The investigational anti-B7-H3 antibody-drug conjugate vobramitamab duocarmazine exerts anti-tumor activity in vitro and in vivo in pediatric sarcoma preclinical models

: Prognosis for pediatric sarcoma (pSC)-affected patients, especially those with relapsed/refractory disease, is dismal. The available treatment options are unsatisfactory, challenging researchers to address this unmet need. The investigational B7-H3 targeted ADC vobramitamab duocarmazine (vobra duo) showed clinical effectiveness towards several B7-H3-positive adult tumors and pre-clinical efficacy in pediatric neuroblastoma models. Cytotoxicity of vobra duo was evaluated in 2D and 3D models toward pSC cell lines expressing B7-H3, showing a dose-dependent cell viability reduction. Proliferation was assessed by time-lapse single-cell segmentation. Compared to controls, vobra duo resulted in a significant increase in the cell doubling time. AKT/mTOR master effectors of cell proliferation were investigated by phospho-specific western blot assays. A down-modulation of phospho-AKT/ -P70 S6K and -4E-BP1 protein expression was detected in both A204 (rhabdomyosarcoma) and U-2-OS (osteosarcoma) cells, the most treatment-sensitive and resistant cell lines, respectively, suggesting their involvement in vobra duo-mediated anti-proliferative effect. In response to treatment, all cell lines underwent apoptotic cell death. A significant increase in the executioner cleaved caspase-3 was detected, and a partial but significant reversion of apoptotic cell death was noted following pre-treatment with the pan-caspase inhibitor, Q-VD-OP-h. Vobra duo also triggered caspase-independent apoptotic events: i) increased AIF nuclear translocation, ii) increased mitochondrial superoxide production, and iii) the depolarization of mitochondrial membrane potential. In vivo, the effectiveness of vobra duo was assayed by single and repeated intravenous administration in the mouse rhabdomyosarcoma model. The single injection of 3 mg/Kg of vobra duo induced a significant tumor growth delay. Repeated vobra duo doses ameliorated this outcome, reverting rhabdomyosarcorma to rhabdomyoma tumor, by increasing Desmin and Myogenin/Myf-4 differentiation markers expression, and reducing both Ki-67 and CD133. In conclusion, the in vitro and in vivo anti-tumor effects towards pSC highlight the need to extend the investigation to patient-derived preclinical models, to pave the way for clinical translation.