In vivo evaluation of a novel 18 F-labeled PET radioligand for translocator protein 18 kDa (TSPO) in monkey brain

Purpose [ 18 F]SF51 was previously found to have high binding affinity and selectivity for 18 kDa translocator protein (TSPO) in mouse brain. This study sought to assess the ability of [ 18 F]SF51 to quantify TSPO in rhesus monkey brain. Methods Positron emission tomography (PET) imaging was performed in monkey brain (n = 3) at baseline and after pre-blockade with the TSPO ligands PK11195 and PBR28. TSPO binding was calculated as total distribution volume corrected for free parent fraction in plasma ( V T / f P ) using a two-tissue compartment model. Receptor occupancy and nondisplaceable uptake were determined via Lassen plot. Binding potential ( BP ND ) was calculated as the ratio of specific binding to nondisplaceable uptake. Time stability of V T was used as an indirect probe to detect radiometabolite accumulation in the brain. In vivo and ex vivo experiments were performed in mice to determine the distribution of the radioligand. Results After [ 18 F]SF51 injection, the concentration of brain radioactivity peaked at 2.0 standardized uptake value (SUV) at ~ 10 min and declined to 30% of the peak at 180 min. V T / f P at baseline was generally high (203 ± 15 mL· cm −3 ) and decreased by ~ 90% after blockade with PK11195. BP ND of the whole brain was 7.6 ± 4.3. V T values reached levels similar to terminal 180-min values by 100 min and remained relatively stable thereafter with excellent identifiability (standard errors < 5%), suggesting that no significant radiometabolites accumulated in the brain. Ex vivo experiments in mouse brain showed that 96% of radioactivity was parent. No significant uptake was observed in the skull, suggesting a lack of defluorination in vivo. Conclusion The results demonstrate that [ 18 F]SF51 is an excellent radioligand that can quantify TSPO with a good ratio of specific to nondisplaceable uptake and has minimal radiometabolite accumulation in brain. Collectively, the results suggest that [ 18 F]SF51 warrants further evaluation in humans.