Evaluation of Candidate Theranostics for²²⁷Th/⁸⁹Zr Paired Radioimmunotherapy of Lymphoma

International audienceTh is a promising radioisotope for targeted a-particle therapy. It produces 5 a-particles through its decay, with the clinically approved 223 Ra as its first daughter. There is an ample supply of 227 Th, allowing for clinical use; however, the chemical challenges of chelating this large tetravalent f-block cation are considerable. Using the CD20-targeting antibody ofatumumab, we evaluated chelation of 227 Th 41 for a-particle-emitting and radiotheranostic applications. Methods: We compared 4 bifunctional chelators for thorium radiopharmaceutical preparation: S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (p-SCN-Bn-DOTA), 2-(4-isothicyanatobenzyl)-1,2,7,10,13-hexaazacyclooctadecane-1,4,7,10,13,16-hexaacetic acid (p-SCN-Bn-HEHA), p-isothiacyanatophenyl-1-hydroxy-2-oxopiperidine-desferrioxamine (DFOcyclo*-p-Phe-NCS), and macrocyclic 1,2-HOPO N-hydroxysuccinimide (L804-NHS). Immunoconstructs were evaluated for yield, purity, and stability in vitro and in vivo. Tumor targeting of the lead 227 Thlabeled compound in vivo was performed in CD20-expressing models and compared with a companion 89 Zr-labeled PET agent. Results: 227 Th-labeled ofatumumab-chelator constructs were synthesized to a radiochemical purity of more than 95%, excepting HEHA. 227 Th-HEHAofatumumab showed moderate in vitro stability. 227 Th-DFOcyclo*-ofatumumab presented excellent 227 Th labeling efficiency; however, high liver and spleen uptake was revealed in vivo, indicative of aggregation. 227 Th-DOTA-ofatumumab labeled poorly, yielding no more than 5%, with low specific activity (0.08 GBq/g) and modest long-term in vitro stability (,80%). 227 Th-L804-ofatumumab coordinated 227 Th rapidly and efficiently at high yields, purity, and specific activity (8 GBq/g) and demonstrated extended stability. In vivo tumor targeting confirmed the utility of this chelator, and the diagnostic analog, 89 Zr-L804-ofatumumab, showed organ distribution matching that of 227 Th to delineate SU-DHL-6 tumors. Conclusion: Commercially available and novel chelators for 227 Th showed a range of performances. The L804 chelator can be used with potent radiotheranostic capabilities for 89 Zr/ 227 Th quantitative imaging and a-particle therapy