Theranostic 89 Zr/ 225 Ac anti-CEACAM5 radioimmunotherapy for metastatic castration-resistant prostate cancer

Background and Purpose: Radiotheranostics targeting prostate-specific membrane antigen (PSMA) and therapies directed at the androgen receptor (AR) pathway have significantly improved outcomes in metastatic castration-resistant prostate cancer (mCRPC). However, there remains a significant unmet need for safe and potent therapies for AR/PSMA-negative or low-expressing disease. Carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM5) has been identified as a promising cell-surface therapeutic target in AR/PSMA-low mCRPC tumors. This study aims to develop a novel CEACAM5-targeted radiotheranostic pair for mCRPC using the high-affinity anti-CEACAM5 IgG1 monoclonal antibody tusamitamab, formulated as [ 89 Zr]Zr-DFO-tusamitamab for PET imaging and [ 225 Ac]Ac-macropa-tusamitamab for alpha-emitter therapy. Methods: Tusamitamab was conjugated with either desferrioxamine B (DFO) or macropa chelators and subsequently radiolabeled with either zirconium-89 (Zr-89) or actinium-225 (Ac-225), respectively. Proof-of-concept studies were performed in nude mice bearing either cell-line derived xenografts (NCI-H660) or neuroendocrine prostate cancer patient organoid derived xenografts (PDOX). Radioimmunoconjugates were evaluated for stability, immunoreactivity, and in vitro binding. Tumor targeting was assessed using PET/CT imaging and/or ex vivo biodistribution studies. Therapeutic efficacy was evaluated in NCI-H660 tumor-bearing mice across a range of administered activities (single intravenous injection of 1.85-18.5 kBq) compared with untreated controls. Results: Tusamitamab-chelator conjugates (chelator-to-antibody ratios of 1-4) retained low-nM affinity comparable to parental tusamitamab, as measured by surface plasmon resonance. [ 89 Zr]Zr-DFO-tusamitamab and [ 225 Ac]Ac-macropa-tusamitamab were radiolabeled in high yields (30 MBq/nmol and 2.8 MBq/nmol, respectively) and remained stable in PBS and human serum for up to 240 hours. Both radioimmunoconjugates demonstrated ~70% immunoreactivity in a magnetic bead radioligand assay. Biodistribution studies showed high CEACAM5-specific tumor uptake at 120 hours (29.2 ± 3.2 %IA/g for [ 89 Zr]Zr-DFO-tusamitamab and 35.9 ± 7.8 %IA/g for [ 225 Ac]Ac-macropa-tusamitamab). Efficient tumor targeting was also observed in PDOX xenografts (14.1 ± 6.0 %IA/g at 144 hours). Therapy studies demonstrated complete responses by week 3 in all [ 225 Ac]Ac-macropa-tusamitamab-treated mice, with treatment well tolerated through week 4 (studies still ongoing). Comprehensive toxicity and dosimetry assessment studies are ongoing. Conclusion: Proof-of-concept evaluation of [ 89 Zr]Zr-DFO-tusamitamab and [ 225 Ac]Ac-macropa-tusamitamab in mCRPC mouse models has been completed. Initial therapeutic results with [ 225 Ac]Ac-macropa-tusamitamab indicate strong antitumor activity and a favorable therapeutic window.