Whole-body biodistribution, radiation absorbed dose, and brain SPET imaging with [¹²³I]5-I-A-85380 in healthy human subjects

The biodistribution of radioactivity after the administration of a new tracer for α4β2 nicotinic acetylcholine receptors (nAChRs), [¹²³I]5-iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380), was studied in ten healthy human subjects. Following administration of 98±6 MBq [¹²³I]5-I-A-85380, serial whole-body images were acquired over 24 h and corrected for attenuation. One to four brain single-photon emission tomography (SPET) images were also acquired between 2.5 and 24 h. Estimates of radiation absorbed dose were calculated using MIRDOSE 3.1 with a dynamic bladder model and a dynamic gastrointestinal tract model. The estimates of the highest absorbed dose (μGy/MBq) were for the urinary bladder wall (71 and 140), lower large intestine wall (70 and 72), and upper large intestine wall (63 and 64), with 2.4-h and 4.8-h urine voiding intervals, respectively. The whole brain activity at the time of the initial whole-body imaging at 14 min was 5.0% of the injected dose. Consistent with the known distribution of α4β2 nAChRs, SPET images showed the highest activity in the thalamus. These results suggest that [¹²³I]5-I-A-85380 is a promising SPET agent to image α4β2 nAChRs in humans, with acceptable dosimetry and high brain uptake.