Abstract
BackgroundWith the recent growth of interest in cell-based therapies and radiolabeled cell products, there is a need to develop more robust cell labeling and imaging methods for in vivo tracking of living cells. This study describes evaluation of a novel cell labeling approach with the positron emission tomography (PET) isotope 89Zr (T1/2 = 78.4 h). 89Zr may allow PET imaging measurements for several weeks and take advantage of the high sensitivity of PET imaging.MethodsA novel cell labeling agent, 89Zr-desferrioxamine-NCS (89Zr-DBN), was synthesized. Mouse-derived melanoma cells (mMCs), dendritic cells (mDCs), and human mesenchymal stem cells (hMSCs) were covalently labeled with 89Zr-DBN via the reaction between the NCS group on 89Zr-DBN and primary amine groups present on cell surface membrane protein. The stability of the label on the cell was tested by cell efflux studies for 7 days. The effect of labeling on cellular viability was tested by proliferation, trypan blue, and cytotoxicity/apoptosis assays. The stability of label was also studied in in vivo mouse models by serial PET scans and ex vivo biodistribution following intravenous and intramyocardial injection of 89Zr-labeled hMSCs. For comparison, imaging experiments were performed after intravenous injections of 89Zr hydrogen phosphate (89Zr(HPO4)2).ResultsThe labeling agent, 89Zr-DBN, was prepared in 55% ± 5% decay-corrected radiochemical yield measured by silica gel iTLC. The cell labeling efficiency was 30% to 50% after 30 min labeling depending on cell type. Radioactivity concentrations of labeled cells of up to 0.5 MBq/106 cells were achieved without a negative effect on cellular viability. Cell efflux studies showed high stability of the radiolabel out to 7 days. Myocardially delivered 89Zr-labeled hMSCs showed retention in the myocardium, as well as redistribution to the lung, liver, and bone. Intravenously administered 89Zr-labeled hMSCs also distributed primarily to the lung, liver, and bone, whereas intravenous 89Zr(HPO4)2 distributed to the liver and bone with no activity in the lung. Thus, the in vivo stability of the radiolabel on the hMSCs was evidenced.ConclusionsWe have developed a robust, general, and biostable 89Zr-DBN-based cell labeling strategy with promise for wide applications of PET-based non-invasive in vivo cell trafficking.
Highlights
With the recent growth of interest in cell-based therapies and radiolabeled cell products, there is a need to develop more robust cell labeling and imaging methods for in vivo tracking of living cells
Cells have been labeled with 18 F-18 F-2-fluoro-2-deoxy-D-glucose (FDG) [12,13,14,15,16] (T1/2 = 109.8 min), 99mTc-hexamethylpropyleneamine oxime (HMPAO) [17] (T1/2 = 6 h), and 64Cu-TETA- or 89Zr-DFO-antiCD45/ hPBSCs 12.7 h (64Cu)-labeled anti-CD45 [8] (T1/2 = 12.7 h), but the short half-lives of these radioisotopes limit their utility for cell tracking to shorter observational periods
The novel method employs the two-step process (Figure 1): 1) preparation of 89Zr-labeled p-isothiocyanato-benzyldesferrioxamine (89Zr-DBN) and 2) random labeling of primary amines of cell surface proteins with 89Zr-DBN. We have evaluated this labeling strategy in three cell types: mouse melanoma cells, human mesenchymal stem cells, and mouse dendritic cells
Summary
With the recent growth of interest in cell-based therapies and radiolabeled cell products, there is a need to develop more robust cell labeling and imaging methods for in vivo tracking of living cells. With the growth of interest in cell-based therapies, there is a need to develop more sensitive, robust, and quantitative imaging methods for in vivo tracking of living cells. A PET-based approach would offer superior quantification and imaging sensitivity characteristics over a SPECT-based approach, which are critical for tracking of small numbers of administered cells [1]. In this regard, 89Zr has emerged as an attractive PET radionuclide for cell labeling applications due to its high spatial resolution and 78.4-h half-life that may allow monitoring of administered cells up to a 2- to 3-week period. This radiotracer yielded poor in vivo imaging characteristics, possibly due to insufficient CD45 molecules on the plasma membrane of stem cells [8]
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