Fluorine‐18 Radiolabeling and Radiopharmacological Characterization of a Benzodioxolylpyrimidine‐based Radiotracer Targeting the Receptor Tyrosine Kinase EphB4

Abstract

Members of the Eph receptor tyrosine kinase family play essential roles in the pathogenesis of cancer and are therefore promising candidates for molecular imaging by positron emission tomography (PET), for example. In this regard, radiochemical access to novel PET radiotracers derived from potent inhibitors that target the EphB4 kinase domain and which bear a benzodioxolylpyrimidine structural motif was developed. A synthetic route was established for a new fluorine-18-containing radiotracer and for the desired precursor based on a high-affinity benzodioxolylpyrimidine receptor tyrosine kinase inhibitor lead structure. The radiotracer [(18)F]15 was obtained in 16 % radiochemical yield with a specific activity of ∼7 GBq μmol(-1) and >95 % radiochemical purity. Due to the implication of EphB4, particularly in the progression, angiogenesis, and metastasis of melanoma, EphB4-overexpressing human melanoma cells were generated and used as a novel in vitro model for radiopharmacological evaluation of the radiotracer. We demonstrate that the corresponding non-radioactive reference compound regained its functionality as an inhibitor for both EphB4 receptor tyrosine kinase and Src kinase. EphB4 was significantly inhibited at compound concentrations >1 μM. Cellular uptake studies with [(18)F]15 revealed substantial uptake in both EphB4-overexpressing and control cells. Moreover, NMRI nu/nu mice bearing both EphB4-overexpressing tumors and control tumors were used for radiopharmacological characterization by biodistribution studies ex vivo and by dynamic small-animal PET experiments in vivo. Despite the high metabolic stability of the novel radiotracer observed in vivo, no substantial binding or accumulation in EphB4-overexpressing and control tumors was observed. Nevertheless, we point out that the approach presented herein gives convenient access to novel (18)F-labeled benzodioxolylpyrimidines and is a promising strategy for the further development of novel radiotracers for imaging Eph receptor tyrosine kinases in cancer.