GPA33-pretargeted radioimmunotherapy with mono- and bivalent DOTA-based Lu-177-labeled radiohaptens in a mouse orthotopic liver xenograft model of metastatic human colorectal cancer

Rationale: Pretargeted radioimmunotherapy (PRIT), which combines systemic antibody-based targeting with ionizing radiation, is promising for treating liver metastases in patients with colorectal cancer (CRC).Previously, we established a three-step DOTA-PRIT regimen to deliver DOTA radiometal payloads to CRC using an anti-tumor/anti-DOTA bispecific antibody (BsAb) targeting cell surface glycoprotein A33 (GPA33), a tumor antigen target expressed on over 95% of primary and metastatic CRC; a clearing agent; and a monovalent 177 Lu radiohapten called [ 177 Lu]Lu-ABD.More recently, we developed a bivalent 177 Lu radiohapten called [ 177 Lu]Lu-Gemini to enhance tumor uptake and radiohapten retention.Here, we aimed to compare the efficacy and safety of bivalent vs. monovalent three-step DOTA-PRIT regimens in orthotopic CRC liver metastasis models, to mimic a clinical path forward.Methods: We established two orthotopic CRC liver metastasis models by inoculating either SW1222-luc (GPA33 high ) or LoVo (GPA33 low ) human CRC cells in athymic nude mice under ultrasonographic guidance.Tumor targeting efficacy and dosimetry of the radiohaptens were compared using ex vivo biodistribution studies, SPECT/CT, and quantitative autoradiography.We also performed a DOTA-PRIT experiment to compare the efficacy and safety profiles of bivalent (single-cycle [ 177 Lu]Lu-Gemini, 48 h pretargeting interval) vs. monovalent (multicycle [ 177 Lu]Lu-ABD, 24 h pretargeting interval) three-step DOTA-PRIT regimens, each designed to deliver comparable total radiation doses to tumors (around 50 Gy).Results: Both radiohaptens demonstrated efficient SW1222-luc tumor targeting, with [ 177 Lu]Lu-Gemini showing superior targeting and tumor activity retention compared with [ 177 Lu]Lu-ABD.In LoVo tumors, [ 177 Lu]Lu-Gemini showed superior targeting, while [ 177 Lu]Lu-ABD showed negligible targeting.Dosimetry estimates revealed higher SW1222-luc tumor mean absorbed doses for [ 177 Lu]Lu-Gemini (119.88 cGy/MBq, 48 h pretargeting interval) compared with [ 177 Lu]Lu-ABD (32.88 cGy/MBq, 24 h pretargeting interval), with more favorable blood and kidney therapeutic indices (50 and 9 for [ 177 Lu]Lu-Gemini, and 15 and 5 for [ 177 Lu]Lu-ABD, respectively).In the DOTA-PRIT experiment, both monovalent (injected activity: 3 44.4 MBq, 133.2 MBq total) and bivalent (injected activity: 44.4 MBq total) radiohapten regimens increased the median survival of treated mice compared with controls: 71 days for [ 177 Lu]Lu-ABD-treated mice, 81 days for [ 177 Lu]Lu-Gemini-treated mice, and 18 days for controls, without a statistical difference between treatment groups.Treatments were well tolerated, without significant weight loss or hematologic changes.Radiation-induced injuries were not identified histologically in the kidneys or bone marrow of mice submitted for necropsy. Ivyspring