Organometallic chemistry volume38

Furthermore, in vivo optical imaging canenable detection of tumours on the basis of the selectivity of the imagingprobe, with high sensitivity yet without exposure to ionising radiation.

Specialist Periodical Reports

Organometallic Chemistry Volume 38

Edited by Ian J S Fairlamb and Jason M Lynam

Organometallic Chemistry

Volume 38

A Specialist Periodical Report

Organometallic Chemistry Volume 38

A Review of the Recent Literature

Editors

I Fairlamb and J Lynam, University of York, UK

Authors

Rory L Arrowsmith, University of Bath, UK

M.P Cifuentes, Australian National University, Canberra, AustraliaSarah B.J Dane, University of Cambridge, UK

Philip J Harford, University of Cambridge, UK

L.J Higham, Newcastle University, UK

M.G Humphrey, Australian National University, Canberra, AustraliaAnant R Kapdi, Institute of Chemical Technology, Mumbai, IndiaTimothy C King, University of Cambridge, UK

Sofia I Pascu, University of Bath, UK

Hubert Smugowski, University of Bath, UK

A.E.H Wheatley, University of Cambridge, UK

D.S Wright, University of Cambridge, UK

ISBN 978-1-84973-376-2

ISSN 0301-0074

DOI 10.1039/9781849734868

A catalogue record for this book is available from the British Library

All rights reserved

Apart from fair dealing for the purposes of research or private study fornon-commercial purposes, or for private study, criticism or review, aspermitted under the Copyright, Designs and Patents Act, 1988 and theCopyright and Related Rights Regulations 2003, this publication may not bereproduced, stored or transmitted, in any form or by any means, without theprior permission in writing of The Royal Society of Chemistry, or in the case

of reproduction in accordance with the terms of the licences issued by theCopyright Licensing Agency in the UK, or in accordance with the terms of thelicences issued by the appropriate Reproduction Rights Organization outsidethe UK Enquiries concerning reproduction outside the terms stated hereshould be sent to The Royal Society of Chemistry at the address printed on thispage

Published by The Royal Society of Chemistry,

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to the chapters online Please contact sales@rsc.org with proof ofpurchase to arrange access to be set up

Thank you

Comprehensive reviews of the organometallic chemistry in this Volumedetail the literature published in 2010 on the chemistry of metal clusterswritten by Mark Humphrey and Marie Cifuentes, the chemistry of the alkaliand coinage metals by Philip Harford and Andrew Wheatley as well asrecent developments in Group 2 (Be-Ba) and Group 12 (Zn-Hg) compounds

by Sarah Dane, Timothy King and Dominic Wright

This Volume therefore covers many synthetic and applied aspects ofmodern organometallic chemistry from various areas of the periodic table

Department of Chemistry, University of York, York YO51 5DD, UK

E-mail: ijsf1@york.ac.uk; jason.lynam@york.ac.uk

Organomet Chem., 2012, 38, v–v | v

Ian J S Fairlamb and Jason M Lynam

New developments in the biomedical chemistry of metal complexes:

from small molecules to nanotheranostic design

1Rory L Arrowsmith, Sofia I Pascu and Hubert Smugowski

Cover Ball and stick representation of Grubbs generation II catalyst.

Organomet Chem., 2012, 38, vii–viii | vii

Organometallics aspects of C–H bond activation/functionalization 48Anant R Kapdi

(s-bond metathesis of C–H bond)

63

Mark G Humphrey and Marie P Cifuentes

Philip J Harford and Andrew E H Wheatley

Sarah B J Dane, Timothy C King and Dominic S Wright

Me6[14]dieneN4

5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetra-deca-4,11-diene

5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetra-decane

3,4,7,8-Me4phen 3,4,7,8,-tetramethyl-1,10-phenanthroline

SCF self consistent field

New developments in the biomedical

chemistry of metal complexes: from small molecules to nanotheranostic design

Rory L Arrowsmith, Sofia I Pascu* and Hubert Smugowski

DOI: 10.1039/9781849734868-00001

Introduction

Molecular imaging is a key area for development worldwide In 2007, thiswas defined by the Society of Nuclear Medicine as a new interdisciplinaryresearch field, which is at the interface between clinical and preclinicalresearch This is highlighted by the increasing demand for new imagingprobes for specific biological targets.1 By the end of 2010 more than 3.2million positron emission tomography (PET) studies have been carried outworldwide It is widely recognised that optimal disease management isachieved by monitoring patient status before, during and after therapy PETagents offer high resolution, non-invasive imaging with provision ofinvaluable diagnosis of biological function at agent concentrations belowthe pharmacological threshold There is currently intense interest in thedevelopment of new PET agents for imaging a wide range of disease states,and of new drugs for targeted radiotherapy Drugs containing a radio-nuclide are known as radiopharmaceuticals and can be used for diagnosisand/or therapy Radiopharmaceuticals chosen for the purpose of diagnosisare usually positron emitters (PET) or gamma emitters (SPECT), whereas

Auger effect causing cell death The choice of radioisotope is also madeaccording to an optimum half-life, which at the same time minimisesradiation doses whilst giving sufficient time for synthesis and accumulation

(Herceptint, a monoclonal antibody therapeutic) in metastatic breastcancer has been completed in USA in 2010.2The choice of radionuclide isdependent on availability, half-life and pharmacokinetics The isotope 18F(t1/2109.8 min) is most widely used for imaging applications, especially as

(t1/2 20.4 min) and 18F (t1/2 109.8 min) have relatively short lives in thecontext of following relatively slow biological processes such as the accu-mulation of a labeled monoclonal antibody at a target site in vivo The mostcommonly used positron emitting isotopes are18F,11C,13N,15O, howeverthere is growing interest in use of metal radioisotopes such as60Cu, 64Cu,Chemistry Department, University of Bath, Claverton Down, Bath, UK, BA2 7AY.

E-mail: s.pascu@bath.ac.uk

Organomet Chem., 2012, 38, 1–35 | 1

Ga and Zr The relatively long half-life of Cu (t1/212.7h) as well asthe availability of68Ga (t1/21.13 h) from commercial, portable, generatorsmakes these attractive radioisotopes for PET imaging as these may be used

at a site remote from a cyclotron Imaging with readily available metallicradioisotopes for Single Photon Emission Computed Tomography(SPECT) such as99mTc (t1/26 h) and111In (t1/22.8 days) are by far the mostwidely used in nuclear medicine on a global scale The first tomographicdevice, SPECT, was developed by Kuhl and Edwards in 1963 In thistechnique detection of the gamma emissions from the radionuclide enable a3D image to be produced.5

Compared with SPECT, PET imaging has the crucial advantage in terms

of sensitivity and resolution Such metallic radionuclides ultimately undergouptake within cells and while the distribution of these complexes can bedetermined in vivo at the 1–2 mm range of resolution, little is known of theirfate once they are in the intercellular environment This often hampers therational design of new diagnostics and therapeutics and ultimately theaccurate diagnosis of cancer There is growing interest in molecular imaging

as a non-invasive, highly sensitive methods capable of both early diagnosisand enhancing the understanding of the molecular basis of the disease.6,7Molecular imaging also combines understanding of molecular functionwith in vivo imaging As a result it can report upon disease mechanisms at acellular and sub-cellular level, as well as the effectiveness and selectivitytowards target cells of a specific therapy Optical imaging, therefore, can beused to follow the uptake of luminescent complexes in both cells andmulticellular organisms In vitro studies not only function as a platform forassessing the suitability of in vivo work and as a drug discovery tool, but alsoreveal uptake of small molecules into components of the cell via colocali-sation studies, which in turn gives an indication of the likely activity that

an investigated compound may show in vivo This in turn enhances themechanistic understanding of pharmacological processes involved Recentpublications of luminescent metal complexes for which their properties wereexplored in biological systems, will be highlighted with a strong emphasis onorganometallic compounds Furthermore, in vivo optical imaging canenable detection of tumours on the basis of the selectivity of the imagingprobe, with high sensitivity yet without exposure to ionising radiation.This review will discuss recent advancements of metal complexes forimaging at a cellular level using optical imaging and at an organism levelwith a focus on multimodality imaging probe design – including thoseapplicable for Single Photon Emission Computed Tomography (SPECT),Positron Emission Tomography (PET), Magnetic Resonance Imaging(MRI) and Near Infra-Red (NIR)

Radiopharmaceuticals and multimodality probe design considerationsRadiopharmaceuticals are designed to answer a specific medical need andare based on the knowledge of molecular biology The first generation ofradiopharmaceuticals involved radioactive isotopes aimed at mimickingnormal biological processes, such as 2-[18F]-fluoro-2-deoxy-D-glucose and[99mTcO4]2, which take advantage of higher glucose uptake by cancer cellsand mimic iodine uptake by the thyroid, respectively.8There is a current

trend towards ‘second generation’ radiopharmaceuticals that use a gically active molecule (BAM), such as a peptide or antibody for specifictargeting Much of the work utilises the labeling of standard chelatingagents such as 1,4,7,10-tetraazacyclododecane-N,N0,N00,N000,-tetraacetic

1,4,8,11-tetra-azacyclododecane-1,4,8,11-tetraacetic acid (TETA), (Fig 1) conjugated to abiomolecule (BAM) via a linker and a chelator.9

Developments such as the scintillation detector and improved detectiontechnology aided the advancement of PET and SPECT Despite thescintillation detector remaining virtually the same as the original designed

by Anger in 1958, the recent development of the dual-headed gammacamera has had a significant positive effect on PET Multiple detectorsystems also achieved better sensitivity and efficiency to enable simulta-neous scanning of multiple sections Combining modalities such as SPECT/

CT, PET/X-ray, PET/MRI, or most frequently PET/CT, allows for betterimage quality, shorter scanning time and reduced costs This results in moreefficient use of radiopharmaceuticals and more facile recognition ofabnormalities The synergistic combination of PET and MRI holds promisefor a successful next generation of dual-modality scanners in medicalimaging These instruments will provide accurate diagnoses thanks to thesensitive and quantifiable signal of PET and the high soft-tissue resolution

of MRI Furthermore, patients will receive reduced radiation doses ever, these new tools require a new class of imaging probes Therefore, therehas recently been increasing interest in the development of dual-modality

based on a PET isotope and gadolinium.10The second generation of dual/multimodal contrast agents are synthesised using MNPs, having a provenrecord of biocompatibility and a track record of extensive use in the clinic asMRI contrast agents.11,12

A combination with optical imaging enables both greater understanding

of the probe both in cells and in an organism as well as enabling theidentification of a tumour; the advantages of the high sensitivity of PET andSPECT, which are not limited by tissue penetration as is optical imagingand presents itself as a very interesting marriage of modalities with potential

to improve both scientific knowledge and patient diagnosis and therapy.There are only very few examples in the literature described as dual/multi-modality hybrid nanomaterial used for PET/MRI or PET/MRI/NIRF.The development of PET radiopharmaceuticals labeled with generator-produced PET radionuclides has facilitated greater use of this imagingmethod in clinical nuclear medicine For example, the68Ge/68Ga parent and

N N

daughter radionuclides are ideal for this: the half life of Ga isotope (68min) is long enough to achieve the synthesis of a wide variety of radio-pharmaceuticals and allow for long data acquisitions, thus enhancing theimages quality (vide infra).

There is significant research effort carried out that focuses on finding newtheranostic targets, of which is beyond the scope of this review (see Ref 13for further information) It can be envisaged that diagnostic radiometals,such as 64Cu, 67Ga,68Ga,99mTc for example may be used as future diag-nostic agents which are simultaneously amenable for coupling with radio-therapeutic agents such as177Lu,90Y,111In or212Pb: this could even allowfor follow-up treatments provided the chemical properties of the complexare not altered significantly by the change in metal A diagnostic agent and atherapeutic agent make a ‘theranostic pair’ However, intrinsically cytotoxicagents could also be radiolabelled with another approach being to utilisenanoparticles filled with a drug targeted with a BAM

Nanomedicines design in imaging applications

chemother-apeutics selectively to tumours have been developed in recent years.Succesful examples reported were based on designs that involved couplingdrugs to receptor-specific ligands and/or protection of the drug by wrap-ping in a polymer or lyposome with enhanced kinetic stability in vitro Theprecise way in which such nanomedicines act within cells remainsunknown

Currently, it is believed that using fluorescence microscopy techniques toimage radiolabelled nanomedicines within cells (nanotheranostics) couldprovide valuable information on the cell behaviour and generate the nextgeneration of contrast agents Molecular imaging probes can act as diag-nostic therapeutics, allowing prediction of response to treatment, dosimetry

to be calculated on an individual basis as well as opening the possibility ofsimultaneous diagnosis and therapy – these theranostics remain a holy grail.Nanoparticles, such as core-shell silica coated magnetic nanoparticles,gold nanoparticles or quantum dots have become very attractive for bio-logical and medical applications because of the progressions in methods fortheir synthesis, coatings and analysis.15 There are various fields within thebiosciences where nanoparticles can be very useful, such as tissue engi-neering; drug, radionuclide and gene delivery; magnetic resonance imagingcontrast enhancement; hyperthermia; detoxification of biological fluids; cellseparation; tissue repair and magnetofection.16–18

In this review, several examples for the use of nanoparticles involvingtransition metal or gallium or indium complexes for biomedical applicationswill be discussed The major disadvantage of most medical treatments isthat they are non-specific The damaging side effects of therapeutic remediesare caused by their administration: they are not targetted specifically, butemploy general distribution systems This makes direct drug delivery themost promising application of magnetic nanoparticles Nanoparticles arecapable of carrying pharmaceuticals on their surface, and by applying anexternal magnetic field, the drugs could be directed to the target organ for

reaching their target, encapsulation within nanoparticles is an attractiveform of drug delivery for release into tumours.14

Magnetic nanoparticles (MNPs) are of particular interest due to theirpotential firstly to enable imaging that unlike gadolinium chelates do notrapidly accumulate in the liver, secondly to act as a drug targeting systemand thirdly that they can be covered with biocompatible coatings preventingthe body’s innate immune system from attacking the drug carriers Addi-tionally, with the use of an external magnetic field and gradient, it is possible

to confine the particles to a designated tissue area.20Their use in MRI is stillunder consideration, however, thanks to new methods of particle synthesis,functionalisation, coatings and analysis, MNPs are even more attractive forall kinds of medical applications in the future For a recent review onmagnetic nanoparticles in theranostics see Ref 21

This review emphasises recent developments of luminescent metal plexes (including the gallium, indium and the transition elements) forimaging in vitro and/or in vivo, whilst highlighting multimodal imaging,theranostics (combined ‘all-in-one’ diagnostics and therapeutics) andselected examples from coordination chemistry and nanotechnology cov-ering molecular imaging probe design and testing

com-Transition metal-based imaging and therapy probes

Iridium Iridium(III) complexes are of great interest due to their highphosphorescence, which is as a result of the rapid intersystem crossing from

a singlet state to a triplet state due to the 5d electronic configuration.Thanks to the largely ligand-based phosphorescence origin of Ir(III) com-plexes, the emission wavelength is tuneable, leading to a large array ofapplications in addition to those in bioimaging.22Iridium complexes displaylarge Stokes shifts, long lifetimes and limited photobleaching when com-pared to organic fluorophores Despite this until recently few iridium(III)complexes were reported to enter cells

Yu et al developed two cationic iridium polypyridine complexes in 2008for cytoplasmic imaging, with low cytotoxicity and with emission in greenand red respectively, both displaying internalisation in the cell.25 Subse-quently iridium(III) polypyridine indole complexes showing high cytotoxi-city and uptake in cells was demonstrated by Lo et al in 2009.26From 2010onwards there have been an growing number of iridium(III) complexesdeveloped and entering cells, with increasingly interesting properties andpotential within this field Li et al developed cationic iridium(III) complexesdisplaying low cytotoxicity as phosphorescent cytoplasm imaging agents,possessing variable emission properties by way of varying the ligandstructure.27 It was possible to achieve colours from blue to red purely bymodifying the pyridine coordinate, with further modifications carried out toattain a NIR probe The large Stokes shifts, exclusive cytoplasmic uptakeand insignificant cytotoxicity bring excellent potential to enhance colocali-sation studies; since the tunable properties ensure the choice of a probe withnon-overlapping emission Furthermore, Williams et al reported an iridiumcomplex, 1 distinguishable from standard organic dyes using a 10 ns delay inlaser pulse and acquisition (see Fig 2).28An iridium(III) complex developed

by Li et al., could luminesce upon entry to the nucleus by way of a

molecular transporter via a reaction-based mechanism.29 This allowsselective and rapid nuclear imaging of live cells with very low cytotoxicity atthe concentration required for imaging Notably, zwitterionic iridium(III)complexes have also been developed and displayed uptake in cells.30 Fur-thermore, photoswitchable iridium complexes were designed that can rever-sibly switch between an open and a closed from when irradiated with light.31Iridium complexes have recently been investigated as luminescent sensorsable to monitor the variation of homocysteine and cysteine levels in cells (ofsignificance to the physiological balance in biology), in this case using acationic iridium(III) complex.32Furthermore, zinc ion sensing in vitro waspossible via a family of cyclometallated iridium(III) polypyridine com-pounds including a di-2-picolylamine.33

Li et al demonstrated the capacity of an iridium complex, unusual in that

it did not includ a pyridine structure, to able to monitor the levels of Hg(II),which was found to be proportional to phosphorescence emission withincells.34 Interestingly, iridium complexes for which luminescence was quen-ched by oxygen were designed for hypoxia imaging in vivo (see Fig 3).35Compound 3 could even be detected within the tumour 6 to 7 mm from theskin surface

The bioconjugation of iridium complexes remains currently underintense exploration For example, two new, cyclometalated iridium(III) andrhodium(III) bis(pyridylbenzaldehyde) complexes were designed by Lo

et al.in 2010 both with and without biotin tags, with their uptake followed

in HeLa cells.36Furthermore, cell-penetrating peptides conjugated to Ir(III)

(b)

1

Ir N

pub-(a) (b) (c) (d)

N

S Ir O O

N

S Ir O O

COOH

Fig 3 Iridium complexes reported by Takeuchi et al where increas ed fluorescence indicates hypoxic tumours in nude mice, where for compound 2 (left) l ex=445–490, l em=580 nm and compound 3 (right) l ex=575–605 nm, l em=645 nm 35

phenylpyridine complexes showed cytoplasmic and vesicular uptake,mitochondrial and nucleoli targeting was achieved using dual-functionalpeptide [Ir-HTat], 4 and [Ir-P450dHTat], 5 and respectively (see Fig 4).37

A peptide-labelled iridium complex was synthesised and successfully found

to visualise, using fluorescence lifetime imaging (FLIM), the expression of

a chemokine receptor, a G protein-coupled membrane receptor with afunction in metastatic spread of cancer.38

Two cyclometallated iridium(III) complexes were developed by Li et al.,

was exploited by inserting the complex into polymer nanoparticles andvisualised in epidermal carcinoma (KB) cells Furthermore, an excitingdevelopment towards multimodal imaging and with theranostic potentialwas carried out by Hsiao et al using multi-purpose silica-coated iron oxidenanoparticles functionalised with an iridium complex for MRI, in vitro

Rhenium and Technetium.99mTc is a metastable gamma-emitting isotopewhich was first isolated in 1959 and played a crucial role in medical diag-nosis as it paved the way to metal-based radiopharmaceuticals applications

applications in imaging such as of the brain, heart, liver, kidney and boneimaging and is the most commonly used radioisotope for SPECT Despitethis,99mTc hinders the binding of organ specific pharmaceuticals, due to itsnon-physiological nature Since there is no stable isotope of technetium, for

in vitro luminescence imaging it is possible to make use of isostructural

As highlighted above, iridium(III) and rhenium(I) polypyridine plexes are of particular relevance as sensors, due to good quantum yieldsand especially so since the high environmental sensitivity of rhenium(I)polypyridine complexes was reported.23,24Rhenium(I) complexes have longluminescent lifetimes and significant Stokes shifts, making them highlysuitable as in vitro probes Coogan et al have developed numerous

Notably reporting in 2011 a Re(I) complex that can act as a carrier of ionssuch as silver and copper The unfilled form of the complex does not entercells, however in the case of Agþ filling it can enter the nucleoli.46 Inter-estingly, dinuclear tricarbonyl rhenium(I) complexes appended to peptide

(a) (b) (c) (d)

HRKKRRQRRR Ir

H

N N N

NH HN

MLAKGLPPKSVLVKGGH

Fig 4 [Ir-HTat], 4 (left) and [Ir-P450dHTat], 5 (right) incubated in HeLa cells and imaged by confocal microscopy, where yellow indicates co-localisation of nucleoli & vesicular structures (with Rhodamine B-Tat) and MitoTracker respectively 37 In each case, green corresponds to the iridium complex.

nucleic acid showed rapid cell uptake, low cytotoxicity and have the ability

to distinguish between the nucleus and cytoplasm via different excitation/

complexes have also been developed by Lo et al for metal ion sensing

in vitroand displayed increased luminescence emission and a larger lifetimeupon Zn(II) or Cd(II) binding.47

Rhenium(I) complexes suitable for bioconjugation and fluorescence

Interestingly, rhenium(I) complexes with an appended a–D-glucose weredeveloped with the potential as glucose uptake monitors, showingmitochondrial uptake and cytotoxicity that did not depend on cell type.50Polypyriderhenium(I) bis-biotin complexes were observed in HeLa cells bylaser scanning confocal microscopy.51,52 Subsequently, rhenium complexeswith polylactide conjugates displayed cell uptake in A2780 cells.53

As mentioned above, isostructural Re/99mTc complexes can be developedfor in vitro and in vivo investigations respectively For example, Pelecanou

et al.designed Re and99mTc complexes incorporating the [M(CO)3(NNO)]unit covalently attached to 2-(40-aminophenyl)benzothiazole (an anticanceragent) for theranostic applications.55Recently they explored new Re and Tccomplexes of the same family with those highlighted above, by optical andSPECT imaging respectively, demonstrating greater uptake in cell lines ofcancerous origin with respect to non-cancerous lines.56The first instance ofsubstituting a well established chelator with a 1,2,3-triazole analogue for

was reported by Mindt et al in 2008.57The isostructural Re/99mTc folic acidanalogues were synthesised using a Cu(I) catalysed cycloaddition method,known as a ‘‘click reaction’’ that allowed chelation and bioconjugation inone step, which the authors named ‘‘click-to-chelate’’ Moreover, Mindt

from a single folic acid based precursor, using67Ga,111In and99mTc agents

N N (OC)3Re

Cl Re(CO)3

Cl

T = Thymine PNA monomer 6

Fig 5 A bimetallic Re(I) compound 6 (d) in the nucleus (a) and in the cytoplasm (b), where (c) is an overlay of (a) and (b) 42

for SPECT, Cy 5.5 for optical imaging and F for PET The In-DTPAfolate complex has recently been reported as with the capacity to quantifymacrophage activation.59The authors demonstrated that the later stages ofosteoarthritis can be correlated to reduced macrophage activation, allowingmonitoring of the disease progression, for which there are no clinicalmeasures at present Furthermore the same group has used click reactions

to design tridentate di-1,2,3-triazole chelator imaging tracers and also

potential purposes including multimodal imaging probe development.60,61This efficient and facile synthesis combined with uptake in folic acidreceptor expressing KB cells and tumour targetting in mice confirms thepromise of this procedure

Rhenium and technetium complexes have also been designed to enter thenucleus and bind to DNA, an example of which was by Santos et al whosynthesised tricarbonyl pyrazolyl-diamine rhenium(I) complexes that showpotential for the development of future targeted radiopharmaceuticals.62Furthermore, tricarbonyl rhenium and technetium complexes with acridinederivatives showed nuclear uptake via fluorescence and activity based

technetium complexes comprising of a DNA interchelator for nucleartargeting, a biologically active molecule (here a bombesin analogue, 7) and alinker, cleavable upon cell entry displaying uptake in both the nucleus andthe cytoplasm (see Fig 6).54,64

Multimodal imaging probe are also under development, such as

technetium complex was designed in 2011 for dual modal fluorescence/SPECT imaging and also has potential for therapy via 188Re.66

An interesting nanocomposite with potential application in multimodalimaging was reported by Hafeli.67 Silica coated magnetite nanoparticleswere modified with an amino silane coupling agent (N-[3-(trimethyoxysilyl)propyl]-ethylenediamine) and histidine This enabled the radiolabelling

The stability of the synthesised nanocomposite was also shown in in vitroexperiments The authors suggested theranostic application of thisnanocomposite, that it could be used in magnetically targeted cancer

O ORe (CO)

N O H O N O H O

NH

O NH O

7

Fig 6 A rhenium tricarbonyl complex conjugated to a bombesin analogue and an calator, compound 7, visualised in fixed PC-3 cells by fluorescence microscopy (left), where green represents the complex and blue the DAPI nuclear stain.54

inter-radiotherapy and also as a dual-modal imaging agent Radioactive netic nanoparticles, a potential tracer for diagnosis in nuclear medicine,were described by Kim et al Radiolabelling was conducted usingtechnetium pertechnetate (99mTcO4 

mag-) and then alginic acid was adsorbed

on the particles.68

Ruthenium The majority of fluorescent ruthenium complexes reported todate do not contain a metal-carbon bond, however a small number ofluminescent ruthenium organometallic complexes with uptake in cells havebeen reported A di-carbonyl tris(2.20-bipyridyl) ruthenium(II) chloridecomplex was encapsulated within the hydrophobic supercages of a zeolite.Fluorescence quenching by dissolved oxygen was monitored as a function ofconcentration and was demonstrated in vitro in macrophage cells.69Sevenorganometallic porphyrin complexes (for example compound 8), five of whichruthenium and one with analogous iridium and rhodium complexes, were

complexes localised in granular structures within the cytoplasm (Fig 7),comparing well to the uptake demonstrated by the porphyrin ligand itself.Furthermore, excellent phototoxicity was observed for all ruthenium com-plexes indicating real potential for the combination of both chemotherapeuticand photodynamic activity against cancer There have been some very inter-esting examples of ruthenium complexes from coordination chemistry, some ofwhich will be discussed here There have recently been a small number ofruthenium complexes designed for the purpose of nuclear uptake and DNAbinding, for example Palaniandavar et al developed a [Ru(phen)2(dppz)]2+

intense staining of the nucleus, showing potential to challenge commercial dyessuch as Hoechst.71A ruthenium beta-cyclodextran complex shown to trans-locate DNA and on the basis of its ability to aggregate DNA was designed foruse of inhibition of DNA enzymes (such as topoisomerase and Hind III).72

A new ruthenium(II) complex attached to a porphyrin was reported toshow potential since it can be imaged and activated therapeutically using

b–carboline ruthenium(II) complexes that are intrinsically fluorescent, enter

N HN N NH

N N

Ru Cl Cl Ru

Cl Cl

Cl

Cl Cl

Cl

8

Fig 7 Fluorescence microscopy of an organometallic ruthenium porphyrin complex (right) in Me300 cells, where blue, green and red correspond to a DAPI stained nucleus, lysotracker green and the ruthenium complex.70

cell nuclei and initiate autophagy and apoptosis in cells Moreover tworuthenium(II)-porphyrin complexes displaying cell uptake observable viafluorescence microscopy were synthesised by Liu et al These have thecapacity to reduce the generation of reactive oxygen species and as a resultactivate apoptosis in a human hepatoma cell line.75

A family of octahedral ruthenium(III) complexes were developed byTra´vnı´cˇek et al and investigated in several cell lines and in mice for cyto-toxicity and antitumour activity respectively.76 Whilst the complexes werenot found to be cytotoxic in vitro, the in vivo evaluation displayed superiorresults when compared with NAMI-A, a related imidazole-containingruthenium complex that has entered clinical trials

An attractive example of dual functional agents for biological imagingapplications was reported by Sun et al A fluorescent ruthenium complexwas immobilised on to the surface of magnetic nanoparticles via 3-(3,4-

polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol

Nanocomposites prepared using aforementioned methodology exhibitedgreat colloidal and photochemical properties The quenching of the ruthe-nium complex 9 (Fig 8) was prevented by polymer coating No cytotoxicity

to the SK-BR-3 cells was observed, indicating good suitability as adiagnostic

Zirconium The long half life (78.4 h) of 89Zr, a positron emitter, is ofparticular interest to immuno-PET since it can enable the longer reactiontimes required for radioimmunodiagnostic applications and has beenexplored for its potential to act as a theranostic pair with 90Y and 177Lu

desferrioxamine B (N-sucDf) with p-benzyl azacyclododecane-1,4,7,10-tetraacetic acid (p-SCN-Bz-DOTA) and p-iso-thiocyanatobenzyl diethylenetriaminepentaacetic acid (p-SCN-Bz-DTPA)for comparisons with 177Lu and88Y (was used in the place of 90Y).78Thedata showed that the zirconium complexes could accurately predict thelutetium and yttrium biodistribution Subsequently, van Dongen et al

isothiocyanate-1,4,7,10-tetra-Ru

N N

N N COOH COOH

HOOC

COOH

NH

NH S

S PPG-PEG-PPG

PPG-PEG-PPG HN

HN O

O

O

O O O

N H H N

9

Fe3O4

Fig 8 Iron oxide nanoparticles 9 developed by Sun et al., labeled with a fluorescent ruthenium complex; reproduced from ref.77

compared the biodistribution of Zr to that of Y each conjugated toZevalin (an anti-CD20 murine monoclonal antibody of ibritumomab tiux-etan) Yttrium was chelated by MX-DTPA, with zirconium chelated byN-sucDf, but with the presence of an empty MX-DTPA Very similar bio-distribution was observed with the exception of bone and liver, suggestingthat89Zr-Zevalin could be a useful tool in the prediction of90Y-Zevalin.79Recently, immuno-PET and optical imaging were combined via Cerenkovluminescence imaging (CLI) in vivo using the labeled antibody 89Zr-DFO-J591.80 Cerenkov emission works on the principle of the electronic excita-tion caused by a charged particle travelling faster than the speed of light of adialectric medium, emission subsequently occurs when these moleculesreturn to the ground state Grimm et al reported the first instance oftumour uptake that could be quantified by CLI and PET indicating thefuture potential of this technique and the benefit of its future developmentand dual-modal imaging (Fig 9).

An intresting theranostic example of zirconium application was recently

des-ferrioxamine B [DFO] was synthesised and tested Such constructed composite was able to deliver therapeutic and imaging radionuclidespecifically to the vessels of a solid tumour and destroy it

nano-Platinum and Palladium Since the anti-cancer properties of cis-platinwere discovered in 1965 there has been a global interest in the development

of new platinum complexes A method of monitoring uptake of cis-platinanalogues used by Tanke et al in 2000 relied on the principle of a con-jugated inactive fluorophore that upon DNA binding would be activatedand therefore would ‘‘light up’’.82 Whilst this technique provides someinsight as to uptake and metabolism the anti-cancer activity and biologicalproperties these complexes could be altered Furthermore, Almendral et al.designed two platinum chloride cholylglycinate complexes possessingfluorescence, which was enhanced upon binding to DNA and investigated

Fig 9 In vivo optical imaging of mice using Cerenkov radiation 96 hours after the tration of the radiotracer89Zr-DFO-J591.80

adminis-using Flow Injection Analysis.83Interestingly Che et al developed a nescent platinum complex that possessed very significant cytotoxicity andselectivity for cancer cells, which unusually was not for DNA targeting, butinstead was found to locate predominantly in the cytoplasm showing closelocalisation with mitotracker.84

lumi-Notably a small number of cyclometalated platinum complexes haverecently been developed for two-photon microscopy in cells.85,86 In 2008,Botchway et al designed cyclometalated platinum complexes for two-photon excitation in cells with good photostability and low cytotoxicity.The complexes (for example compound 10) could rapidly enter cells and

platinum complexes with potential for photodynamic therapy of tumourswere developed by Selke et al and imaged by fluorescence microscopyshowing some uptake in the mitochondria and the nucleus.88

Furthermore, two-photon fluorescence microscopy was used to showthat a cyclometalated platinum(II)-rhodamine complex 11 can act as aselective indicator for mercury(II) ions in aqueous solution, includingwhen incubated in cells (Fig 11).89 Luminescent cyclometalated platinumcomplexes were also designed for potential in photodynamic therapy, with a2-(20thienyl)polypyridyl complex synthesised by Selke et al displayingpotent photoactivity and localisation in mitochondria and the nucleus.88The development of second generation of imaging agents coupled withbiomolecules and/or monoclonal antibodies (BAMs) and their incorpora-tion into nanostructures is in progress A primary monoclonal antibody forICAM-1 was labeled with platinum and showed a quantitative increase influorescence when incubated with tumour necrosis factor a (TNF-a) Incombination with terbium conjugated to a b1-integrin antibody (unre-sponsive to TNF-a) and europium appended to an E-selectin antibody(quantitative response to TNF-a) these complexes could be developed fur-ther for automated screening in the drug development process.90Further-

incorporated into phospholipid micelles of ca 100 nm in diameter andretaining phosphorescent properties The probe was found to be suitable foroptical imaging of tumours in vivo.91

Pt

High intensity

Low intensity

Mesoporous silica nanoparticles were functionalised with a NIRfluorescent marker, ATTO647N, a palladium-porphyrin and a cyclic RDG

sensing and enables photodynamic therapeutic applications, whilst thepeptide ensures uptake in cancer cells, which could then be monitored by

in vitrofluorescence (Fig 12) These tri-functional nanoparticles show realpotential as theranostics due to their specific binding to avb3 integrin andhence their strong therapeutic effect

Furthermore, Inouye et al reported on the synthesis of water-soluble,blue-emitting platinum nanoclusters.93 These [Pt5] nanoclusters have an18% quantum yield in water, and were used to label chemokine receptors inliving HeLa cells Recongnition was achieved via the binding of the [Pt5]cluster to an antibody through a conjugated protein and cells were imagedusing fluorescence techniques The authors proposed this material as auseful fluorescent probe for bioimaging and subcellular targeting

Manganese Tricarbonyl cymantrene-peptide conjugated manganesecomplexes were synthesised by Neundorf et al., showing cytotoxicitytowards cancer cells.94 Attachment of carboxyfluorescein enabled in vitrofluorescence uptake to be observed, which indicated localisation in thelysosome Furthermore, Lippard et al synthesised dipicolylamine 5-phenyl-10,15,20-tris(4-sulfonatophenyl) porphyrins with an outlook towards dualfluorescence/MRI detection of zinc.95 The free ligand acts as a powerfulfluorescence ‘‘turn on’’ zinc sensor, whereas in the presence of zinc,

(a) (b)

N N Pt

NH S

NH N O

O N N

N N Pt

NH

N N O

O N N

–HgS

Fig 11 A platinum(II) complex (a) showing selectivity for Hg 2 þ (b) in HeLa cells by photon fluorescence microscopy, where (c) is before and (d) is after addition of mercury.89

two-the manganese complex magnetic relaxation is modified, reducing two-the T1effect, whilst amplifying the T2 effect Interestingly, a manganese tris-(1-pyrazolyl)methane) tricarbonyl chloride complex was designed forlive cell raman microspectroscopy imaging, without addition of a label(Fig 13).96Authors suggest a common limitation of fluorescence microscopy

in that compounds of interest can require the addition of a fluorophore.The carbonyl vibrations in the region of 2000 cm1could be detected withinthe cell indicating the future of applications with metal complexes usingthis modality

Fig 13 An HT29 colon cancer cell incubated with a manganese carbonyl complex, visualised A) by optical imaging, B) Raman imaging of integration range ca 2800  3050 cm1, C) Raman imaging of integration range 1945  1965  2 cm 1 , D) is an overlay of B and C, E–G) are cross-sections of the above Raman images 96

(a)

(b)

Fig 12 The method of development of A647@MSN-RGD-PdTPP (reproduced with permission from Ref 92) (a) and visualised in MCF-7 cells where blue represents Hoechst nuclear stain and red the nanoparticles (b) Image reproduced with permission from Ref 92.

The synthesis of a new MRI contrast agent with fluorescence properties,based on manganese was reported by Yang et al.97Silica-coated manganeseoxide nanoparticles were synthesised and funtionalised with amino groups,which enable the covalent conjugation with a fluorescent dye, Rhodamine Bisothiocyanate (RBITC) It was mentioned that this nanocomposite waswater-dispersible, stable, biocompatible and was able to target cancer cellsspecifically The authors suggested that the material could provide a plat-form for bimodal imaging.

Cobalt New molecules with potential for boron neutron capture therapyincorporating porphyrins conjugated to boron-containing cobalt complexeswere synthesised via the Sonogashira reaction.98 The uptake of the com-plexes was followed by laser scanning microscopy demonstrating entry intolung adenocarcinoma cells as well as low cytotoxicity – an essential feature

in absence of neutron flux Interestingly, Hambley et al developedcoumarin based fluorescent ligands, whereby upon chelation of cobalt(III)

nuclear uptake was observed, which was not for the respective ligandindicating that the complex stability was sufficient for nuclear entry.Dissociation resulted in the observed fluorescence Conversly, only very fewcobalt complexes have been investigated in cells by luminescence micro-scopy However there is a growing trend towards use of cobalt basednanoparticles, highlighting the emergence of this metal in molecularimaging, and several examples are highlighted below

A small number of dual-modal fluorescent/MR active probes werereported recently, a particularly interesting example of which by Sung et al.,where in vitro and in vivo uptake in stem cells of silica coated cobalt ferritemagnetic nanoparticles containing rhodamine B isothiocyanate was

localisation in stem cells injected in nude mice indicating very good potentialfor these probes as contrast agents Furthermore, in 2010 cobalt ferritenanoparticles possessing a sequence enabling binding to microRNAs wereexamined in the mouse cell line, P19 and in nude mice This work showed forthe first time the in vivo neuronal differentiation dependent on fluorescence

Fig 14 Cobalt ferrite magnetic nanoparticles visualised in human stem cells by microscopy (a) and in nude mice using fluorescence (b) where the darkest areas above indicate presence of stem cells.100

quenching The authors suggest future applications of these ials to facilitate the observation of in vivo target gene expression, opening up

nanomater-an exciting nanomater-and broad rnanomater-ange of potential biomedical applications

Reecntly, iron cobalt graphitic-carbon nanocrystals were developed byMcConnell et al for in vivo fluorescence and MR imaging This enabledmonitoring of vascular macrophages, with future potential for theranostics

of vascular inflammation.102

Zinc Zinc(II) is a d10metal ion with coordination geometries ranging forexample from tetrahedral, square pyramidal or trigonal bipyramidal thusgiving rise to versatile probes for the development of luminescence imagingusing transition metal complexes: some recent developments will be high-lighted hereby.103A series of zinc salen complexes (for example 12) weredesigned by Xu et al and investigated by single and two-photon microscopy

in cells displaying colocalisation in the lysosome, endosome and ER as well

as excellent photostability and low cytotoxicity (Fig 15).104

in live cells for oxalic acid, presence of which resulted in quenching

Histological studies demonstrated that the complex could distinguishneurofibrillary tangles of hyperphospohylated tau proteins and amyloidplaques in the hippocampus of a patient who had Alzheimer’s disease.Keppler et al studied triapine, which is under investigation as an anti-cancer agent, by fluorescence microscopy The corresponding zinc complex[Zn(Triapine)Cl2] HCl was also synthesised and imaged in colon carcinomacells to show uptake in nucleoli.107 The unusual attribute of intrinsicfluorescence of this particular anticancer drug may help to facilitatepersonalised medicine via monitoring the uptake within patient biopsysamples and subsequent morphological changes

In 2005, trimetallic zinc meso-to-meso ethyne-bridged zinc(II)] complexes were designed as NIR fluorophores with potential for

CN NC

N N Zn

in vivo imaging; and showed uptake by fluorescence in murine melanomacells.108Furthermore, near IR in vivo fluorescence imaging of cell death hasbeen possible via a bimetallic zinc(II)-dipicolylamine (Zn-DPA) complex, 14conjugated to a ligand with affinity for phosphatidylserine, which is present

on dying and dead cell surfaces (Fig 17).109When compared to Vivo 750 the complex possessed higher target to non-target ratio on account

Annexin-of lower bladder uptake, giving complementary information as two probesand good clearance indicating potential for radiolabelling studies due to aresultant reduced radiation exposure.110Further, this probe shows promisefor the assessment of patient response to therapy as well as evaluation of novelanticancer treatments The probe has potentially broader applications and wasdemonstrated as possessing the capacity to detect tissue infected by

of which the hydrophilic variants are non-toxic to mammalian cells yet toxic to

S aureus, including antibiotic resistant strains Moreover, the uptake of afluorescent analogue was imaged in E coli and S aureus.112Hamachi et al.developed Zn-DPA complexes containing four zinc nuclei and demonstratedthat their D4 tag/Zn-DPATyr pair can successfully label proteins.113

New Zn-DPA complexes were developed by Hamachi et al for therecognition of nucleoside polyphosphates (including ATP), whereby

(b)

(a)

O N (CH 2 ) 4 SO 3 H

Zn 2+

+ –

4 NO3

14

Fig 17 Near-IR fluorescence image of tumour cell death after radiation therapy (a) as a result

of treatment with imaging probe 14, (b) Image reproduced with permission from Ref 109.

N N

O HN

fluorescence is activated by presence of the desired compound via cence resonance energy transfer (FRET) yielding a ratiometric response(Fig 18).114 A zinc(II) dipicolylamine complex with ATP sensitivity andconjugated to nanoparticles with a silica core was imaged in fixed non-cancerous rat epithelial (NRK) cells with potential use in metabolicstudies.115 Furthermore, targeted cancer imaging of oral epithelial cancerwas reported by Menon et al via the conjugation of a mannose ligand tochitosan-zinc sulphide nanocrystals.116

fluores-Copper Radionuclides of copper include60Cu,61Cu,62Cu and64Cu, whichare all positron emitters, and67Cu, a b emitter Copper-64, with a half-life

of 12.7 h, decays 41% by electron capture, 40% by b and 19% by bþ,117making this radioisotope highly appropriate for simultaneous diagnosis andtherapy Despite this, a challenge to synthesis, characterisation and fluores-cence imaging of dual-modal imaging agents incorporating copper(II) com-plexes is the paramagnetic behaviour occurring due to the d9electron count

A significant complex for molecular imaging, aliphatic diacetyl-bis (N(4)-methylthiosemicarbazone) (Cu[ATSM], 15) has beenshown to be selective for hypoxic tissue and has progressed to phase IIclinical trials for cervical cancer diagnosis (Fig 19) Hypoxic tissue is low inoxygen concentration and has been correlated to cancer, strokes and heartdisease.118,119Early diagnosis of hypoxia would allow a change in treatmentplan and therefore better patient prognosis

copper(II)-There are currently no approved non-invasive diagnostic methods forhypoxic tumours and therefore there is a substantial need for an appropriatemolecular imaging probe The potential for Cu[ATSM] to act as a

Fig 18 Representation of Zn-DPA complexes for sensing of nucleosides via FRET duced with permission from Ref 114.

N H

H S

therapeutic agent was recently investigated by Fujibayashi et al., and was

Mechanism of cell uptake of bis(thiosemicarbazones) can be investigateddue to the weak intrinsic fluorescence of thiosemicarbazones The first

in vitrofluorescence study of zinc bis(thiosemicarbazones) in human cancercells was carried out in 2005 by Dilworth et al using the Zn[ATSM]analogue in a number of cell lines.121Cu[ATSM] is not fluorescent thereforeand zinc analogues have been used as models for Cu[ATSM] in vitro SinceZn(II) and Cu(II) are isoelectronic the mechanism of cell uptake can beexpected to be similar Recently a series of new zinc bis(thiosemicarbazo-nato) complexes were reported showing in vitro fluorescence uptake incells.122Copper and zinc bis(thiosemicarbazonato) complexes incorporatingpyrene and styrene appended as the fluorescent tags have been proposed ashaving potential for dual modal imaging.123,124

Furthermore, Ghosh et al developed a copper(II)-fluorescein complex(not pertaining to the thiosemicarbazonato family) for the imaging of nitricoxide.125 On the other hand, Pascu et al have successfully synthesisedcopper bis(thiosemicarbazone) complexes, without using conjugated lumi-nescent tags due to intrinsic fluorescence according to the addition of a 1,8-napthyl backbone and showed the first images of copper bis(thiosemi-carbazonato) complexes in cancer cells (Fig 20).126,127

Bioconjugation to copper-64 complexes is currently under investigationwith some examples discussed here Glucose was appended to bis(thio-semicarbazonato) complexes by Christlieb et al., retaining hypoxia

selectivity yet not displaying significant uptake in the heart and brain as in

bombesin conjugates were developed displaying binding to PC-3 cells andappear suitable for PET applications Furthermore, Bayly et al recentlycoupled a nitroimidazole to a copper bis(thiosemicarbazonato) complexwith a purpose of enhancing the hypoxia selective of the complex.129Theconjugates displayed superior selectivity and reduced non-target uptakewhen compared with propyl derivatives used as controls and CuATSM/Aderivatives respectively

Receptors for the E coli enterotoxin, STh, are often expressed in ectal cancers; analogues of which have potential for imaging and therapy

color-A STh analogue was conjugated to DOTcolor-A, TETcolor-A and NOTcolor-A at the

tumour uptake with respect to organs, when investigated by biodistribution

icosane-1-ylamino) methyl) benzoic acid (AmBaSar), a cage like tional chelator; was synthesised in high yield and showed increased stability

visualisation of PC-3 xenografts.132Moreover, Maecke et al developed andevaluated 4 new64Cu and68Ga complexes with somatostatin antagonist p-Cl-Phecyclo(D-Cys-Tyr-D-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)D-Tyr-NH2.133There are a number of recent examples of labeling nanoparticles

nano-composties for dual/multimodal imaging Jarrett et al developed the way

oxide nanoparticles The labeling was achieved via coordinating the 64Cu

(S-2-(4-iso-thiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)and the subsequent covalent attachment to the nanoparticle.135

Lee et al described amino modified magnetic nanoparticles and coupled

to cyclic arginine-glycine-aspartic (RGD) peptides for integrin avb3 geting and macrocyclic DOTA chelators for PET and labeled with64Cu.136

ends of PEG arms attached to superparamagnetic iron oxide nanoparticleswere developed by Gong et al., with an anticancer drug conjugated via pH-

Fig 21 Multifunctional supermagnetic nanoparticles for MRI/PET and drug delivery (Ref 134).

especially for peptide-conjugated nanocarriers and with good tumourcontrast Despite notable liver uptake these multifunctional carriers possesspotential for combined therapy and PET/MRI imaging.

Nanoparticles incoporating [64Cu]CuS were designed by Li et al forPET imaging and photothermal coupling These showed potential as bothdiagnostic agents according to good tumour uptake and renal clearance and

as therapeutic agents due to successful photothermal ablation of U87 cancercells.137 For another notable example of loading nanoparticles with 64Cuwith potential for acting as a drug carrier in addition to an imaging agentsee Ref 138

Chen et al developed iron oxide nanoparticles coated with human serum

PET, NIRF and MRI showing high tumour uptake, significant liver uptakeand potential for drug loading as a theranostic agent (Fig 22).139

Main group metal-based imaging and therapy based probes

Gallium The PET radioisotope 68Ga, with its 68 minute half-life, has amajor advantage in that it is generated from68Ge (half-life of 270.8 days) isrelatively simple to obtain via commercial generators and does not requirethe availability of a cyclotron There are two additional gallium isotopes ofinterest for molecular imaging:66Ga (9.5 hour half-life), which is a positronemitter and 67Ga (3.26 d half-life), a gamma emitter.140 Gallium-67 wasused in scintigraphy141 for imaging of a number of conditions including,among others, towards the diagnosis of Hodgkin’s or non-Hodgkin’slymphomas For recently reported examples of67Ga scintigraphy see Refs.142–145 There have been a number of bioconjugated gallium complexeswith an outlook towards theranostics, some examples of which will bediscussed here

A DOTA-GlyGlu-Cyc lactam bridged cyclised a-melanocyte stimulatinghormone peptide was radiolabelled with67Ga and allowed visualisation of

Fig 22 Trimodal imaging via PET/MRI/NIRF imaging, a drug could be loaded using the same method as dopamine (used to modify the hydrophilicity, Ref 139).

primary and metastatic melanoma, showing potential as a theranostic agentsince the versatility of DOTA could allow chelation of a radiotherapeutic

tumour uptake in PC-3 xenografted nude mice, where67/68Ga can act as adiagnostic radionuclide and177Lu as a therapeutic.147 68Ga DOTATOC wasutilised as a diagnostic and indicated the necessity for177Lu-DOTATATE

Moreover, a patient with very significant uptake of67Ga was treated withgallium maltolate, which appeared to cause tumour necrosis and increasepatient mobility and quality of life.149

A quadruple nanoparticle imaging system based on67Ga was reported byKim et al.150Rhodamine modified, silica coated cobalt–ferrite nanoparticlewere decorated with luciferase (MFB) and p-SCN_bn_NOTA (2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclonane-1,4,7-triacetic acid) followed

by 68Ga incorporation This example, together with related nanoparticlesstudies reinforce the view that such systems could become promising mul-timodal imaging agents for bioluminescence, fluorescence, MRI and PETimaging techniques.151

Indium.111In is a relevant radionuclide for SPECT imaging, with a life of 2.8 days and photon energies of 171 and 245 keV Particular appli-cations to dates are in tumour-targeted imaging

half-There have been a number of reported peptide/antibody targeted smallmolecules involving indium-111, therefore some selected examples will bediscussed, in particular those including multimodal imaging or theranostics

N-[[4-[[[2-ethylphenyl)amino]carbonyl]amino]phenyl]acetyl]-(1-amino-1-cyclohexane)carboxamide (LLP2A) was coupled to DOTA and

How-ever, the drawback of renal retention would require further modification,

simultaneous imaging and therapy Glucagon-like peptide receptor imaging

receptor status and the imaging of benign insulinomas, which contrary tomalignant insulinomas have been known to lack the somatostatin receptorsubtype 2 (sst2) and to express GLP-1 receptors.153DOTATE radiotherapycould be applied to the patients with sst2 suppressing tumours Ibritumo-

scans possessing the capacity to predict disease progression and could be auseful tool to tailor therapy towards more aggressive therapeutic strate-

with111In and90Y, which showed high tumour uptake due to specificity for

avb3integrin and therefore promise as a theranostic pair.155

designed by Gruaz-Guyon et al for the purpose of targeted radiotherapy

tumour and renal uptake than the DOTA-LB119, with very low ground in tissues in exception of the kidney Yttrium displayed greateraffinity than indium for DOTA-NT, indicating potential for tumour tar-geting as a radiotherapeutic.

back-Indium complexes are currently being developed for multimodal imaging

In 2006 Li et al designed a dual-labelled probe for tumour imaging,111DTPA-Lys(IRDye800)-cyclic(KRGDf), which bind to integrin avb3, pre-sent in cancerous melanoma cells.157Tumours could be viualised using NIRoptical imaging, with good resolution and sensitivity, which was compli-mented by the tissue penetration of g-scintigraphy (Fig 23) Furthermore,

In-in 2007, another cyclic peptide was applied for combIn-ined near In-infraredfluorescence and SPECT imaging of tumours, allowing unambiguousvisualisation of the tumour by both modalities.158

Pascu et al developed intrinsically fluorescent gallium and indiumbis(thiosemicarbazonato) for rapid radiolabelling.159 Uptake in cells indi-cated localisation in mitochondria, lysosomes and additionally for indiumcomplexes in the nucleus, therefore opening up the possibility Auger elec-tron emission therapy via111In, using a complex such as 17 (Fig 24)

Fig 23 111In-DTPA-Bz-SA-Lys(IRDye800)-c(KRGDf) in nude mice visualised by (from left

to right) white light, g-scintigraphy and NIR.157

(a)

17 (b)

Fig 24 An indium bis(thiosemicarbazonato) complex (b) observed in HeLa cells using focal microscopy (a) by Pascu et al., indicating nuclear uptake by Hoechst staining (blue), where green signifies the complex and cyan denotes colocalisation.159

con-Monoclonal antibodies (trastuzamab or cetuximab) were conjugated tochelators (CHX-A00) for the incorporation of radioisotopes (in this case

111

In), the uptake of the respective ligands were followed in LS174T,

anti-bodies for the targeting of tumours via epidermal growth factor receptorshas been carried out by Kobayashi et al allowing for three tumour types

to be distinguished between (EGFR-1, EGFR-2 and a tumour that did

conjugated to a linker and to a fluorophore (Rhodamine Green, Cy5.5 orCy7) and the wavelength of in in vivo emission therefore indicatedresponse to the antibody Additionally Kobayashi et al developed a

utilising antibodies, here panitumumab and separately trastuzumab to

could be activated upon internalisation into the cell giving a good cation of tumours, which was well complimented by the ‘‘always on’’nuclear imaging

indi-Small molecules containing copper-64 and indium-111 have been geted using peptides/antibodies, giving the possibility of both PET and

many malignancies) fab was tested by in vitro binding and cellular

(fab) c-kit was evaluated in vivo by PET, allowing clear tumour sation and indicating use as a tool to enable an informed decision to bemade before beginning c-kit targeted therapy.163 64Cu and111In DOTA-HSA-Z(HER2:342) were studied by PET and SPECT respectively, bothindicating high tumour and liver uptake This ligand is likely to be sui-table for labeling with radionuclides such as90Y and177Lu, which could

bivalent single-chain antibody dimer fragment was attached to a PEG and labeled with 125I, 111In and 64Cu and investigated by biodis-

possible after 22 h, therefore having potential as a theranostic pair with

125

I An example of antibody (IgG) labeled with111In or125I for targeting

of cancer and fab labeled with 64Cu for PET imaging.166

loaded onto nanoparticles An immunoliposome anchored with

selectivity and due to the possibility of loading with a drug, potential for

liposomal drugs where radioisotopes were combined with

were 111In labeled for biodistribution studies.168 The nanotargeted 111vinorelbine liposomes showed increased suppression of tumour growth,reduced toxicity and increased survival rate, indicating that the combination

In-of nanomedicine and molecular imaging shows promise for tumourtargeting