First preclinical SPECT/CT imaging and biodistribution of [165Er]ErCl3 and [165Er]Er-PSMA-617

165Er (t1/2 = 10.4 h, Ex-ray = 47.1 keV (59.4%) and 54.3 keV (14.3%)) is a promising radionuclide suitable for targeted Auger electron therapy of cancer. 165Er can be produced at a relatively low cost, high yield, and high purity using small medical cyclotrons. As a late lanthanide, 165Er is easy to label and can be used as a surrogate for other lanthanides or Ac in proof-of-concept studies. In this report, we explore the radiochemistry, in vitro, and in vivo behavior of [165Er]ErCl3 and [165Er]Er-PSMA-617 to showcase the application of this radionuclide. Particularly, we report the first phantom and preclinical SPECT imaging of this radionuclide leveraging its characteristic X-ray photon emissions.