Head-to-Head Comparison of 11 C-PBR28 and 18 F-GE180 for Quantification of the Translocator Protein in the Human Brain.

¹⁸ F-GE180 is a third-generation PET tracer for quantifying the translocator protein (TSPO), a biomarker for inflammation. The aim of this study was to perform a head-to-head comparison of ¹⁸ F-GE180 and the well-established TSPO tracer ¹¹ C-PBR28 by scanning with both tracers during the same day in the same subjects. Methods: Five subjects underwent a 90-min PET scan with ¹¹ C-PBR28 in the morning and ¹⁸ F-GE180 in the afternoon. A metabolite-corrected arterial input function was obtained in each subject for both tracers, and the brain uptake was quantified with a 2-tissue-compartment model. Results: The rate of metabolism of ¹⁸ F-GE180 in arterial blood was slower than that of ¹¹ C-PBR28 (the percentages of nonmetabolized parent in plasma at 90 min were 74.9% ± 4.15% [mean ± SD] and 11.2% ± 1.90%, respectively). The plasma free fractions were similar for both tracers: 3.5% ± 1.1% for ¹⁸ F-GE180 and 4.1% ± 1.1% for ¹¹ C-PBR28. The average total volume of distribution ( V _T ) of ¹⁸ F-GE180 was about 20 times smaller than that of ¹¹ C-PBR28 (0.15 ± 0.03 mL/cm ³ for ¹⁸ F-GE180 and 3.27 ± 0.66 mL/cm ³ for ¹¹ C-PBR28). ¹⁸ F-GE180 was characterized by poor transfer from the vascular compartment to the brain (its plasma-to-tissue rate constant [K ₁ ] was about 10 times smaller than that of ¹¹ C-PBR28). Moreover, kinetic modeling was more difficult with ¹⁸ F-GE180, as its V _T values were identified with a lower precision than those of ¹¹ C-PBR28 and outlying values were more frequent. Conclusion: The V _T of ¹⁸ F-GE180 was about 20 times smaller than that of ¹¹ C-PBR28 because of low penetration into the brain from the vascular compartment. In addition, kinetic modeling of ¹⁸ F-GE180 was more challenging than that of ¹¹ C-PBR28. Therefore, compared with ¹¹ C-PBR28, ¹⁸ F-GE180 had unfavorable characteristics for TSPO imaging of the brain.
(© 2018 by the Society of Nuclear Medicine and Molecular Imaging.)