Synthesis, radiosynthesis and first in vitro evaluation of novel PET-tracers for the dopamine transporter: [(11)C]IPCIT and [(18)F]FE@IPCIT

Introduction: Present data indicate that merging beneficial structural elements from previously published DAT-ligands highest DAT affinity, selectivity and a suitable metabolic profile should be achieved. This combination led to the development of IPCIT and FE@IPCIT.

Methods: Precursor synthesis was done starting from cocaine in a six step reaction. O-[(11)C]-methylation was established using [(11)C]methyl iodide, optimized and subsequently automated. Small scale (18)F-fluroroethylation as well as optimization of reaction parameters and automation were performed. Affinity and selectivity of the candidate substances were tested in standard binding experiments on human membranes. Metabolic stability and blood-brain-barrier (BBB) penetration were determined.

Results: Precursor compound, IPCITacid, and reference compounds, IPCIT and FE@IPCIT, were obtained in 4.9%, 12.7% and 4.1% yield, respectively. Automated radiosynthesis of [(11)C]IPCIT yielded 1.9 ± 0.7 GBq (12.5 ± 4%, corrected for decay). Optimum parameters for (18)F-fluoroethylation were 110 °C for 15 min under TBAH catalysis, yielding 67 ± 16 % radiochemical incorporation. Affinity was determined as 1.7 ± 0.6 nM for IPCIT, 1.3 ± 0.2 nM for FE@IPCIT and 37 ± 13 nM for the precursor molecule, IPCIT-acid. Results from in vitro and in silico evaluations revealed high stability but also high lipophilicity.

Conclusion: Present data indicate high affinity and stability of both IPCIT and FE@IPCIT. Radiolabelling, optimization of reaction parameters and automation succeeded. On the other hand, data concerning BBB-penetration are not promising.