Increasing sensitivity of cell tracking by changing labeling and detection from SPECT to PET

Cell tracking by gamma imaging with radionuclides has been performed clinically for over 30 years and is used for tracking autologous leukocytes to detect sites of infection/inflammation. The standard radiolabelling methodology has been non-specific assimilation of lipophilic, metastable complexes of indium-111 (with oxine) or technetium-99m (with HMPAO). Regenerative medicine and immune cell-based therapies are creating new roles for clinical tracking of these cells. Conventional cell radiolabelling methods have been applied for some of these cell types, but for clinical use new applications will require detection of small lesions and small numbers of cells beyond the sensitivity of traditional gamma camera imaging with In-111 or Tc-99m (e.g. coronary artery disease, diabetes, neurovascular inflammation and thrombus), creating a need for positron-emitting radiolabels to exploit the better sensitivity, quantification and resolution of clinical PET.

So far the search for positron emitting (PET) radiolabels for cells has met with limited success. The near-ubiquitous presence of glucose transporters allows labelling with [18F]-FDG but labelling efficiencies are highly variable, the radiolabel is prone to rapid efflux, and the short half-life (110 min) of F-18 allows only brief tracking. Copper-64 offers a longer (12 h) half-life and efficient cell labelling using lipophilic tracers but rapid efflux of label from cells is a persistent problem and a still longer half-life would be preferred. A “PET analogue” of In-111 oxine, capable of cell tracking over 7 days or more, would be highly desirable but is not yet available.

Zr-89 Oxine: a PET cell radiolabelling agent for long term in vivo cell tracking

This paper describes the first synthesis of Zr-89 oxine, and comparison with In-111 oxine for labelling several cell lines, human leukocytes and tracking of the cancer cell line GFP-5T33 cells in mice. The new lipophilic, metastable complex of Zr-89 can radiolabel a range of cells, independently of specific phenotypes, providing a long-sought solution to the unmet need for a long half-life positron-emitting radiolabel to replace In-111 for cell migration imaging. In addition to the expected advantages (enhanced sensitivity, resolution and quantification) of cell tracking with PET rather than scintigraphy or SPECT, Zr-89 shows less efflux from cells in vitro and in vivo than In-111. GFP-5T33 is a syngeneic murine multiple myeloma model originating from the C57Bl/KaLwRij strain, engineered to express green fluorescent protein (GFP). It was chosen for this work because the fate of the cells after i.v. inoculation is known from the literature. Intravenously injected cells migrate exclusively to the liver, spleen and bone marrow. Furthermore as the radiolabelled cells were GFP positive it was possible to validate the non-invasive images by using flow sorting of the GFP positive cells and negative cells. After flow sorting the authors were able to show that after 7 days in vivo the Zr-89 Oxine cells remained viable for the duration of the study, and that ~95% of radioactivity was present in viable GFP+ cells. The excellent in vivo survival and retention of radioactivity by the cells at 7 days, coupled with the demonstrated ability to acquire useful PET images up to 14 days, significantly extend the typical period over which cells can be tracked by radionuclide imaging with directly labelled cells.

The use of PET Zr-89 oxine for cell tracking could have a dramatic impact in the investigation of infection, inflammation and cell-based therapies in humans.

 Original article in EJNMMI