IGF2R-Targeted Alpha Therapy of Osteosarcoma Enabled by Conjugated [²²⁵Ac]Ac-Macropa-PEG₄-IF3

The purpose of this investigation was to formulate and evaluate a chemically optimized [²²⁵Ac]Ac-labeled radioimmunoconjugate targeting the insulin-like growth factor-2 receptor (IGF2R) for targeted alpha therapy of osteosarcoma, a disease with unchanging survival rates for over 40 years. The anti-IGF2R antibody IF3 was thiol-selectively conjugated with Macropa-PEG₄-maleimide and subsequently radiolabeled with ²²⁵Ac by incubation at 37 °C for 60 min. Radiochemical purity, serum stability, receptor binding kinetics, and internalization were evaluated in vitro. Biodistribution, dosimetry (MIRD scheme), and therapeutic capacity were studied in SaOS-2 xenograft-bearing mice by a single i.v. injection of PBS, unlabeled IF3, nontarget IgG-²²⁵Ac, and 185 or 370 kBq [²²⁵Ac]Ac-Macropa-PEG₄-IF3. Conjugation resulted in a well-defined antibody conjugate with a chelator-to-antibody ratio of 3.1 ± 0.2 and >97% monomer purity. Radiolabeling was 98.6% radiochemically pure and stable in human serum (>92% at 168 h). The radioimmunoconjugate had a strong IGF2R affinity (K_d = 3.8 nM) and efficient receptor-mediated internalization. Tumor uptake reached 18.7%ID/g after 72 h, with a persistent retention of 15.2%ID/g at 168 h and tumor-to-blood ratio of 19.0. Uptake in bone marrow was low (1.2%ID/g). A single 370 kBq dose caused complete tumor regression in 75% of treated mice (6/8) up to day 42, whereas no responses were noted in vehicle groups (P < 0.001). No significant systemic toxicity was observed. [²²⁵Ac]Ac-Macropa-PEG₄-IF3 is a translational orphan alpha therapeutic for osteosarcoma with excellent radiochemical stability and potency and low off-target toxicity.