Abstract 2467: SPECT-imaging guided development and evaluation of targeted alpha therapy (TAT) for colorectal liver metastasis
Authors: Suurs Frans V. , Kristian Alexander , Petros Gebregziabher , Feng Yuan Zeng , Fleten Karianne G. , Bjerke Roger M. , Cuthbertson Alan , Flatmark Kjersti 
Published: June 2022

Abstract
BACKGROUND: Only a small fraction of patients with colorectal liver metastases (CLM) is eligible for curative resection, and novel treatment options are highly needed. The high linear-energy transfer of short-ranged alpha particles can induce complex double-stranded DNA breaks, leading to cell death. With no known resistance mechanism, alpha-particle emitting isotopes represent a promising tool in cancer management. In targeted alpha therapy (TAT), an alpha-particle emitting isotope is attached to a tumor-targeting antibody (hereafter TAT antibody), ensuring specific delivery to the tumor. By substituting the alpha-particle emitting isotope (Actinium-225, 225Ac) with a diagnostic isotope (Zirconium-89, 89Zr), the in-vivo distribution of the TAT antibody can be monitored using PET/SPECT/CT. The aim of this work was to set up robust labeling and PET/SPECT protocols to evaluate TAT in CLM using novel experimental models.
METHODS: For SPECT-imaging of 225Ac daughter isotopes, the combined gamma emission spectra were gated at 218 keV ± 20% and 440 keV ± 20% for Francium-221 and Bismuth-213, respectively. 89Zr was labeled to a desferrioxamine* (DFO*) conjugated TAT antibody, which recognizes both the murine and human antigen. Indium-111 (111In) labeling was performed through diethylenetriaminepentaacetic acid (DTPA) conjugated to an isotype antibody. Radiochemical purity and protein integrity were assessed by size exclusion HPLC, iTLC and SDS-PAGE. For orthotopic CLM model development, 106 cells of human colorectal cancer cell lines HT-55 or LS1034 were injected in the spleen of female Rj:NMRI-Foxn1nu mice, followed by splenectomy. Tumor growth was monitored by T2-weighted MR-imaging. Non-tumor bearing mice (n = 12, 3 per group) were injected with 4 MBq of 10, 20, 50 or 100 µg 89Zr-TAT antibody to evaluate biodistribution and the mice scanned on day 2, 4 and 6 after iv injection.
RESULTS: Francium-221 and Bismuth-213 could be separately visualized after free Actinium-225 administration (i.v.). 89Zr and 111In were routinely labeled to their respective antibody with up to 500 MBq/mg with &gt;95% radiochemical purity. 89Zr-DFO*-TAT antibody showed &gt;95% radiochemical purity and &gt;95% protein integrity after 7 days stability test in serum, shown by iTLC and SDS-PAGE. SPECT/CT of 89Zr-DFO*-TAT antibody in non-tumor bearing mice showed little uptake in healthy tissues apart from the liver due to hepatic antibody clearance. Orthotopic models of CLM show reproducible development of metastases for HT-55 and LS1034 within 3 weeks.
CONCLUSION: Robust labeling methods, PET/SPECT imaging protocols and suitable animal models of CLM were established. Results of our dual-isotope SPECT/CT imaging of our 89Zr-DFO*-TAT antibody in comparison with an 111In-labeled isotype antibody, together with efficacy data of the TAT, will be presented at the meeting.
Citation Format: Frans V. Suurs, Alexander Kristian, Gebregziabher Petros, Yuan Zeng Feng, Karianne G. Fleten, Roger M. Bjerke, Alan Cuthbertson, Kjersti Flatmark. SPECT-imaging guided development and evaluation of targeted alpha therapy (TAT) for colorectal liver metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2467.