In vivo quantification by SPECT of [ ¹²³I] ADAM bound to serotonin transporters in the brains of rabbits

A novel radioiodine ligand [ ¹²³I] ADAM (2-((2-((dimethylamino) methyl)phenyl)thio)-5-iodophenylamine) has been suggested as a promising serotonin transporter (SERT) imaging agent for the central nervous system. In this study, the biodistribution of SERTs in the rabbit brain was investigated using [ ¹²³I] ADAM and mapping images of the same animal produced by both single-photon emission computed tomography (SPECT) and microautoradiography. A semiquantification method was adopted to deduce the optimum time for SPECT imaging, whereas the input for a simple fully quantitative tracer kinetic model was provided from arterial blood sampling data. SPECT imaging was performed on female rabbits postinjection of 185 MBq [ ¹²³I] ADAM. The time-activity curve obtained from the SPECT images was used to quantify the SERTs, for which the binding potential was calculated from the kinetic modeling of [ ¹²³I] ADAM. The kinetic data were analyzed by the nonlinear least squares method. The effects of the selective serotonin reuptake inhibitors fluoxetine and p-chloroamphetamine (PCA) on rabbits were also evaluated. After scanning, the same animal was sacrificed and the brain was removed for microautoradiography. Regions-of-interest were analyzed using both SPECT and microautoradiography images. The SPECT images were coregistered manually with the corresponding microautoradiography images for comparative study. During the time interval 90-100 min postinjection, the peak specific binding levels in different brain regions were compared and the brain stem was shown to have the highest activity. The target-to-background ratio was 1.89±0.02. Similar studies with fluoxetine and PCA showed a background level for SERT occupation. Microautoradiography demonstrated a higher level of anatomical details of the [ ¹²³I] ADAM distribution than that obtained by SPECT imaging of the rabbit brain. SPECT imaging of the rabbit brain with [ ¹²³I] ADAM showed high affinity, high specificity, and favorable kinetics. The time-activity curve showed that the accumulation of the [ ¹²³I] ADAM in the brain stem reached a maximum between 90 and 100 min postinjection. The microautoradiography provides high-resolution images of the rabbit brain. Our results for the [ ¹²³I] ADAM biodistribution in the rabbit brains demonstrate that this new radioligand is suitable as a selective SPECT imaging agent for SERTs.