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 micr oglia. PK 11195 is a high-affinity ligand for the PER. The aim of this stud y 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 o f the whole-body images were analysed, and geometric mean count rates were used to determine organ activity. Organ absorbed doses and effective dose w ere calculated using the MIRD method. [I-123]iodo-PK 11195 was rapidly clea red from the blood, mainly by the hepatobiliary system. Approximately 22% w as voided in urine after 48 h. Average organ residence times were 0.74, 0.4 4 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 eff ective dose was 40.3 mu Sv/MBq. In conclusion, [I-123]iodo-PK 11195 is a su itable agent for the visualisation of the PER and indirectly for the imagin g of neuro-inflammatory lesions. Taking into account the radiation burden o f 7.46 mSv following an administration of 185 MBq, a [I-123]iodo-PK 11195 i nvestigation has to be considered an ICRP risk category IIb investigation.