Biodistribution and dosimetry of [I-123]iodo-PK 11195: a potential agent for SPET imaging of the peripheral benzodiazepine receptor

The highest concentrations of the peripheral benzodiazepine receptor (PBR) are found in the kidneys and heart. In addition, the PER has been reported to reflect neuro-inflammatory damage by co-localisation with activated microglia. PK 11195 is a high-affinity ligand for the PER. The aim of this study was to investigate in humans the biodistribution and dosimetry of [I-123]iodo-PK 11195, a potential single-photon emission tomography tracer for the PER. Five healthy volunteers were injected with 112 MBq of [I-123]iodo-PK 11195. Sequential whole-body scans were performed up to 72 h post injection. Multiple blood samples were taken, and urine was collected to measure the fraction voided by the renal system. Decay-corrected regions of interest of the whole-body images were analysed, and geometric mean count rates were used to determine organ activity. Organ absorbed doses and effective dose were calculated using the MIRD method. [I-123]iodo-PK 11195 was rapidly cleared from the blood, mainly by the hepatobiliary system. Approximately 22% was voided in urine after 48 h. Average organ residence times were 0.74, 0.44 and 0.29 h for the liver, upper large intestine and lower large intestine, respectively. The testes received the highest dose. 109.4 mu Gy/MBq. All other organs investigated received doses of less than 50 mu Gy/MBq. The effective dose was 40.3 mu Sv/MBq. In conclusion, [I-123]iodo-PK 11195 is a suitable agent for the visualisation of the PER and indirectly for the imaging of neuro-inflammatory lesions. Taking into account the radiation burden of 7.46 mSv following an administration of 185 MBq, a [I-123]iodo-PK 11195 investigation has to be considered an ICRP risk category IIb investigation.