Improving the Therapeutic Selectivity of Trastuzumab Deruxtecan Using 8C2 Fab Fragments

DXd, a camptothecin derivative, is employed as the payload molecule for the antibody-drug conjugate (ADC) trastuzumab deruxtecan (T-DXd), which is in current clinical use for the treatment of breast and gastric cancer. Despite the clinical success of ADCs for treatment of cancer, exposure to released payload in plasma and extracellular fluids contributes to off-target toxicities, which limit ADC dosage and efficacy. In this study, we evaluated an anti-DXd antibody fragment, 8C2 Fab, for utility in mitigating DXd toxicity in cell culture and for improving the therapeutic index of T-DXd in vivo in mice. 8C2 Fab was produced, purified, and characterized for binding to DXd and T-DXd. In vitro competitive cytotoxicity assays showed that 8C2 Fab blocked the cell-killing effect of DXd, increasing the DXd concentration associated with 50% growth inhibition (IC₅₀) by 50-fold; however, 8C2 did not decrease the cytotoxicity of T-DXd. Co-administration of 8C2 Fab to mice with 600 mg/kg T-DXd significantly decreased the percentage body weight loss at nadir in mice compared to results found with T-DXd alone (12.8 ± 3.66% vs 7.08 ± 4.24%, p = 0.0331). In contrast, co-administration of 8C2 Fab with T-DXd (1 or 10 mg/kg) did not negatively impact the anti-tumor efficacy in mice bearing NCI-N87 xenograft tumors. Our results demonstrate that 8C2 Fab decreases DXd cytotoxicity in vitro, and decreases T-DXd-induced toxicity in vivo, while not inhibiting T-DXd anti-tumor efficacy in vitro or in vivo, supporting utility for improving the therapeutic index of T-DXd.