“New Kid on the Block” 161Tb-FAPI-2286 in Metastatic Papillary Thyroid Cancer

Background: 161 Tb has decay properties similar to those of 177 Lu, but also emits a substantial number of conversion and Auger electrons. The aim of this study was to apply 161 Tb in a clinical setting and investigate the feasibility of visualizing the physiological and tumor biodistributions of 161 Tb-FAPI-2286 in metastatic papillary thyroid cancer (PTC). Patients and Methods: 161 Tb was used for the radiolabeling of FAPI-2286. 161 Tb-FAPI-2286 was administered to a 69-year-old male patient with an invasive encapsulated follicular variant of PTC. Whole-body planar γ-scintigraphy images were acquired at 2, 23, 47, and 96 hours post injection for dosimetry calculations. SPECT images were reconstructed using a recently established protocol and visually analyzed. The patient was observed for immediate and delayed adverse events after the administration of 161 Tb- FAPI-2286. Results: Radiolabeling of FAPI-2286 with 161 Tb was readily achieved with a high radiochemical purity suitable for patient application. Planar images and dosimetry revealed the expected time-dependent biodistribution of 161 Tb-FAPI-2286 in the liver, kidneys, spleen, and urinary bladder. The application of 161 Tb-FAPI-2286 was well tolerated, and no therapy-related adverse events were observed. Conclusions: This study demonstrated the feasibility of 161 Tb-FAPI-2286 using γ-scintigraphy and SPECT imaging. Based on this essential first step in translating 161 Tb to clinical use, further efforts will be directed toward the application of 161 Tb for therapeutic purposes.