Preparation of a novel radiotracer targeting the EphB4 receptor via radiofluorination using spiro azetidinium salts as precursor

The visualization of Eph receptors, which are overexpressed in various tumor entities, using selective small molecule Eph inhibitors by means of PET is a promising approach for tumor imaging. Indazolylpyrimidineamines represent a class of compounds, which are known to have high affinity especially for the EphB4 receptor. Radiofluorination of these compounds could provide a highly specific imaging agent and was investigated using a classical nucleophilic introduction of [18F]fluoride as well as a less common nucleophilic ring opening reaction of azetidinium salts. In the past, radiofluorinations using azetidinium precursors were demonstrated to result in high radiochemical yields in short periods of time. For this purpose, an azetidinium precursor based on the indazolylpyrimidineamine lead compound was developed and radiofluorination was successfully accomplished. The respective [18F]radiotracer was quickly prepared with high radiochemical purity >97% and in a radiochemical yield of 34%.