Specificity of translocator protein-targeted positron emission tomography in inflammatory joint disease

Expression of the translocator protein (TSPO) on inflammatory cells has facilitated imaging of synovitis with TSPO-targeted positron emission tomography (PET). We aimed to quantitatively assess the specificity of the second-generation TSPO PET radioligand, [¹¹C]PBR28, and to generate simplified PET protocols in patients with inflammatory joint disease (IJD) in this pilot study.

Three IJD patients (two rheumatoid arthritis and one osteoarthritis) with knee involvement underwent dynamic [¹¹C]PBR28-PET scans before and after administration of 90 mg of oral emapunil (XBD-173), a TSPO ligand the same day. Radial arterial blood sampling was performed throughout the scan, and total radioactivity and radioactive metabolites were obtained. A semi-automated method was used to generate regions of interest. Standardized uptake value (SUV) and SUV ratio corrected for activity in bone and blood between 50 and 70 min (SUVr_50-70 bone, SUVr_50-70 blood, respectively) and PET volume of distribution (V_T) of the radioligand were calculated.

A mean [¹¹C]PBR28 radioactivity of 378 (range 362-389) MBq was administered. A significant decrease (p < 0.05) in V_T, SUVr_50-70 bone and SUVr_50-70 blood observed after oral emapunil confirmed the TSPO specificity of [¹¹C]PBR28. A decrease in SUV was not observed in the post-block scan.

[¹¹C]PBR28 is TSPO-specific radioligand in IJD patients. Simplified PET protocols with static PET acquisition can be used in the management and evaluation of novel therapeutics that target TSPO overexpressing cells.