Macrophage cell tracking PET imaging using mesoporous silica nanoparticles via in vivo bioorthogonal F-18 labeling
Graphical abstract
Introduction
Non-invasive cell tracking in vivo for monitoring of the delivery and transplantation of various therapeutic cells (e.g., immune cells or stem cells) is an area of intense investigation [1]. For this purpose, cells should be labeled directly or indirectly to enable detection by in vivo imaging modalities such as fluorescence or bioluminescence imaging (BLI), magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), or positron emission tomography (PET) [2,3]. The reporter gene approach to indirect cell-labeling, whereby live cells are transfected with a reporter gene before injection, has been widely used to visualize live cells in vivo. This technique is based on interaction between the encoded reporter protein and an exogenously administered tracer [4]. Since reporter genes are expressed as long as cells remain viable and are not diluted by cell division, this approach is well suited to the long-term monitoring of cell viability [1,5]. However, the reporter gene approach has several limitations; i) it requires a sophisticated genetic engineering procedure and related safety precautions; ii) is subject to signal loss due to gene silencing; iii) may induce immunological responses or exhibit poor pharmacokinetic properties related to the exogenously administered tracer; and iv) when BLI is used may be limited by poor tissue penetration by light [6].
Over the past decade, a variety of biocompatible nanoparticles have been intensively investigated in the contexts of early diagnostics and therapy [7,8]. Depending on the imaging modality used, nanoparticles such as quantum dots, superparamagnetic iron oxide nanoparticles (SPIONs), and gold nanoparticles have been employed for direct cell-labeling approaches as a surrogate measure of cell numbers [[9], [10], [11]]. Of these nanoparticles, SPIONs are the most popular for direct cell-labeling in MRI cell-tracking studies [12]. Although direct labeling using SPIONs is relatively straightforward and well established, and MRI has excellent spatial resolution, cell tracking strategies using SPIONs with MRI are restricted in terms of long-term monitoring due to the SPIONs-dilution effect resulting from cell division as well as the low sensitivity on MRI [12,13].
As a leading molecular imaging modality, PET can provide biological information at the molecular level in living subjects with high sensitivity [14], and thus, enables the visualization of different in vivo processes using specific molecular imaging probes labeled with positron-emitting radioisotopes [15,16]. Fluorine-18 (F-18) is most commonly used to produce such probes, because of its favorable physiological and nuclear properties and its easy accessibility [[17], [18], [19]]. Unfortunately, despite its many merits, the short half-life of F-18 (t1/2 = 109.8 min) makes it unsuitable in terms of long-term monitoring in cell tracking studies using PET.
The bioorthogonal conjugation protocol based on strain-promoted alkyne azide cycloaddition (SPAAC) plays a crucial role in a range of biological applications [[20], [21], [22], [23]]. In a recent significant advance, mesoporous silica nanoparticles (MSNs) were successfully radio-labeled with F-18 and loaded into living subjects via the SPAAC reaction between aza-dibenzocyclooctyne (DBCO)-tethered MSNs and 18F-labeled aza-dibenzocycloocta-triazole (18F-DBCOT)-tethered MSNs using 18F-labeled azide [24]. This in vivo F-18 labeling protocol allows F-18 to be used for the long-term monitoring of nanoparticles and provides good pre-targeting PET images using MSNs beyond the short half-life of F-18.
Macrophage cells can home to tumor sites in response to macrophage chemoattractants excreted from tumor tissues, which play many key roles in the promotion of tumor cell survival, metastasis, and proliferation [25,26]. Similarly, chemokines and adhesion molecules can also cause macrophage accumulation in atherosclerotic plaques, which may facilitate potentially fatal events [26]. In this regard, tracking of macrophage cells and other immune cells is considered a promising means for assessing responses to treatment in cancer and atherosclerosis and for understanding their progressions at a molecular level [26]. Here, we introduce an extremely efficient PET based cell-tracking protocol. In this study, given that macrophages home to tumors and atherosclerotic plaques [25], the PET tracking images of RAW 264.7 macrophage cells were successfully obtained at tumor and atherosclerotic plaque sites in tumor-bearing and ApoE−/- mouse models by in vivo F-18 labeling achieved using a bioorthogonal SPAAC conjugation reaction.
Section snippets
Preparation and characterization of DBCO-MSNs
MSNs have characteristics that favor their use as drug delivery systems, such as high drug loading capacities, facile surface modification, multi-functionality, biochemical stability, and low cytotoxicity [[27], [28], [29]]. In particular, 50–100 nm PEGylated MSNs are known to penetrate cells easily to have excellent biocompatibilities [[30], [31], [32]]. In the present study, DBCO-ligated PEGylated MSNs (DBCO-MSNs) with a size range of ≈60–80 nm were designed for labelling in vivo by
Conclusions
We developed an efficient cell tracking protocol based on an in vivo bioorthogonal F-18 labeling reaction using a PET system. For this study, DBCO-functionalized MSNs were prepared, and incubated with macrophage RAW 264.7 cells to produce the DBCO-MSNs-loaded RAW cells, which could home and accumulate at tumor and atherosclerotic plaque sites, and react rapidly with the 18F-labeled azide synthon. In our PET imaging study, a high signal intensity was detected at the tumor site in the
Preparation of DBCO-MSNs
NH2-MSNs were synthesized as previously described [33,34]. FmocNH-PEG24-COONHS (4.8 mg, 2.3 μmol) was added to a suspension of NH2-MSNs (20 mg) in phosphate buffered saline (PBS, pH 7.2, 3 mL) at 25 °C and stirred for 12 h. The products, FmocNH-PEG-MSNs, were purified by centrifugation (13200 rpm, 5 min) and washed several times with EtOH/water (3:1). In order to remove the Fmoc group, piperidine (30 μL) was added to the suspension of FmocNH-PEG-MSNs (10 mg) in CH2Cl2 (3 mL) and stirred at
Author contributions
†H.J.J., R.J.Y., and J.K.K. contributed equally to this work.
Acknowledgments
This work was supported by the Basic Science Research Program (grant code: NRF-2017R1A2A2A10001451), a grant of KIRAMS (No. 50536-2019) funded by the Ministry of Science and ICT (MSIT), and the MOTIE (Ministry of Trade, Industry & Energy) and KSRC (Korea Semiconductor Research Consortium) support program (project number: 10080450), Republic of Korea.
References (35)
- et al.
Non-invasive cell tracking in cancer and cancer therapy
Curr. Top. Med. Chem.
(2010) Nuclear imaging of cancer cell therapies
J. Nucl. Med.
(2009)- et al.
Long-term tracking of cells using inorganic nanoparticles as contrast agents: are we there yet?
Chem. Soc. Rev.
(2012) - et al.
Genetically engineered whole-cell sensing systems: coupling biological recognition with reporter genes
Chem. Rev.
(2000) - et al.
Trafficking mesenchymal stem cell engraftment and differentiation in tumor-bearing mice by bioluminescence imaging
Stem Cell.
(2009) - et al.
Clinical imaging in regenerative medicine
Nat. Biotechnol.
(2014) - et al.
Nanocarriers as an emerging platform for cancer therapy
Nat. Nanotechnol.
(2007) - et al.
Nanomedicine applications of hybrid nanomaterials built from metal–ligand coordination bonds: nanoscale metal–organic frameworks and nanoscale coordination polymers
Chem. Rev.
(2015) - et al.
Nanomedicine for cancer immunotherapy: tracking cancer-specific T-cells in vivo with gold nanoparticles and CT imaging
ACS Nano
(2015) - et al.
Cholera toxin B conjugated quantum dots for live cell labeling
Nano Lett.
(2007)
Nanoparticles for cell labeling
Nanoscale
Design considerations for the synthesis of polymer coated iron oxide nanoparticles for stem cell labelling and tracking using MRI
Chem. Soc. Rev.
Molecular imaging in living subjects: seeing fundamental biological processes in a new light
Genes Dev.
Positron emission tomography provides molecular imaging of biological processes
Proc. Natl. Acad. Sci. U.S.A.
Molecular Imaging: Radiopharmaceuticals for PET and SPECT
Molecular imaging with PET
Chem. Rev.
Recent developments and trends in 18F-radiochemistry: syntheses and applications
Mini-Rev. Org. Chem.
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2022, Materials Today BioCitation Excerpt :A further candidate used in PET imaging is Fluorine-18 (18F) that have several favorable physiological and nuclear properties, but its application is limited by its short half-life. To overcome this drawback, Kim and co-workers [216] proposed the radiolabeling of nanostructure silica material via an in vivo strain-promoted alkyne azide cycloaddition (SPAAC) covalent labeling reaction between aza-dibenzocyclooctyne-tethered PEGylated mesoporous silica nanoparticles (DBCO-MSNs) and 18F inside the macrophage cells. The developed macrophage cell-tracking protocol increased radioisotope half-life time, providing a persistently strong tumor imaging until 8 days after treatment.
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2022, BiomaterialsCitation Excerpt :Direct radiolabeling of macrophages suffers from high background signals because macrophages will stay in blood stream, different tissues and organs for a relatively long time. A clickable mesoporous silica nanoparticle was used to label macrophages, which could capture 18F-azide-radiotracers via the strain-promoted alkyne azide cycloaddition reaction [129]. PET/CT imaging revealed higher signals in mice tumor or atherosclerosis plaques several days post-injection in comparison with normal macrophages.
- 1
These authors contributed equally to this work.
- 2
Prof. Dong Wook Kim, Department of Chemistry, Inha University, 100 Inharo, Namgu, Incheon 402–751, Korea.