Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Objectives:

The aim was to investigate whether fluorocyclobutyl rings can be introduced into targeting probes to improve metabolic stability, while maintaining its binding affinity, using tyrosine as a model system for the LAT transporters.

Methods:

The precursor, cis-cyclobutane-1,3-diol ditosylate, its corresponding F-19 reference compound trans-3-fluorocyclobutanol (FCB), along with the cis-(3-fluorocyclobutyl)-tyrosine (3FCBT), were synthesized using standard organic chemistry methodologies. The non-radioactive 3FCBT was tested in competition and efflux stimulation cell assays using A549 human lung carcinoma cells with [³H]-D-Tyrosine. The metabolic stability of reference compound 3FCBT was studied in both rat hepatocytes and human plasma. Radiosynthesis methods using standard radiofluorination of the prosthetic group [¹⁸F]FCB and its conjugation to tyrosine gave the desired 3[¹⁸F]FCBT after chromatographic purification. In vitro studies were performed in A549 cells using 3[¹⁸F]FCBT and incubated at 37°C for 10, 20, 30 and 60 minutes with and without inhibitors fluoroethyl-tyrosine (FET) and non-radioactive
3FCBT.

Results:

The syntheses of cis-cyclobutane-1,3-diol ditosylate, trans-3-fluorocyclobutanol (FCB), along with the cis-(3-fluorocyclobutyl)-tyrosine (3FCBT) were established. 3FCBT was shown to block the uptake of [³H]-D-tyrosine in the competition cell assay and could stimulate the release of ³H]-D-Tyrosine from the cell in an efflux stimulation cell assay. 3FCBT showed very high stability in both rat hepatocytes (> 95%) and human plasma (> 95%). The unoptimized radiosynthesis gave the desired 3[¹⁸F]FCBT, via the prosthetic group [¹⁸F]FCB, in moderate yield (12%) with high radiochemical purity (> 99%). The cell uptake showed an increase of 3[¹⁸F]FCBT over time and reached a plateau of 5.87% after 30 minutes.

Conclusions:

The radiosynthesis of the prosthetic group [¹⁸F]FCB and its conjugation to tyrosine to give 3[¹⁸F]FCBT were successfully established. The introduction of 3[¹⁸F]FCBT into the LAT-targeting vector D-Tyr was characterized by a significant in vitro uptake in A549 cells and was actively transported into these cells. The encouraging results warrant further investigations of this tracer in the in vivo setting.