Evaluation of Candidate Theranostics for<sup>227</sup>Th/<sup>89</sup>Zr Paired Radioimmunotherapy of Lymphoma
Diane S. Abou, Mark S. Longtine, Amanda Fears, Nadia Benabdallah, Ryan Unnerstall, Hannah E. Johnston, Kyuhwan Shim, A. Hasson, Hanwen Zhang, David Ulmert, Floriane Mangin, Şerife Özen, Laurent Raibaut, Stéphane Brandès, Michel Meyer, Jean‐Claude Chambron, David S. Tatum, Darren Magda, Richard L. Wahl, Daniel L.J. Thorek
Abstract
<sup>227</sup>Th is a promising radioisotope for targeted α-particle therapy. It produces 5 α-particles through its decay, with the clinically approved <sup>223</sup>Ra as its first daughter. There is an ample supply of <sup>227</sup>Th, allowing for clinical use; however, the chemical challenges of chelating this large tetravalent <i>f</i>-block cation are considerable. Using the CD20-targeting antibody ofatumumab, we evaluated chelation of <sup>227</sup>Th<sup>4+</sup> for α-particle–emitting and radiotheranostic applications. <b>Methods:</b> We compared 4 bifunctional chelators for thorium radiopharmaceutical preparation: S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (<i>p</i>-SCN-Bn-DOTA), 2-(4-isothicyanatobenzyl)-1,2,7,10,13-hexaazacyclooctadecane-1,4,7,10,13,16-hexaacetic acid (<i>p</i>-SCN-Bn-HEHA), <i>p</i>-isothiacyanatophenyl-1-hydroxy-2-oxopiperidine-desferrioxamine (DFOcyclo*-<i>p-</i>Phe-NCS), and macrocyclic 1,2-HOPO <i>N</i>-hydroxysuccinimide (L804-NHS). Immunoconstructs were evaluated for yield, purity, and stability in vitro and in vivo. Tumor targeting of the lead <sup>227</sup>Th-labeled compound in vivo was performed in CD20-expressing models and compared with a companion <sup>89</sup>Zr-labeled PET agent. <b>Results:</b><sup>227</sup>Th-labeled ofatumumab-chelator constructs were synthesized to a radiochemical purity of more than 95%, excepting HEHA. <sup>227</sup>Th-HEHA-ofatumumab showed moderate in vitro stability. <sup>227</sup>Th-DFOcyclo*-ofatumumab presented excellent <sup>227</sup>Th labeling efficiency; however, high liver and spleen uptake was revealed in vivo, indicative of aggregation. <sup>227</sup>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%). <sup>227</sup>Th-L804-ofatumumab coordinated <sup>227</sup>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, <sup>89</sup>Zr-L804-ofatumumab, showed organ distribution matching that of <sup>227</sup>Th to delineate SU-DHL-6 tumors. <b>Conclusion:</b> Commercially available and novel chelators for <sup>227</sup>Th showed a range of performances. The L804 chelator can be used with potent radiotheranostic capabilities for <sup>89</sup>Zr/<sup>227</sup>Th quantitative imaging and α-particle therapy.