Abstract 5440: Development of a novel PSMA-targeting small molecule-Thorium-227 conjugate for the treatment of prostate cancer
Authors: Boehnke Niels , Indrevoll Bård , Hammer Stefanie , Papple Alex , Kristian Alexander , Cuthbertson Alan , Zitzmann-Kolbe Sabine 
Published: June 2022

Abstract
Aim/Introduction: PSMA (prostate-specific membrane antigen) is a membrane-bound N-carboxypeptidase that is highly expressed on prostate cancer cells and has been used extensively as a homing target for the treatment of prostate cancer. Peptidomimetic inhibitors of the enzymatic functionality of PSMA have been shown to be able to deliver conjugated cargo into prostate cancer cells. Most prominently PSMA-617 carrying a DOTA chelator has been labelled with therapeutic radionuclides like beta-emitting lutetium-177 or alpha-emitting actinium-225 and has shown clinical activity in late stage prostate cancer patients. We sought to develop a new PSMA-targeting small molecule conjugate that shows high uptake in PSMA-expressing tumors, is quickly cleared from the circulation, and can easily be labeled with the alpha emitter thorium-227 (half-life 18.7d).
Materials and Methods: Using PSMA-617 as a starting point for our venture we were able to identify a novel linker motif that showed improved competition against 3H-PSMA-617 on PSMA-expressing LNCaP cells. Furthermore, we developed a new 2,3-hydroxypyridinone chelator (carboxy-HOPO) modified with carboxyl groups to increase hydrophilicity. The carboxy-HOPO conjugates were labelled with thorium-227 and evaluated in cellular binding, internalization, and antiproliferation assays. Furthermore, the thorium-227 labelled conjugates were tested in vivo for pharmacokinetics and efficacy in mouse xenograft models.
Results: The conjugates were readily labeled with thorium-227 at room temperature and possessed high stability and improved binding, internalization and antiproliferative activity in vitro. In vivo therapeutic studies with our new thorium-227-labelled carboxy-HOPO-PSMA small molecule conjugate in mice with PSMA-expressing tumor LNCaP xenografts showed high uptake and retention in the tumors and very fast clearance from the blood. Besides uptake in the kidneys no major uptake or retention into other organs was observed. Low uptake of free thorium-227 into the bones confirms the high complex stability in vivo with no leakage of thorium-227 from the carboxy-HOPO chelator. Single dosing with 1.5MBq/kg led to good treatment efficacy. No signs of toxicity were observed in the treated mice during the course of the study encouraging further development of the compound.
Conclusion: In summary we developed a novel PSMA-targeting carboxy-HOPO small molecule conjugate that readily complexed thorium-227, showed high uptake into PSMA-expressing cells and gave statistically significant inhibition of tumor growth in tumor xenograft models compared to control groups.
Citation Format: Niels Boehnke, Bård Indrevoll, Stefanie Hammer, Alex Papple, Alexander Kristian, Alan Cuthbertson, Sabine Zitzmann-Kolbe. Development of a novel PSMA-targeting small molecule-Thorium-227 conjugate for the treatment of prostate cancer [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 5440.