Radiolabelled nanoparticles: novel classification of radiopharmaceuticals for molecular imaging of cancer.

http://informahealthcare.com/drt ISSN: 1061-186X (print), 1029-2330 (electronic) J Drug Target, Early Online: 1–11 ! 2015 Informa UK Ltd. DOI: 10.3109/1061186X.2015.1048516

REVIEW ARTICLE

Radiolabelled nanoparticles: novel classification of radiopharmaceuticals for molecular imaging of cancer Seyedeh Fatemeh Mirshojaei1, Amirhossein Ahmadi2, Enrique Morales-Avila3, Mariana Ortiz-Reynoso3, and Horacio Reyes-Perez3

Journal of Drug Targeting Downloaded from informahealthcare.com by Nyu Medical Center on 06/22/15 For personal use only.

1

Nuclear Science and Technology Research Institute, Atomic Energy Organization of Iran, Tehran, Iran, 2Pharmaceutical Sciences Research Center, Faculty of Pharmacy, Mazandaran University of Medical Sciences, Sari, Iran, and 3Facultad de Quı´mica Toluca-Me´xico, Universidad Autońoma del Estado de Me´xico, Toluca, Mexico Abstract

Keywords

Nanotechnology has been used for every single modality in the molecular imaging arena for imaging purposes. Synergic advantages can be explored when multiple molecular imaging modalities are combined with respect to single imaging modalities. Multifunctional nanoparticles have large surface areas, where multiple functional moieties can be incorporated, including ligands for site-specific targeting and radionuclides, which can be detected to create 3D images. Recently, radiolabeled nanoparticles with individual properties have attracted great interest regarding their use in multimodality tumor imaging. Multifunctional nanoparticles can combine diagnostic and therapeutic capabilities for both target-specific diagnosis and the treatment of a given disease. The future of nanomedicine lies in multifunctional nanoplatforms that combine the diagnostic ability and therapeutic effects using appropriate ligands, drugs, responses and technological devices, which together are collectively called theranostic drugs. Co-delivery of radiolabeled nanoparticles is useful in multifunctional molecular imaging areas because it comprises several advantages based on nanoparticles architecture, pharmacokinetics and pharmacodynamic properties.

Multimodality tumor imaging, nanotargeted radiopharmaceuticals, radiolabelled nanoparticles, SPECT & PET imaging

Introduction The development of nanoparticles has had a remarkable influence on a host of scientific areas, introducing new potential capabilities and functionalities across a wide range of applications. As small structures, nanoparticles are delivery vehicles that can extravasate through the endothelial cell layers and interact with the cell structures of various tissues. But, they are also large enough to transport high payloads of therapeutic or diagnostic agents (e.g. radioisotopes for molecular imaging) [1]. Long-circulating nanoparticles are ideal vehicles for targeted drug delivery because their surface can be functionalized with different types of ligands in a multimeric configuration (Figure 1a); they also have high affinity for receptors that are over-expressed by tumor cells, which could increase the potential for a multivalent interaction with the target (Figure 1b) [2]. Nanomaterials can greatly improve diagnostic imaging qualities even at the level of single cells before the discovery of the earliest signs of disease [3,4]. The combination of nanomaterials with molecular imaging

Address for correspondence: Amirhossein Ahmadi, Pharmaceutical Sciences Research Center, Faculty of Pharmacy, Mazandaran University of Medical Sciences, 18 kilometer of Farah Abad Road, Sari, P. Box: 48175-861, Iran. E-mail: Amirhossein_pharma@ yahoo.com

History Received 13 December 2014 Revised 29 April 2015 Accepted 2 May 2015 Published online 10 June 2015

devices allows recognition and recording of functional, dynamic and spatiotemporal processes at both molecular and cellular levels in humans and other living systems (Figure 1c) [3]. Radiolabeled nanoparticles for molecular targeting are engineered comprising three main components, the core, the targeting biomolecule (which must be able to recognize a specific biological target) and the radiotracer group. The ability of nanoparticles to bypass biological limitations enhances their targeting efficacy [5,6].

Nanoparticles for medical purposes Nanotechnology is an emerging subject in which medicine and engineering closely cooperate with other fields of science, such as physics and chemistry. The application of nanoscience and nanotechnologies has substantial potential advantages in information and communication technologies, electronics, transportation, biology, medicine, pharmacy, production of more potent, stronger and lighter materials and so on [7–10]. The application of nanotechnology to improve human health is called ‘‘nanomedicine’’, which stands for the use of nanoparticles for diagnosis, physical and pathological processes monitoring, as well as for therapy on metabolic control [11]. The main uses of such nanomaterials would be (a) specific targeting of a disease site, (b) recalling information from the site, (c) delivering a therapeutic payload at the site, (d) increasing the patient tolerability and (e) diminishing toxic


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S. F. Mirshojaei et al.

J Drug Target, Early Online: 1–11

Figure 1. (a) Combination of nanomaterials, diverse types of ligand and radionuclides gives a great variety of multimeric devices for molecular imaging. (b) High affinity interaction between ligands and receptors overexpressed in cell surfaces increases the multivalent interaction. (c) Recognition and recording dynamic and spatiotemporal procedures at the molecular or cellular level.

effects of chemotherapy drugs. Such nano constructs that possess bifunctional and multifunctional capabilities would certainly be of medical interest [12]. Over the last three decades, scientists have developed special strategies to synthesize nanomaterials in a systematic way and characterize their unique, size-dependent properties [13–15]. An understanding of these crucial physical and chemical properties is essential for the leading use of nanomaterials in medical applications. Nanomaterials consist of metal atoms, nonmetal atoms or a mixture of these, which are commonly referred to as organic or inorganic particles. The surface of nanomaterials is usually coated with polymers or ligands to improve its biocompatibility and select specific targets [4]. The final size and structure of nanomaterials depend on the salt concentrations, surfactant additives, reactant concentrations, reaction temperatures and solvent conditions used during their synthesis [15]. Nanoparticles used as drug delivery systems are nanosized particles that are smaller than one micrometer and mostly smaller than 200 nm, and devices or systems can be made with different types of materials, such as polymers (polymeric nanoparticles, micelles or dendrimers), lipids (liposomes), viruses (viral nanoparticles), organometallic compounds (nanotubes) and inorganic nanoparticles (fullerenes, carbon nanotubes, quantum dots and magnetic nanoparticles) as well as so-called ‘‘polymer therapeutics’’, such as polymer–protein conjugates, polymer–drug conjugates, polymeric micelles and polymeric drugs [16]. Nanoparticles can be designed and synthesized as multimeric systems to cover multivalent effects; however, multimeric is not synonymous with multivalent, and these interactions are consequence of factors, such as the density of ligands and their receptors, selectivity and binding affinity for cell receptors. The physical and chemical properties of nanoparticles play a critical role in determining particle–cell interactions, cellular trafficking mechanisms, biodistribution, pharmacokinetics and optical properties [6,17]. Cancer nanotechnology is expected to transform current cancer treatment systems by providing more efficient

diagnostics and therapeutics drugs. Today, nanomaterials allow the intersection of both paradigms in the engineering of multifunctional nanoparticles, creating effective formulations for tailored made therapies for a specific patient or groups of patients, allowing for detecting cancer at an early stage, delivering anticancer drugs specifically to malignant cells and monitoring whether these drugs kill malignant cells, all within a single platform [18,19]. Common types of nanoparticles and their main applications are summarized in Table 1. Each nanoparticle type shows certain advantages and disadvantages that are an inherent feature of a particular material, such as solubility, thermal conductivity, ability to bind biomolecules or linkers, chemical stability and capacity to incorporate and release compounds, as well as biocompatibility, toxicity, immunogenicity and controlled drug release rate.

Radionuclides for diagnostic tumor imaging Radioactive tracers are extensively used for mapping physiological functions and metabolic activity, because they render thorough information about any given organ function and its eventual disease. The type of radioactive decay, energy yield and physical half-life influence the selection of potential radionuclides for diagnostic tumor imaging (Table 2); moreover, the biochemical and physiological properties of a radiotracer are dictated by the chemical form of the radionuclide. Gamma emitters with energy in the 150 keV range are applicable for gamma imaging or single photonemission computed tomography (SPECT), while high-energy positron-emitters with energy of 511 keV can be used for positron-emission tomography (PET) [20]. The term theranostics has been identified in recognition of heterogeneous diseases in personalized medicine strategies. Theranostics embodies the fusion of therapy and diagnostics for the purpose of optimizing efficacy and safety, as well as updating the process of drug development. The increasing number of scientific inventions in these fields has made the development of theranostics possible. Theranostic agents have

Radiolabelled nanoparticles

DOI: 10.3109/1061186X.2015.1048516

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Table 1. Common types of nanoparticles for multifunctional purposes. Nanoparticle type

Description

Applications

Metallic NP’s

Noble metals, iron oxides (5100 nm), functionalized with biomolecules and bifunctional chelating agents

Carbon nanoparticles

Allotropes of carbon at nanometric level with specific 3D configuration, fullerenes, single wall nanotubes, multiple wall nanotubes (55000 nm to the mayor axis) Semiconductor crystal with a cadmium core and metal shell, with fluorescent properties (510 nm)

Molecular imaging (PET/SPECT/MIR) Targeted radiotherapy Thermotherapy Targeted drug delivery Multifunctional targeted delivery and molecular imaging Chemo-photo thermal therapy In vitro and in vivo imaging probes Fourier resonance energy transfer (FRET) Angiogenesis PET/NIRF imaging Carriage of biomolecules In vivo fluorescence imaging Molecular imaging Passive imaging (PET/SPECT) Controlled drug delivery


Quantum dots (QD)

Inorganic NPS

Ceramic and porous materials (silica, titania, alumina) (5100 nm)

Liposomes

Spherical assemblies of amphiphilic phospholipid bilayers (50–100 nm)

Polymeric NP’s

Colloidal particles of synthetic and natural polymers (10–1000 nm)

Dendrimers

Highly repetitive branched molecules, with a central core, interior dendritic surface and functional surface groups

Table 2. The characteristics of potential radionuclides for nanotargeted imaging [88]. Imaging Emission type Radionuclide type SPECT

PET

99m

Tc In 67 Ga 131 I 123 I 11 C 15 O 13 N 124 I 18 F 76 Br 64 Cu

111

g g, e auger g g, b g b+ b+ b+ b+ b+ b+ b+

Emax (keV)/ abundance (%)

t1/2 (h)

140 (89) 6.0 245 (94), 19 (16) 67.2 93 (40), 393 (5) 78.3 364 (81), 636.9 (7.1) 192 159 (83) 13.2 960 0.34 1720 0.033 1190 0.166 2134 100.2 635 1.83 3941 16 656 12.7

a number of advantages, including the combination of passive and active targeting, allowing for concomitant agent localization by molecular imaging and other efficient therapeutic functions, such as hyperthermia, radiation, free radical generation and environmentally responsive drug release [20]. Considering that there is no ‘‘magic bullet’’, the aim of combining therapy is focused on the induction of damage in pathways that are not mutually exclusive, have multiples action sites, evade resistance mechanisms, sensitize cells and diminish side effects in healthy tissues. The diagnostic quality and tumor therapeutic efficacy are determined by the percentage uptake at the target site and the quality of radiation; in the case of radiolabeled drug delivery systems, almost the entire efficacy is dependent on the selectivity and specificity of the delivery mechanism [6,21–25].

Common properties of radiolabeled nanoparticles Radiolabeled nanoparticles represent a new class of agents that has enormous potential for clinical applications.

Radiolabeled nanoparticles conjugated to target specific molecules can be directly used as agents for diagnosis. Many types of radiolabeled nanoparticles have three main components: the core, the targeting biomolecule and the radiotracer group (Figure 2). The targeting biomolecule includes a component with high affinity for target epitopes; radiolabeling can be performed with or without slight modifications of the original nanoparticle surface. For ligands to bind effectively, each radionuclide can be conjugated directly on the nanoparticle surface, with or without a spacer, or can be attached to the nanoparticle during chemical synthesis. The spacer groups between the nanoparticle surface and the radionuclide or the biomolecule can be a simple hydrocarbon chain, a peptide sequence or a PEG linker [2]. In some cases, a bifunctional chelating group (BFC), like 1,4,7,10-tetraazadodecaneN, N0 , N00 , N000 -tetraacetic acid (DOTA), has to be conjugated to the nanoparticle and then a radioactive metal needs to be attached. These require modification of nanoparticles before radiolabeling [29]. Recently, there are increasing utilizations of radioisotopes to study nanoparticles due to high sensitivity detection of radioactivity. Different from other molecular imaging modalities where typically the nanoparticle itself is detected, radionuclide-based imaging detects the radiolabel rather than the nanoparticle. The nanoparticle distribution is measured indirectly by assessing the localization of the radionuclide, which can provide quantitative measurement of the tumor targeting efficacy and pharmacokinetics only if the radiolabel on the nanoparticle is stable enough under physiological conditions.

Molecular imaging with radiolabeled nanoparticles Medical imaging techniques are noninvasive techniques that allow the recording of spatiotemporal events at the cellular and subcellular levels. Methods used in the clinical diagnosis require a sufficient intensity of the signal emitted from the region of interest so that the difference between the structure


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Figure 2. Schematic structure of a radiolabeled nanoparticle design for molecular imaging (BFC: bifunctional chelating group) [17].

Table 3. Characteristics of different imaging modalities [31,57]. Imaging modality PET SPECT MRI CT Ultrasound Luminescence Fluorescence

Resolution (mm) 2 2 50 100 250 5 2.5

Sensitivity

Limited deep

High High Low Low Low High High

Unlimited Unlimited Unlimited Unlimited Unlimited 5 cm 1 cm

under observation and the surrounding tissues can be detected. Imaging methods with medical diagnostic purposes include the use of positron-emitting (particulate b+ radiation) and gamma-emitting (g radiation) radionuclides to achieve the necessary concentration of a radiopharmaceutical on the desired site. Several carriers, such as nanoparticles, liposomes and microspheres, have been proposed. Current techniques, such as computed tomography (CT), magnetic resonance imaging (MRI), ultrasound and optical imaging (fluorescence and bioluminescence detection) among other alternatives, associated with PET or SPECT increase the probability of having an earlier disease diagnosis and more effective intervention. Additionally, radionuclides nanotechnology devises are able to carry and accumulate contrast agents for non-nuclear imaging techniques, therefore increasing the strengths of individual modalities, providing information about the biological structure and metabolic processes in a single high sensitivity and high-resolution imaging. Table 3 shows characteristic differences between common imaging modalities [23]. Radionuclide-based imaging techniques, such as PET and SPECT, are highly sensitive (in nanomolar order), but they commonly have a resolution of more than several millimeters. In the case of SPECT, its principal advantages are wide availability and ability to acquire simultaneous images of two radiopharmaceuticals with different energies, allowing for an increased observational time window (using longer half-life single photon emitters). However, the SPECT is unable to quantify processes that lack attenuation correction and shows

poor temporal and spatial resolution, due to the low geometric efficiencies of collimators. PET imaging can detect and record a high percentage of the emitted events, improving the image quality by increasing signal-to-noise ratio as well as allowing for shorter scans, multiple field of view scanning in a reasonable period, and improved temporal resolution in dynamic studies. Although there are a great variety of radiotracers used as natural substrates and chemical analogs, only one process could be evaluated at a time. This can be a limiting factor, especially in small animal models in which higher resolution is even more essential. In contrast, MRI and CT are much less sensitive, but they have a resolution of approximately less than 1 mm. MRI can be used to image deep tissues; its main advantage is not exposing the patient to any radiation, yet one disadvantage is that the data acquisition time is slow compared to other approaches. On the other hand, optical imaging penetration is also restricted to only several centimeters [26,27]. SPECT imaging utilizes high sensitive cameras systems with multiple cadmium zinc telluride detectors and a pinhole or multi-pinhole collimators, providing high quality images with good count statistics despite shorter acquisition times or lower radioactive tracer levels [28–30]. For each of the above imaging modalities, novel nanoparticles can increase tissue contrast or recognize specific biological changes. For molecular imaging, special probes are needed to monitor physiological or biochemical changes in vivo. Usually these probes require two characteristics, one that promotes the accumulation of the probe at the site of interest and a second that allows them to be imaged. Multifunctional probes that use two or more imaging modalities are currently being developed. These devises could potentially overcome the sensitivity or resolution restrictions of a single imaging approach [31]. Nanoparticles can be used to image the current anatomy-based level at the molecular level. In sum, nanoparticle imaging techniques include advanced optical and luminescence imaging as well as spectroscopy and ultrasound combined with magnetic resonance imaging and nuclear imaging depending on the targeting agents [32,33].


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Nanotargeted radiopharmaceuticals for SPECT & PET imaging Hultborn et al. reported the preoperative use of colloidal gold injections in breast cancer patients. Further investigations introduced 99mTc, a substitute for radioactive gold because it is readily available, inexpensive and has ideal imaging and dosimetry characteristics compared to the latter [34]. The most common radionuclides used for SPECT imaging include 99m Tc (t1/2 ¼ 6 h), 111In (t1/2 ¼ 2.8 days) and 131I (t1/2 ¼ 8.0 days), whereas the most common agents for PET are 64 Cu (t1/2 ¼ 12.7 h), 18F (t1/2 ¼ 109.8 min) and 68Ga (t1/2 ¼ 68.1 min) [35]. The targeting biomolecule needs a component with high affinity for target epitopes, and some biomolecules require structural modifications before they are attached to the nanoparticle surface (Figure 3). Radiolabeling of these biomolecules can be performed with or without slight design modifications of the original nanoparticle surface. For ligands to bind effectively, each radionuclide can be conjugated directly to the nanoparticle surface, with or without a spacer, or can be attached to the nanoparticle during chemical synthesis. The spacer groups are located between the nanoparticle surface and the radionuclide or biomolecule. They can be formed by a simple hydrocarbon chain, a peptide sequence or a polyethylene glycol (PEG) linker [17]. In some cases, a bifunctional group, such as 1,4,7,10-tetraazadodecane-tetraacetic acid (DOTA), hydrazino nicotinamide (HYNIC), diethylene and triamine pentaacetic acid (DTPA), among others, has been conjugated to the nanoparticle as a linker. However, none of these entirely fulfills the desired criteria of an ideal imaging agent. The search for an optimal nanoparticle radiopharmaceutical is still

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in progress. Some examples of radiolabeled nanoparticles with diagnostic applications are described below. The main characteristic, tumor uptake and therapeutic efficacy of an 111 In-labeled, chimeric L6 (ChL6) monoclonal antibody attached to an iron oxide nanoparticle were studied in athymic mice bearing human breast cancer tumors [23]. The 111In-labeled ChL6 was conjugated to the carboxylated-PEG on dextran-coated iron oxide nanoparticles with 1–2 ChL6 antibodies per nanoparticle. The time this nanoparticle remained in the circulatory system was long enough to provide a considerable opportunity for it to exit the blood vessels and reach to the cancer cells. SPECT imaging was carried out to quantify the nanoparticle uptake in the tumor, which was approximately 14% of the injected dose per gram of tumor (%ID/g) at 48 h post-injection [36–38]. In another report, recombinant antibody fragments were tested for tumor targeting of these nanoparticles. Whole-body autoradiography studies showed that only 5% of the injected dose is targeted to the tumor after 24 h [39]. Cell adhesion molecules, integrins, are involved in a wide range of cell–extracellular matrices and cell–cell interactions [40]. The avb3 integrin, which binds to arginine–glycine– aspartic acid (RGD)-containing components of the interstitial matrix, such as vitronectin, is over-expressed in different tumor types and plays a critical role in tumor angiogenesis. Among all 24 integrins discovered to date, integrin avb3 is the most intensively studied [41,42]. In another report, integrin avb3targeted 111In-labeled perfluorocarbon (PFC) nanoparticles were tested for the detection of tumor angiogenesis in New Zealand tumor-bearing white rabbits [43]. The PFC nanoparticles with approximately 10 111In per particle were found to have better tumor-to-muscle ratio than those containing

Figure 3. Strategies for nanoparticle design for intracellular delivery [6].


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approximately one 111In per particle. At 18 h post-injection, the measured tumor radioactivity in rabbits receiving integrin avb3-targeted PFC nanoparticles was approximately 4-fold higher than the controls. Based on biodistribution studies, the spleen was the initial organ for clearance. The use of radionuclides to obtain functional and morphological information regarding lymphatic drainage is called lymphoscintigraphy, and Ocampo-Garcıá et al. recently reported a gold nanoparticle conjugated for mannose receptors in sentinel lymph node detection in breast cancer radiolabelled for SPECT/CT imaging [44]. Nanoshells were first developed by West and Halas at Rice University. Nanoshells are a new class of optically tunable nanoparticles (designated nanoparticles) that are composed of a dielectric core (silica) coated with an ultrathin metallic layer (gold) [45–47]. Nanoshells can be divided into two groups, those formed from oxide core–shell particles with a hollow core (hollow nanoshells) and those with a dielectric core and a metallic shell (core–shell nanoshells). Both types have applications in drug delivery, imaging, photothermal therapy and microenvironmental studies where they are placed. Xie et al. reported a new procedure for labeling gold nanoshells with the radionuclide labels copper-64 (64Cu) and indium111 (111In) through a bifunctional PEG and chelating agent [54]. According to their report, radiolabeling with these agents permits determination of the biodistribution of the radiolabeled nanoshells in live rats with head-and-neck squamous-cell carcinoma xenografts by noninvasive PET and SPECT imaging. PET and SPECT images of the rats showed that both 64Cu and 111In appeared to be useful for tracking the in vivo distribution of nanoshells, and both 64 Cu-nanoparticle and 111In-nanoparticle showed clear uptake of the radiolabeled nanoshells in the tumor 20 h after injection. Carbon nanotubes are geometric hexagonal cylinder of pure carbon units called C60 fullerene, produced as single(SWNTs) or multi-walled (MWNT) networks. Single-walled carbon nanotubes (SWNTs) have individual size, shape and physical properties that make them promising tools for biological/biomedical purposes [48–50]. In an early study, water-soluble hydroxylated SWNTs were labeled with 125 I (t1/2 ¼ 60.2 days) to examine their distribution in mice [51]. Subsequently, Singh et al. functionalized water-soluble SWNTs with the chelating molecule diethylenetriaminepentaacetic (DTPA) and labeled them with 111In for imaging purposes [52]. Impressively enough, both reports proposed that these WNTs were not retained in any of the RES organs (e.g. liver or spleen) and were cleared rapidly from the blood circulation through the renal route. Recently, the biodistribution of radiolabeled SWNT-oligonucleotide conjugates was also evaluated and reported in mice [35]. Radioactive Cd125mTe/ZnS (t1/2 ¼ 57.4 days) quantum dots were targeted to the mouse lung with an antibody [53]. Animals were sacrificed at different post-injection time intervals and the biodistribution in major organs was evaluated. The target-specific antibody conjugated Cd125mTe/ZnS quantum dots principally targeted the lungs, while the quantum dots linked to a control antibody were mainly accumulated in the liver and spleen. This report provided for the first time a quantitative measurement of the


in vivo targeting efficacy of a quantum dot–antibody system. In a follow-up study, these Cd125mTe/ZnS quantum dots were used to document the competition between vascular targeting and interaction of quantum dots with the RES [54]. Due to the use of lead collimators to define the incidence angle, SPECT imaging has very low detection efficiency [55]. In drug discovery research, radiolabeling can be helpful for evaluating in vivo biodistribution of nanoparticles via imaging techniques. In this respect, researchers covalently labeled the conjugate with (18F)-fluorobenzoate to study the in vivo distribution of gold nanoparticles by PET. The results showed that PET imaging allows the in vivo biodistribution of the gold nanoparticles to be studied in a noninvasive and sensitive way using a reduced number of animals, and those gold nanoparticles can be covalently and radioactively labeled for PET biodistribution studies [56].

Nanotargeted radiopharmaceuticals for dual modality imaging Among all molecular imaging modalities, none of them is lonely perfect or sufficient to obtain all necessary information for a particular problem [57]. For example, it is difficult to accurately quantify fluorescence signal in living subjects, particularly in deep tissues; MRI has a high resolution, but it suffers from low sensitivity. Radionuclide-based imaging techniques have very high sensitivity, but they have relatively poor resolution. The combination of multiple molecular imaging modalities can offer synergistic advantages over any modality itself. Multimodality imaging using a small, molecule-based probe is very challenging due to the limited number of attachment points and the potential interaction with its receptor binding affinity [2]. However, nanoparticles have large surface areas where multiple functional moieties can be incorporated for multimodality molecular imaging [35]. Recently, radiolabeled nanoparticles bearing individual properties have increased in popularity in multimodality tumor imaging. The ultimate generation of dual modality instruments currently available is based on the combination of SPECT/PET and MRI [58–62]. The use of MRI as a substitute for CT is due to the following: (i) No radiation exposure in patients (it has been estimated that the radiation dose of a full-body CT scan is equivalent to that of more than 500 X-ray scans). (ii) Higher anatomical soft-tissue contrast. (iii) The possibility of simultaneous acquisition of modalities (PET–MR), reducing the time patients spend inside the scanner and assuring that the two modalities are being used under the same physiological conditions and spatial positioning. (iv) More effective MRI contrast agents based on paramagnetic metals (opening up the attractive idea of using dual modality SPECT–MR or PET–MR imaging agents) [63–67]. A few other reports have focused on the radiolabeling of quantum dots with PET isotopes, such as 18F (t1/2 ¼ 110 min) and 64Cu. However, neither the incorporation of a targeting moiety nor optical imaging was carried out in these studies [68–70].



DOI: 10.3109/1061186X.2015.1048516

Figure 4. The structure of the DOTA–QD–VEGF conjugate. DOTA can chelate

64

Cu, which allows for PET imaging [76].

Figure 5. Whole-body coronal PET images of tumor-bearing mice at 1, 4, 16 and 24 h p.i. of approximately 300 mCi of DOTA–QD–VEGF. The arrows indicate the tumor [76].

Angiogenesis plays a pivotal role in facilitating malignant tumor growth. Vascular endothelial growth factor (VEGF) and its receptors (VEGFRs), as the main angiogenic regulators, have recently been investigated as tumor imaging and cancer therapeutic targets [5,71–75]. Amine-functionalized QDs have been conjugated with VEGF protein and the macrocyclic chelating agent 1,4,7,10-tetraazacyclododecane-1,4,7,10tetraacetic acid (DOTA) for VEGFR recognition and 64 Cu (t1/2 ¼ 12.7 h) labeling for PET imaging, respectively (Figure 4) [76]. According to Kai Chen et al., 64Cu-labeling yield was greater than 90% for both conjugates (based on 60 pmol of QD per mCi of 64Cu; n ¼ 3), and the level of 64 Cu-labeled QD conjugate injected into each mouse was approximately 22 pmol (200–400 mCi based on 64Cu). Due to PET’s high sensitivity, much less 64Cu-labeled QD conjugate was needed to observe the tumor uptake than with NIRF imaging (typically 200 pmol). The mice were scanned at

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64

Cu–DOTA–QD and

64

Cu–

multiple time points p.i.; Figure 5 shows representative coronal slices containing the tumor. In another study, the future of MRI/PET scanners was posited to greatly benefit from the application of bifunctional nanoprobe conjugates. In yet another study, we developed polyaspartic acid (PASP)–coated iron oxide (IO) nanoparticles conjugated with cyclic arginine–glycine–aspartic acid (RGD) peptides and the macrocyclic chelating agent 1,4,7,10tetraazacyclododecane-tetraacetic acid (DOTA) for integrin avb3 recognition and positron-emitting radionuclide 64Cu labeling (Figure 6) [77]. The first example of in vivo dual-modality PET/MR imaging using a single agent has been reported. Recently, a 124I (t1/2 ¼ 4.2 days)-labeled IO nanoparticle was also reported as a dual-modality PET/MR probe for lymph node imaging in rats. This nanoparticle may be useful in the clinical field for accurate localization and characterization of


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Figure 6. (a) Synthesis of the PET/MRI dual functional probe DOTA–IO–RGD. DOTA–IO was prepared similarly, except that no RGD peptide was used. (b) Illustration of a PET/MRI probe based on an IO nanoparticle [77].

lymph nodes, which are critical for cancer staging because the lymphatic system is an important route for cancer metastasis [78,79]. The identification of hypoxic tissue has therapeutic applications for multiple diseases, including myocardial ischemia, stroke and tumors [80]. PET tracers used for the hypoxia imaging are, for example, [18F] fluoromisonidazole ([18F]-FMISO) and diacetyl-bis-(N4-methylthiosemicarbazone) ([64Cu]-ATSM) [81]. Monitoring glucose metabolism yields valuable information about energetic processes in vivo. [18F] Fluorodeoxyglucose ([18F]-FDG) is the most widely used PET tracer, and it is routinely applied to measure glucose consumption in cardiology, neurology and oncology. The example of PET/MRI measurements for assessing glucose metabolism shows that multi-functional imaging allows monitoring different stages of metabolic processes. Another example of a multiple-stage process would be the imaging of brain function as a result of a drug–receptor interaction. The interaction of a drug with neuroreceptors can be visualized with PET tracers that specifically bind to receptors of the dopaminergic system [82]. A bimodal agent with a ‘‘single pharmacological behavior’’ has the advantages of both imaging modalities, such as the high sensitivity of PET and the high resolution of MRI, and can be combined into a single image, thus providing confidence that both images represent the same biological process, while at the same time, the individual disadvantages of the modalities are minimized. The synergistic effect of dual-modality imaging agents and techniques should make quantification of the signal more accurately, allowing clinicians to diagnose, plan, treat and monitor their outcomes more precisely.

Theranostic radiolabeled nanoparticles as a new radiopharmaceuticals device Theranostic refers to the use of molecular targeting vectors labeled either with diagnostic or therapeutic radionuclides within the same platform of radiopharmaceuticals. Therefore molecular imaging and diagnosis of the disease can be effectively followed by personalized treatment utilizing the same molecular imaging vectors, some recent examples include SPECT-therapy and PET-therapy. The success of personalized medicine depends on a property choice of radionuclide for imaging. Reported theranostic devices are based on matched-pair radioisotopes, for example 99mTc/90Y, 68 Ga/177Lu, 99mTc/177Lu, 68Ga/90Y, etc. Jimeńez-Mancilla et al. [83] reported a new class of multifunctional theranostic radiopharmaceutical based on gold nanoparticles capped with Tat-Bombesin, HYNIC-TOC and DOTA-GGC peptides and 99mTc/177Lu match-pair radionuclide, where 99mTc acts as SPECT contrast agent and simultaneously cellular damage can be generated. Targets of bombesin analogues are limited to cell membrane and cytoplasm, whereas the peptide sequence TAT (49–57) promotes their internalization and routing in to the cell nucleus. Santos-Cuevas et al. [84] demonstrated that the 99m Tc-N2S2-Tat (49–57)-Lys3-bombesin (99mTc-Tat-BN) peptide is highly internalized in nuclei of breast and prostate cancer cells. At a single-cell level, short-range charged particles, such as IC electrons and Auger electrons (99mTcHYNIC-TOC), impart a dense ionizing energy deposition pattern associated with increased radiobiological effectiveness, one characteristic that becomes important when a nanoparticle is capable of reaching the nucleus. Beta radiation (177Lu-DOTA-GGC) offers an efficient crossfire effect on


DOI: 10.3109/1061186X.2015.1048516

cancer cells. Additionally, optical and thermoablative properties of gold cores are suitable for photothermal therapy. For these reasons, authors suggest that 99mTc/177Lu-AuNP-Tat-Bn radiopharmaceutical has a potential application in medical diagnosis and therapy treatments, laying the groundwork for preclinical studies to determine the in vivo tumor uptake, the radiation dose and lastly the diagnostic and therapeutic potential.


Conclusion Nanotechnology has been used in every single modality of the molecular imaging arena for imaging purposes. The most relevant advantages of radionuclide-based imaging techniques, such as SPECT and PET, are that they are highly sensitive and quantitative. Furthermore, the emerging field of multimodality imaging with radiolabeled nanoparticles can help researchers to detect and evaluate the same radiolabeled nanoparticle with multiple imaging techniques. The advantage of cross-modality validation is that it can provide more accurate and reliable data from the radiolabeled nanoparticle than that obtained with SPECT or PET imaging alone. Multifunctional nanoparticles can combine diagnostic and therapeutic capabilities for target-specific diagnosis and the treatment of disease. Medical imaging modalities, such as SPECT, PET and MRI, can non-invasively identify tumors, but radiolabeled nanoparticle probes with multivalent properties can provide multimodal images with enhanced signal and sensitivity. The future of nanomedicine lies in multifunctional nanoplatforms that combine both therapeutic components and multimodality imaging. This integration of the diagnostic imaging capability with therapeutic interventions, termed ‘‘theranostics’’, is critical to addressing the challenges of cancer heterogeneity and adaptation [85,86]. Compared to recent conventional targeted radionuclide therapy or radioimmunotherapy, the application of nanocarriers can allow the specific multivalent attachment of targeted molecules, such as antibodies, peptides or even ligands, to the surface of nanocarriers, which can deliver a high payload of radionuclides, chemotherapeutics and imaging agents to achieve multifunctional and multimodality targeting of tumor cells as well as to increase the efficacy and safety of targeted therapy. Combining the localization of an agent to the desired target with both minimization of the agent’s immunotoxic effects and bypass of sequential biological barriers is a major challenge in the development of passively and actively nanotargeted drug delivery systems. Future studies should be aimed at designing and optimizing these novel approaches, extending them to combine potent radionuclides, imaging agents, chemotherapeutics and radiosensitizing agents [87].

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18. 19. 20. 21. 22. 23. 24. 25.

Declaration of interest The authors report no conflicts of interest.

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Multifunctional radiolabeled nanoparticles: strategies and novel classification of radiopharmaceuticals for cancer treatment.

Polymeric nanoparticles for molecular imaging.

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Molecular classification of gastric cancer.

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Towards a molecular classification of colorectal cancer.

Molecular Classification of Triple-Negative Breast Cancer.

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Solid silica nanoparticles: applications in molecular imaging.

Anti-cancer radiopharmaceuticals.

Cyclotrons and positron emission tomography radiopharmaceuticals for clinical imaging.

Targeted diagnostic magnetic nanoparticles for medical imaging of pancreatic cancer.

Targeted fluorescent magnetic nanoparticles for imaging of human breast cancer.

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Molecular Imaging of Ovarian Cancer.

Molecular-genetic imaging of cancer.

Development of Novel Magnetic Nanoparticles for Hyperthermia Cancer Therapy.

Molecular imaging using PET for breast cancer.