title: Single-Cell Characterization of 18F-FLT Uptake with Radioluminescence Microscopy.
abstract: The radiotracer 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) is commonly used to measure cell proliferation in vivo. As a marker of cell proliferation, (18)F-FLT is expected to be differentially taken up by arrested and actively dividing cells, but PET measures only aggregate uptake by tumor cells and therefore the single-cell distribution of (18)F-FLT is unknown. We used a novel in vitro radioluminescence microscopy technique to measure the differential distribution of (18)F-FLT radiotracer with single-cell precision.
  
  
  
  Using radioluminescence microscopy, we imaged the absolute uptake of (18)F-FLT in live MDA-MB-231 cells grown under different serum conditions. We then compared (18)F-FLT uptake with a standard measure of cell proliferation, using fluorescence microscopy of 5-ethynyl-2'-deoxyuridine incorporation in fixed cells.
  
  
  
  According to 5-ethynyl-2'-deoxyuridine staining, few cells (1%) actively cycled under serum deprivation whereas most of them (71%) did under 20% serum. The distribution of (18)F-FLT reflected this dynamic. At 0% serum, uptake of (18)F-FLT was heterogeneous but relatively low. At 20% serum, a subpopulation of (18)F-FLT-avid cells, representing 61% of the total population, emerged. Uptake of (18)F-FLT in this population was 5-fold higher than in the remainder of the cells. Such a dichotomous distribution is not typically observed with other radiotracers, such as (18)F-FDG.
  
  
  
  These results suggest that increased (18)F-FLT uptake by proliferating cells is due to a greater fraction of (18)F-FLT-avid cells rather than a change in (18)F-FLT uptake by individual cells. This finding is consistent with the fact that (18)F-FLT uptake is mediated by thymidine kinase 1 expression, which is higher in actively dividing cells. Overall, these findings suggest that, within the same patient, changes in (18)F-FLT uptake reflect changes in the number of actively dividing cells, provided other parameters remain the same.
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Palo Alto, California.
authors.forename: Debanti
authors.initials: D
authors.lastname: Sengupta
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Palo Alto, California pratx@stanford.edu.
authors.forename: Guillem
authors.initials: G
authors.lastname: Pratx
mesh_terms: D045744:Cell Line, Tumor; D049109:Cell Proliferation; D003857:Deoxyuridine / Q000031:analogs & derivatives / Q000378:metabolism; D015224:Dideoxynucleosides / Q000493:pharmacokinetics*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008853:Microscopy / Q000379:methods; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics*; D059010:Single-Cell Analysis; D013937:Thymidine Kinase / Q000096:biosynthesis / Q000378:metabolism
chemical_list: D015224:Dideoxynucleosides; D019275:Radiopharmaceuticals; D013937:Thymidine Kinase; C092717:thymidine kinase 1; C031086:5-ethynyl-2'-deoxyuridine; C002854:alovudine; D003857:Deoxyuridine
keywords: cancer proliferation; single cell radionuclide imaging; thymidine analogs