Preclinical ^203/212 Pb-DOTA–Based Pretargeted Radioimmunotherapy in Nude Mice Bearing Established Human Colorectal Cancer Xenografts

The ²⁰³Pb (half-life [t_1/2], 51.9 h) and ²¹²Pb (t_1/2 = 10.6 h) theranostic pair shows great potential for radiopharmaceutical therapy. We have developed a highly versatile pretargeted radioimmunotherapy (PRIT) platform that utilizes antitumor antigen/anti-DOTA bispecific antibodies (BsAbs) in combination with rapidly clearing, low-molecular-weight DOTA-radiohaptens (DOTA-PRIT). In this study, we tested the hypothesis that high-therapeutic index (TI) targeting of ²¹²Pb (an in vivo generator of α-emitting progeny, specifically ²¹²Bi (t_1/2 = 1.01 h) and ²¹²Po (t_1/2 = 0.3 µs), and its imaging surrogate, ²⁰³Pb (γ-emitter, 279 keV), would be feasible with anti-glycoprotein A33 (GPA33) DOTA-PRIT for treatment of human colorectal cancer (CRC). <b>Methods:</b> For efficient and stable chelation of lead and high-affinity recognition by BsAbs, we synthesized a TCMC-based radiohapten precursor named TCMC-<i>Proteus</i> (TCMC-Pr). We prepared [²¹²Pb]Pb-TCMC-Pr and confirmed its stability in vitro and recognition by BsAbs. Using the ²⁰³Pb analog, we conducted serial biodistribution and SPECT/CT imaging studies with a 3-step GPA33 DOTA-PRIT approach and extrapolated dosimetry for ²¹²Pb. We then established anti-GPA33 ²¹²Pb-DOTA-PRIT in mouse xenografts of human CRC (GPA33-expressing SW1222). <b>Results:</b> TCMC-Pr was successfully radiolabeled with ²⁰³Pb/²¹²Pb and verified stable in serum, and specific BsAb binding was confirmed. For GPA33-pretargeted [²⁰³Pb]Pb-TCMC-Pr, the blood, SW1222 tumor, and kidney uptakes at 24 h after injection were 0.27 ± 0.14 %IA/g, 8.04 ± 2.94 %IA/g, and 0.88 ± 0.14 %IA/g, respectively. ²¹²Pb doses to tumor, blood, and kidneys were 12,723 mGy/MBq, 313 mGy/MBq (TI, 40.6), and 1,075 mGy/MBq (TI, 11.8), respectively. During ²¹²Pb-DOTA-PRIT therapy studies, we established efficacy and identified a double-cycle treatment regimen (938 kBq + 938 kBq separated by 48 h; total, 1.88 MBq) that led to prolonged survival including 3 of 5 histologic cures at the study endpoint of 137 d. Mice in this treatment group exhibited normal bone marrows and moderate tubulointerstitial lesions, with overall preserved renal function. <b>Conclusion:</b> We have developed a safe, curative GPA33-targeted ²¹²Pb-DOTA-PRIT treatment in a preclinical model of human CRC. This research underscores the potential of ^203/212Pb-DOTA-PRIT in managing CRC and provides a road map for its modular application to other human tumor target antigens compatible with DOTA-PRIT.