To select the most suitable chelate for ²²⁵ Ac radiolabeling of the anti-FZD10 antibody OTSA101, we directly compared three chelates: S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (p-SCN-Bn-DOTA), 2,2′,2″-(10-(1-carboxy-4-((4-isothiocyanatobenzyl)amino)-4-oxobutyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl) triacetic acid (p-SCN-Bn-DOTAGA), and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono-N-hydroxysuccinimide ester (DO3A-NHS-ester). We evaluated the binding affinity of the chelate-conjugated OTSA101 antibodies, as well as the labeling efficiency and stability in murine serum of ²²⁵ Ac-labeled OTSA101 as in vitro properties. The biodistribution, intratumoral distribution, absorbed doses, and therapeutic effects of the chelate-conjugated OTSA101 antibodies were assessed in the synovial sarcoma mouse model SYO-1. Of the three conjugates, DOTAGA conjugation had the smallest impact on the binding affinity (p < 0.01). The labeling efficiencies of DOTAGA-OTSA101 and DO3A-OTSA101 were 1.8-fold higher than that of DOTA-OTSA101 (p < 0.01). The stabilities were similar between ²²⁵ Ac-labeled DOTA-OTSA101, DOTAGA-OTSA101, and DO3A-OTSA101in serum at 37 and 4°C. The dosimetric analysis based on the biodistribution revealed significantly higher tumor-absorbed doses by ²²⁵ Ac-labeled DOTA-OTSA101 and DOTAGA-OTSA101 compared with ²²⁵ Ac-DO3A-OTSA101 (p < 0.05). ²²⁵ Ac-DOTAGA-OTSA101 exhibited the highest tumor-to-bone marrow ratio, with bone marrow being the dose-limiting tissue. The therapeutic and adverse effects were not significantly different between the three conjugates. Our findings indicate that among the three evaluated chelates, DOTAGA appears to be the most promising chelate to produce ²²⁵ Ac-labeled OTSA101 with high binding affinity and high radiochemical yields while providing high absorbed doses to tumors and limited absorbed doses to bone marrow.

Yusuke Nakamura is a stockholder and scientific advisor at OncoTherapy Science. Toyomasa Katagiri is a stockholder and external board member at OncoTherapy Science and is a current Editorial Board Member of Cancer Science. Yosuke Harada is a stockholder and employee at OncoTherapy Science. The other authors have no financial or other competing interests to declare in relation to this study.