-time monitoring of cell fate.A schematic diagram of multimodal imaging using multimodal

optical probes is shown in Fig. 3.

1

5. Conclusions

Optical probes have been successfully used in biochemical andcellular studies. They are becoming more and more important inthe field of small animal research. Some optical probes have beenused for the visualization of surgical targets. However, systemi-cally assessment of toxicity need to be done, and more effortsshould be made to improve their biostability.Multimodal imaging probes contain more than one imaging

unit and offer synergistic advantages over single-modalityimaging. Multimodal imaging probes bearing an optical imagingunit have particular importance in molecular imaging. There areplenty of optical facilities available, and one can easily obtaininformation with high sensitivity and different spatial resolutions.Multimodal probes will greatly benefit research into thedistribution and kinetics of molecules, drugs, therapeutic cellsand gene expressions in vivo. On the other hand, the boom in


multimodal imaging will accelerate the development of opticalimaging probes, which is a ‘double-win’.The clinical potential of multimodal optical probes is

promising. More and more carefully designed optical probeshave been synthesized and evaluated, and they are playing anincreasingly important role in biomedical research. In the future,multidisciplinary approaches from chemistry, biology, engineer-ing, pharmacology and medicine will lead to the development ofclinically useful imaging probes.

Acknowledgements

This work is partly sponsored by grants from the National ScienceFoundation of China (nos 30672396, 81000630) and the Ministryof Science and Technology of China (nos 2006DFB32940,2011CB504400).

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