ATG-106, a novel “2+1”format CDH6-targeted T-cell Engager (TCE), shows potent T cell dependent cytotoxicity and in vivo anti-tumor efficacy

Background CDH6 is crucial in embryonic kidney development but shows negligible expression in the adult kidney. Its overexpression in cancers like ovarian and renal cancer, contrasted with limited normal tissue expression, makes CDH6 an attractive target for cancer therapy, as supported by promising ADC clinical efficacy. However, T-cell engagers (TCEs) show limited efficacy and carry cytokine release syndrome (CRS) risks in solid tumors. In this study, we developed a novel "2+1", stericly-masked CDH6xCD3 bispecific TCE, ATG-106, demonstrating potent anti-tumor activity with a potentially reduced CRS risk. Method ATG-106 was developed by introducing a novel conformational epitope-targeted anti-CD3 single chain fragment variable (scFv) antibody to the hinge region of a novel humanized CDH6 monoclonal antibody. The CD3 binding site is concealed by the anti-CDH6 Fab arm before binding to CDH6, due to the steric hindrance. It was evaluated through a series of preclinical in vitro and in vivo assays for efficacy and safety, including binding affinity, cell based CD3 signal pathway activation, T cell dependent cytotoxicity (TDCC) and cytokine release. The in vivo efficacy of ATG-106 was evaluated in human PBMC reestablished 786-O kidney cancer and OVCAR-3 ovarian cancer xenograft mouse model. The safety profile of ATG-106 was characterized in intermittent intravenous dosing studies in rhesus monkeys using surrogate antibody (ATG-106-RM). Results ATG-106 exhibited reduced binding affinity to CD3+ cells before CDH6 crosslinking, while inducing 100-400 folds more potent cytotoxicity against CDH6-positive tumor cells compared to a 1+1 control TCE. It induced minimal ex-vivo cytokine release in the whole blood assay. In PBMC humanized 786-O xenograft model, ATG-106 demonstrated promising in vivo efficacy, resulted in tumor shrinkage in all mice dosed, with complete remission observed in 0.1 mg/kg (4 out of 6 mice) and 0.3 mg/kg (6 out of 6 mice) treatment groups . Notably, the serum concentration of pro-inflammatory cytokines was very low in ATG-106 treated group, suggesting low risk of CRS . ATG-106 also induced tumor shrinkage and complete remission in OVCAR-3 model. ATG-106-RM was well tolerated in rhesus monkey at doses up to 10mg/kg. Conclusions ATG-106 demonstrated powerful T cell dependent cytotoxicity and in vivo anti-tumor efficacy against ovarian and renal cancer preclinically, which warrants further clinical evaluation.