Comparative in vivo study of iodine-123-labeled beta-CIT and nor-beta-CIT binding to serotonin transporters in rat brain

Both iodine-123-labeled beta-CIT (2 beta-carbomethoxy-3 beta-(4-iodophenyl) tropane) and nor-beta-CIT (2 beta-carbomethoxy-3 beta-(4-iodophenyl)nortrop ane) have shown to be suitable radioligands for imaging serotonin (5-HT) tr ansporters. [I-123]nor-beta-CIT has the highest in vitro affinity for 5-HT transporters among beta-CIT analogs reported so far. However, no direct com parison-studies of these two radiotracers as to their in vivo binding to 5- HT transporters have been reported so far. Therefore, it is still unclear w hich of the two radiotracers is more suitable for single photon emisison co mputed tomograpy (SPECT) imaging of 5-HT transporters. The purpose of this study was to compare directly in a controlled design the in vivo [I-123]bet a-CIT and [I-123]nor-beta-CIT binding to 5-HT transporters under the same c onditions in rats with the focus on brain kinetic characteristics by means of a two-compartment analysis. We observed that [I-123]beta-CIT has a highe r binding potential and faster kinetics for 5-HT transporters than [I-123]n or-beta-CIT, suggesting that [I-123]beta-CIT may be a more suitable radioli gand than [I-123]nor-beta-CIT for imaging 5-HT transporters with SPECT. (C) 1999 Wiley-Liss, Inc.