Aqueous colloidal nanoemulsion of perfluorocarbon polymers

Cell tracking with magnetic resonance imaging (MRI) is achieved by labeling cells of interest with a contrast agent or by transducing cells to express reporter genes (1, 2). In both preclinical and clinical situations, iron oxide–based agents are typically used and provide a strong change in signal per unit of metal in T2-weighted images without significant effect on labeled cells and on the host. However, MRI quantification of iron oxide concentration is still not reliable because there is no clear correlation between the iron oxide signal and the number of labeled cells and measuring a decrease in signal has always been a problem (2, 3). An alternative development for quantitative cell tracking is the application of F in MRI imaging (4, 5). Endogenous fluorine is found primarily in bones and teeth as solid fluorides, and it has a very short T2 relaxation time and results in an undetectable signal with F MRI. Cell tracking studies with fluorinated tracers have shown that the labeled cells can be detected on the order of 10–10 cells per voxel for clinical MRI systems and 10–10 cells per voxel for high-field animal scanners (3, 6). The MRI images positive for the F signal are extremely selective for the labeled cells. The absolute number of the labeled cells could be measured directly from the F images, thus providing a quantitative biomarker. The positive F signal is also of great advantage for cell tracking in regions such as lungs, tissues and organs of the abdominal cavity, and bones. Although cell division and subsequent dilution of the tracers limit long-term cell tracking and decrease the accuracy of cell quantification, it has been shown that the absolute cell number can be quantified for up to 3 weeks in actively dividing cells in mice and that the underestimation of the cell number is within tolerable limits (usually two- to four-fold lower depending on the cell division rate and length of time) (6). This error range can be reduced if the cell division rate is known. Similar to what has been observed with iron oxide agents, the death of the labeled cells may result in transfer of the tracers to phagocytes, resulting in false positive signals (3, 7). CS-1000 is a commercially available reagent specifically formulated to facilitate its internalization into various cell types regardless of the cell's inherent phagocytic ability (3, 7). CS-1000 is an aqueous colloidal nanoemulsion of perfluorocarbon polymers. Perfluorocarbon is both hydrophobic and lipophobic, and it does not become associated with cell membranes (3, 8). The perfluorocarbon component maintains its structure at typical lysosomal pH values and is not degraded by cell enzymes found in the body, which allows it to provide long-lasting intracellular labeling. Bonetto et al. investigated the utility of CS-1000 for tracking and quantification of human dendritic cells (DCs) directly from image data (3). The investigators showed that the minimum number of labeled cells detectable at 7 T was ~2,000 per voxel, with 1.7 × 10 fluorine atoms per cell (3). DCs are antigen-presenting cells of the immune system with key roles in inducing immunity, which forms the rationale of DC immunotherapy (5). Monocyte-derived DCs do not divide (3), which makes DC quantification and tracking studies particularly suitable because there is no signal reduction and data misinterpretation is less likely because the results are not affected by the uncontrolled process of cell division.