Human biodistribution and radiation dosimetry of 11C-(R)-PK11195, the prototypic PET ligand to image inflammation

The positron emission tomography (PET) radiotracer 11C-(R)-PK11195 allows the in vivo imaging in humans of the translocator protein 18 kDa (TSPO), previously called peripheral benzodiazepine receptor (PBR), a marker of inflammation. Despite its widespread use, the radiation burden associated with 11C-(R)-PK11195 in humans is not known. To examine this, we performed dynamic whole-body imaging with PET and 11C-(R)-PK11195 in healthy humans. Five healthy male volunteers were scanned with PET and 11C-(R)-PK11195, using a dynamic whole-body imaging protocol. An organ-specific method was used to measure accumulated radioactivity in source organs, and residence times were calculated as areas under the curve of time-activity curves expressed as percentage of injected radioactivity. Residence times were used as input for OLINDA/EXM 1.0 software to model the equivalent organ doses and the effective dose for the 70-kg man. After intravenous injection of 11C-(R)-PK11195, radioactivity accumulated in organs rich in TSPO as well as routes of excretion: the hepatobiliary system and the urine. The mean effective dose was 4.8 μSv/MBq according to International Commission on Radiological Protection (ICRP) Publication 60 and 5.1 μSv/MBq according to ICRP Publication 103, and the highest equivalent organ doses were observed in the kidneys (14.0 μSv/MBq), spleen (12.5 μSv/MBq) and small intestine (12.2 μSv/MBq). Imaging of TSPO with PET using 11C-(R)-PK11195 is associated with modest radiation exposure, similar in magnitude to most other 11C-labelled PET tracers, suggesting feasibility of 11C-(R)-PK11195 imaging in clinical human studies involving multiple scans in the same subjects per year. (orig.)