A Novel PET Probe for Brown Adipose Tissue Imaging in Rodents

Objectives Brown adipose tissue (BAT) is a highly vascularized, mitochondria-rich type of adipose tissue. The study of BAT has generated high interest as a target for the treatment of disorders such as obesity. In this study, we tested the hypothesis that the BODIPY-based PET agent may afford an effective in vivo approach to detect metabolically active BAT depots.

Methods A BODIPY fluorescence dye was radiolabeled with 18F. The resulting 18F-BODIPY probe was used to detect mass and activation of BAT in two mice species and its imaging capability was compared with that of 18F-FDG. The tracer uptake in BAT and major organs were calculated and presented as %ID/g. The ratios between radiotracer uptake in BAT and muscle were compared. All animals were killed at the end of the imaging study for biodistribution. The H&E staining of BAT tissue was performed. The confocal microscopy imaging was conducted to investigate fluorescence signals from BOPIDY. The mechanistic link between 18F-BODIPY and thermogenesis was studied in surrogate cancer cell lines which have abundant mitochondria as brown adipocytes by co-staining with Mitotracker.

Results In Balb/c mice, 18F-BODIPY accumulation in BAT was higher in stimulated animals (3.93± 0.98; n=6) than in un-stimulated animals (2.804 ± 0.54; n=8) (p<0.05), respectively. In C57bl6 mice, 18F-BODIPY accumulation in BAT was significantly higher in stimulated animals (6.205 ± 1.284; n=6) than in un-stimulated animals (3.162 ±0.5311; n=8) (p<0.001). 18F-FDG PET imaging was performed in Balb/c mice and showed significantly increased uptake in thermogenic BAT (10.08 ± 2.52%ID/g, n=3) than in inactive BAT (3.803 ±0.70%ID/g; n=3), which is consistent with literature report. The biodistribution results were consistent with PET imaging results. Because 18F-BODIPY maintained its fluorescent property, BAT tissues were collected and studied using fluorescence microscope. The higher fluorescence signal was observed in thermogenic BAT than in un-stimulated BAT. Co-staining of tumor cell lines with BODIPY and Mitotracker showed intracellular co-localization of BODIPY dye and mitochondria. This result, combined with our previous research indicated that 18F-BODIPY , upon membrane potential-dependent cellular uptake, accumulated extensively in thermogenic BAT.

Conclusions The newly developed 18F-BODIPY showed relatively low accumulation in unstimulated BAT, but significantly higher accumulations in stimulated thermogenic BAT. In contrast to non-specific higher uptake of 18F-FDG BAT imaging which demonstrated the increased glycolysis in thermogenic condition, 18F-BODIPY which going through oxidization pathway in mitochondria could serve as a valuable supplement to current BAT PET imaging agents.

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