*Article* **A High Separation Factor for <sup>165</sup>Er from Ho for Targeted Radionuclide Therapy**

**Isidro Da Silva 1,2 , Taylor R. Johnson <sup>1</sup> , Jason C. Mixdorf <sup>1</sup> , Eduardo Aluicio-Sarduy <sup>1</sup> , Todd E. Barnhart <sup>1</sup> , R. Jerome Nickles <sup>1</sup> , Jonathan W. Engle 1,3 and Paul A. Ellison 1,\***


**Abstract: Background:** Radionuclides emitting Auger electrons (AEs) with low (0.02–50 keV) energy, short (0.0007–40 µm) range, and high (1–10 keV/µm) linear energy transfer may have an important role in the targeted radionuclide therapy of metastatic and disseminated disease. Erbium-165 is a pure AE-emitting radionuclide that is chemically matched to clinical therapeutic radionuclide <sup>177</sup>Lu, making it a useful tool for fundamental studies on the biological effects of AEs. This work develops new biomedical cyclotron irradiation and radiochemical isolation methods to produce <sup>165</sup>Er suitable for targeted radionuclide therapeutic studies and characterizes a new such agent targeting prostate-specific membrane antigen. **Methods:** Biomedical cyclotrons proton-irradiated spot-welded Ho(m) targets to produce <sup>165</sup>Er, which was isolated via cation exchange chromatography (AG 50W-X8, 200–400 mesh, 20 mL) using alpha-hydroxyisobutyrate (70 mM, pH 4.7) followed by LN2 (20–50 µm, 1.3 mL) and bDGA (50–100 µm, 0.2 mL) extraction chromatography. The purified <sup>165</sup>Er was radiolabeled with standard radiometal chelators and used to produce and characterize a new AE-emitting radiopharmaceutical, [165Er]PSMA-617. **Results:** Irradiation of 80–180 mg natHo targets with 40 <sup>µ</sup>A of 11–12.5 MeV protons produced <sup>165</sup>Er at 20–30 MBq·µA−<sup>1</sup> ·h −1 . The 4.9 <sup>±</sup> 0.7 h radiochemical isolation yielded <sup>165</sup>Er in 0.01 M HCl (400 <sup>µ</sup>L) with decay-corrected (DC) yield of 64 <sup>±</sup> 2% and a Ho/165Er separation factor of (2.8 <sup>±</sup> 1.1) · <sup>10</sup><sup>5</sup> . Radiolabeling experiments synthesized [165Er]PSMA-617 at DC molar activities of 37–130 GBq·µmol−<sup>1</sup> . **Conclusions:** A 2 h biomedical cyclotron irradiation and 5 h radiochemical separation produced GBq-scale <sup>165</sup>Er suitable for producing radiopharmaceuticals at molar activities satisfactory for investigations of targeted radionuclide therapeutics. This will enable fundamental radiation biology experiments of pure AE-emitting therapeutic radiopharmaceuticals such as [165Er]PSMA-617, which will be used to understand the impact of AEs in PSMA-targeted radionuclide therapy of prostate cancer.

**Keywords:** targeted radionuclide therapy; auger emission; radionuclide production; lanthanide separation; erbium-165; <sup>165</sup>Er
