Page - 84 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Conniotet al. Nanocarriers for immunecell targetingand tracking photon emission tomography (SPECT) and X-ray computed tomography(CT)are the imaging techniquesapproved formed- ical applications (Bernsen et al., 2014). PET and SPECT are high-sensitivity and low-resolution techniques, while MRI and CTprovidehigh-resolution images (LiuandLi, 2014).However, the use of radionuclide-based techniques, as PET and SPECT, has brought questions regarding their safety (Laskey et al., 2010). In addition, their combination with additional meth- ods is fundamental to obtain an anatomical image. Therefore, the combination of these different imaging modalities consti- tutes a multimodality imaging method that has been explored inpreclinicalandclinicaldevelopment, includingSPECT/CTand MRI/PET(Naumovaetal., 2014). Among these techniques,MRI is themost versatile and sensi- tivemethodallowing the studyof immune cellmorphology and function (Ahrens andBulte, 2013). In fact, innovative and safer techniques are emerging fromtheuseofdifferentbiocompatible cell labeling probes andMRI to obtain high-resolution images without using ionizing radiation (Sosnovik and Nahrendorf, 2012; Thu et al., 2012). The signal used forMRI arises from the water protons (1H)ordifferent fluorinatedmolecules (e.g., 19F) underastaticmagneticfieldandafterpulsedbyaradio-frequency radiation,which alters the equilibriumof their nuclei. TheMRI signalwill thenresult fromatransientvoltagedeterminedby the propertiesof labeled tissue(AhrensandBulte,2013). This non-invasive and safe imaging technique has been expected to track immune cells in vivo, enabling the characteri- zationof their biodistribution and fate.MRI also seems suitable for the detection/quantification of surfacemarkers and secreted factors resultant frombiological processes occurred in vivo at a particular disease stage (Lu et al., 2013; Naumova et al., 2014). The rapid evolution in this field, advanced by the potential effi- cacy of next-generation cellular-based therapeutic approaches (e.g., immunotherapyandstemcell-basedtherapy),will certainly make thismethodacrucial tool to followdetailedbiological and immunologicalprocesses invivo. The successful application of these in vivo cell-tracking tools canpotentiallyoptimizeimage-guideddiagnosticsandtheoverall efficacyofdifferent therapeuticoptions.Particularly, thosebased onthemodulationofendogenouscells support the selectionofa specifictreatment, thechoiceofthebestadministrationrouteand also theuseof a correctdose for eachpatient (Ahrens andBulte, 2013). Different exogenous cell-labeling probes have been explored but superparamagnetic ironoxide(SPIO)nanoparticlesandper- fluorocarbon(PFC)nanoemulsionsseemtobe themostpromis- ing for those advancedMRI-based techniques (Supplementary Material). Moreover, these are the unique in vivo MRI cell- labeling techniques approved for human clinical trials, and thus willbe furtherdiscussed(AhrensandBulte,2013). Nano-based systems for MRI real-time tracking of immune cells.Differentnanosystems(SupplementaryMaterial)havebeen developed for MRI-based in vivo cell tracking, but the nega- tive contrast agents basedonSPIOandPFCconstitute themost explored ways to control MRI signal and consequent detec- tion (Hawrylak et al., 1993; Bulte andKraitchman, 2004). SPIO contrast agents are small particles composedby ferrous and fer- ric oxides, usually coated by dextran. Even though, these ionic NPs have beenmodified by other biodegradable polymer (e.g., chitosan, PEG, siloxanes, polyaniline, glycerylmonooleate) and labeled with targeting moieties to potentiate their delivery to certain tissues (SupplementaryMaterial) (Shubayev et al., 2009; Dilnawazet al., 2010).TheseMRI-basedcontrast agents strongly perturbthemagneticfieldof theregioninwhichtheyareembed- ded. Thewatermolecules will sense that alteration in themag- neticfieldandtheresultant lossofsignalwill leadtoadarkimage (Ahrens andBulte, 2013).On theother hand, fluorinated-based probes directly label targeted cells and thus the MRI signal is dependent on the number of fluorine atoms and labeled cells, which canbeobserved in their biological environment (Srinivas etal., 2012). The labeling of cells using these nano-based systems can be performed ex vivo or in vivo, through their direct administra- tion in the body. The labeling of immune cells ex vivo with SPIONPshas been explored to track and clarifymigratory pat- ternsofdiverse immunecells, asNK(Daldrup-Linket al., 2005), cells from T lineage (Kircher et al., 2011), and DCs (De Vries et al., 2005; Rohani et al., 2011) used during immunotherapeu- tic cancer approaches. Innovative immunotheranostic strategies underdevelopment combine thesemetal ion-basedNPwith tar- getednanoparticulate cancer vaccines.One interesting studyhas shownmultifunctional iron oxide NPs formulated in order to deliver carcinoembryonic antigens to DCs and be detected by MRI (Choet al., 2011).Alternatively, someSPIONPshavebeen developed to label DCsmembranes bymodifying their surface withCD11cantibodies,promotingreceptor-mediatedendocyto- sis(Ahrensetal.,2003;Yuetal.,2012).Despitebeingapromising approach against cancer disease, their clinical translation is still unclear. Theexvivo labelingofaDC-basedcancervaccinebySPIONP wasused inthefirstclinical trial that involvedthecell trackingby MRI techniques,where itwaspossible todetect the target lymph nodeonly inhalf of thepatientswithmelanoma(DeVries et al., 2005). T cells have been sorted and culturedwith SPIONPs,mostly coatedby transfection agents, as poly-L-lysineorprotamine sul- fate, to promote their capture due to thenon-phagocytic nature of these immune cells (Arbab et al., 2005;Thorek andTsourkas, 2008; Thu et al., 2012). These intracellular labeling was also attempted through the use cell-penetrating peptides and HIV- TAT(Torchilin,2008). The invivo labelingof immunecellsbySPIONPsisoftenused to trackmonocytes andmacrophages to characterize inflamma- toryevents,duetotheirphagocyticbehavior(Settlesetal.,2011). The in vivo labeling can be achieved by the intravenous admin- istrationof SPIONPs, or alternatively after their direct injection into tumor tissue. Both options were successfully used to label immune cells and track their migration pattern toward lymph nodes,whichallows for example thedefinitionof tumor specific stage(Harisinghanietal., 2003). It is important to emphasize that the cell labeling strategy must not alter the function and normal phenotype of immune cells, which could limit the efficacy of cellular-based therapies. www.frontiersin.org November2014 |Volume2 |Article105 | 84