212 Pb-based CEA pretargeted radioimmunotherapy demonstrates tumor targeting and potent TGI in immunodeficient and humanized mouse models, informing a FIH study in mCRC

Background: Alpha particles (alpha) are exceptionally cytotoxic, inducing complex DNA damage and bystander effects. Targeted alpha-therapy demonstrated a favorable therapeutic index (TI) in certain cancers. To improve the TI and expand to other indications, we developed a 212 Pb-based carcinoembryonic antigen-related cell adhesion molecule 5 pretargeted radioimmunotherapy (CEA-PRIT 2.0), involving two complementary SeParated v-domains LInkage Technology antibodies (SPLIT Abs) and 212 Pb-DOTAM. Each CEA-targeted SPLIT Ab carries half of a DOTAM binding v-domain that, when combined upon target binding, forms concentration-dependent stable complexes with 212 Pb-DOTAM. While unbound 212 Pb-DOTAM undergoes rapid renal clearance, 212 Pb-DOTAM captured by the SPLIT Abs leads to alpha emission at CEA-expressing cells with minimal systemic irradiation. Here, we report key preclinical data informing a planned first-in-human (FIH) CEA-PRIT 2.0 study in metastatic colorectal cancer (mCRC) patients. Methods: We assessed CEA-PRIT 2.0-induced tumor growth inhibition (TGI), biodistribution, and tolerability in 3 CEA-expressing human xenograft models. SCID mice bearing BxPC3 (pancreatic) or LS174T (colorectal) tumors received up to 5 treatment cycles, and BRGS-CD47 mice (humanized and non-humanized) bearing HPAF-II (pancreatic) tumors received up to 3 cycles. Each cycle consisted of the two SPLIT Abs (1-5 mg/kg each) given on day 1 to allow for accumulation on CEA-expressing cells before giving 212 Pb-DOTAM (20 µCi) on day 8. Results: Average 212 Pb tumor uptake in all models was 10-43% injected activity per gram of tissue (IA/g) at 24 h, with low blood and kidney retention (<3% IA/g). In the BxPC3 model, TGI was SPLIT Ab dose-dependent, while the LS174T model showed potent TGI already at the lowest dose (1 mg/kg). The SPLIT Ab dose-dependence of the TGI in the HPAF-II model was abrogated in huBRGS-CD47, suggesting secondary immune responses contribute to the therapeutic effect.Manageable body weight (BW) loss was observed in the SCID BxPC3 model. More pronounced BW loss in LS174T (SCID) and HPAF-II (BRGS-CD47/huBRGS-CD47) models was observed, but minimal BW gain or actual BW loss in controls suggests tumor burden toxicity or strain-specific sensitivity were contributing factors.Utilizing the preclinical data, we designed a FIH study, starting with 203 Pb-DOTAM as a surrogate for therapeutic 212 Pb-DOTAM tumor uptake and healthy tissue distribution, SPLIT Ab pharmacokinetics, and tumor CEA expression to inform optimal SPLIT Ab dosing and interval to 212 Pb-DOTAM, before initiating the 212 Pb-DOTAM activity escalation and potential cancer immunotherapy combinations. Conclusions: CEA-PRIT 2.0 showed favorable tumor-to-healthy tissue radiation exposure and potent TGI with a favorable toxicity profile. A FIH study in mCRC is planned in H1 2026.