Novel 89Zr cell labeling approach for PET-based cell trafficking studies

Background: 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. This study describes evaluation of a novel cell labeling approach with the positron emission tomography (PET) isotope Zr-89 (T-1/2 = 78.4 h). Zr-89 may allow PET imaging measurements for several weeks and take advantage of the high sensitivity of PET imaging. Methods: A novel cell labeling agent, Zr-89-desferrioxamine-NCS (Zr-89-DBN), was synthesized. Mouse-derived melanoma cells (mMCs), dendritic cells (mDCs), and human mesenchymal stem cells (hMSCs) were covalently labeled with Zr-89-DBN via the reaction between the NCS group on Zr-89-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 Zr-89-labeled hMSCs. For comparison, imaging experiments were performed after intravenous injections of Zr-89 hydrogen phosphate (Zr-89(HPO4)(2)). Results: The labeling agent, Zr-89-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/10(6) 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 Zr-89-labeled hMSCs showed retention in the myocardium, as well as redistribution to the lung, liver, and bone. Intravenously administered Zr-89-labeled hMSCs also distributed primarily to the lung, liver, and bone, whereas intravenous Zr-89(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. Conclusions: We have developed a robust, general, and biostable Zr-89-DBN-based cell labeling strategy with promise for wide applications of PET-based non-invasive in vivo cell trafficking.