Kinetic analysis of the translocator protein positron emission tomography ligand [¹⁸F]GE-180 in the human brain

Purpose: PET can image neuroinflammation by targeting the translocator protein (TSPO), which is upregulated in activated microglia. The high nonspecific binding of the first-generation TSPO radioligand [¹¹C]PK-11195 limits accurate quantification. [¹⁸F]GE-180, a novel TSPO ligand, displays superior binding to [¹¹C]PK-11195 in vitro. Our objectives were to: (1) evaluate tracer characteristics of [¹⁸F]GE-180 in the brains of healthy human subjects; and (2) investigate whether the TSPO Ala147Thr polymorphism influences outcome measures.

Methods: Ten volunteers (five high-affinity binders, HABs, and five mixed-affinity binders, MABs) underwent a dynamic PET scan with arterial sampling after injection of [¹⁸F]GE-180. Kinetic modelling of time-activity curves with one-tissue and two-tissue compartment models and Logan graphical analysis was applied to the data. The primary outcome measure was the total volume of distribution (V _T) across various regions of interest (ROIs). Secondary outcome measures were the standardized uptake values (SUV), the distribution volume and SUV ratios estimated using a pseudoreference region.

Results: The two-tissue compartment model was the best model. The average regional delivery rate constant (K ₁) was 0.01 mL cm^-3 min^-1 indicating low extraction across the blood-brain barrier (1 %). The estimated median V _T across all ROIs was also low, ranging from 0.16 mL cm^-3 in the striatum to 0.38 mL cm^-3 in the thalamus. There were no significant differences in V _T between HABs and MABs across all ROIs.

Conclusion: A reversible two-tissue compartment model fitted the data well and determined that the tracer has a low first-pass extraction (approximately 1 %) and low V _T estimates in healthy individuals. There was no observable dependency on the rs6971 polymorphism as compared to other second-generation TSPO PET tracers. Investigation of [¹⁸F]GE-180 in populations with neuroinflammatory disease is needed to determine its suitability for quantitative assessment of TSPO expression.