Dose-response efficacy of cell adhesion molecule 1 (CADM1)-GGFG-Exatecan vs CADM1-PEG3-VC-Exatecan in preclinical osteosarcoma models

Background: Osteosarcoma is the most common primary bone malignancy in children and young adults. Despite multimodal therapy, survival rates have remained unchanged since the 1980s. Antibody-drug conjugates (ADCs) targeting CADM1 show promise as targeted treatments. This study compares the dose-response effectiveness of two CADM1-targeted ADC formulations, CADM1-GGFG-Exatecan and CADM1-PEG3-VC-Exatecan, in preclinical osteosarcoma models. Both ADCs are conjugated with Exatecan, a potent topoisomerase I inhibitor, but differ in their linker structures (GGFG vs PEG3), which may affect payload release and tumor penetration. Methods: We assessed the cytotoxicity of both ADCs in osteosarcoma cell lines (HOS, OS17, OS31) to determine IC 50 values. Internalization studies compared uptake kinetics in the same lines. In vivo testing evaluated both ADCs in six patient-derived xenograft (PDX) osteosarcoma models (OS1, OS2, OS9, OS17, OS31, OS33) at doses of 3 mg/kg and 6 mg/kg. Human-IgG1-GGFG-Exatecan, Human-IgG1-PEG3-VC-Exatecan, and PBS served as isotype and vehicle controls. Tumor growth inhibition and event-free survival (EFS) were measured. Results: CADM1-GGFG-Exatecan showed significantly greater anti-tumor activity than CADM1-PEG3-VC-Exatecan, with lower IC 50 values and higher internalization in CADM1-expressing cells. Internalization of CADM1-GGFG-Exatecan reached a steady plateau by day 4, whereas CADM1-PEG3-VC-Exatecan showed minimal uptake. In vivo, both ADCs were well tolerated (≤10% body weight loss). CADM1-GGFG-Exatecan significantly prolonged EFS across all six PDX models (P < 0.05), inducing maintained complete responses (MCR) in OS1, OS31, and OS33, a partial response (PR) in OS2, and progressive disease (PD) in OS9 and OS17. In contrast, PEG3-VC-Exatecan and all controls resulted in PD in all models. Conclusion: CADM1-GGFG-Exatecan demonstrates superior dose-response efficacy versus CADM1-PEG3-VC-Exatecan in both in vitro and in vivo osteosarcoma models. Its enhanced anti-tumor activity, likely driven by improved internalization and potency, supports further development in CADM1-expressing osteosarcoma, including optimization of pharmacokinetics and combination strategies.