Radiosynthesis of 5-(2-[¹⁸F]fluoroethyl)-sunitinib as inhibitor of VEGFR-2-first results

Aim:

Radiolabeled inhibitors of receptor tyrosine kinases (RTK) might be suitable probes for monitoring pathophysiological situations related to enhanced expression of the vascular endothelial growth factor receptor (VEGFR). Imaging of angiogenesis with PET could facilitate for the individual patient the evaluation of e.g. the success of corresponding anti-angiogenic chemotherapy or monitoring the stimulation of the endogenous adaptive vessel growth after implantation of
bioMaterials. For this purpose we developed an 18F-radiolabeled probe, 5-(2-[18F]fluoroethyl)-sunitinib basing on the lead structure of sunitinib®, a multi-kinase inhibitor selective to VEGFR-2.
Materials and Methods:
The non-radioactive reference compound 5-(2-fluoroethyl)-sunitinib was synthesized by Knoevenagel condensation of 5-(2-fluoroethyl)-indoline-2-one with N-[2-(diethylamino)ethyl]-2,4-dimethyl-5-formyl-1H-pyrrole-3-carboxamide. Two suitable precursors for radiolabeling, A and B were obtained by reacting a 5-(2-bromoethyl)-substituted sunitinib derivative with silver 4-toluenesulfonate and silver methanesulfonate, respectively. [18F]Fluoride was produced by the 18O(p,n)18F reaction from [18O]H2O in a 18/9 cyclotron (IBA), separated by an anion exchange cartridge (QMA, Waters) and activated by azeotropic drying with acetonitrile in a stream of nitrogen before use. In a set of radiolabeling experiments 4 mg of precursor A or B were reacted with [18F]fluoride in 500μL of solvent at a scheduled temperature regime for 20 min. The yield of 5-(2-[18F]fluoroethyl)-sunitinib was determined by radio-TLC (silicagel, THF/TEA=9/1).
Results:
The non-radioactive 5-(2-fluoroethyl)-sunitinib was investigated in a competition binding assay against VEGFR-2; a Kd value of 9 nM is justifying its classification as specific inhibitor. The radiolabeling reaction of the precursors A and B was performed at temperatures varying from 60, 90, and 120°C in one of the following solvents: acetonitrile, DMF, and DMSO. As result it turned out that for both precursors in DMF and DMSO only poor labeling yields about 2% could be achieved, whereas by using acetonitrile at 90°C the yield of 5-(2-[18F]fluoroethyl)-sunitinib increased to 7-9%. Reaction temperatures higher than 90°C lead to fast and complete decomposition of the precursors as monitored by several non-radioactive by-products on TLC. Notably no difference in yield was observed by using the methanesulfonyl- or the 4-toluenesulfonyl-precursor.
Conclusions:
The new VEGFR-2 targeted radiolabeled probe 5-(2-[18F]fluoroethyl)-sunitinib was successfully synthesized by radiofluorination of the corresponding methanesulfonyl- or 4-toluenesulfonyl-substituted precursor with [18F]fluoride. First attempts to transfer the labeling method to a remote-controlled system were successful. By now a procedure for the purification of the radiotracer by semipreparative HPLC and SPE is under development to enable the radiopharmacological evaluation.