Targeting AML with Long-Lasting Bispecific Antibodies: Unleashing T Cells for Potent Cytotoxicity

CD123, a marker on acute myeloid leukemia (AML) stem cells and blasts, is a potential target for antibody therapies. However, existing anti-CD123 antibodies like CSL-362 and KHK2823 do not stimulate T cell-mediated destruction of CD123+ AML cells. To address this, a new bispecific antibody was designed to mobilize T cells against CD123+ AML stem and blast cells through a mechanism termed "redirected T cell-cytotoxicity" (RTCC). This approach does not depend on T cell antigen specificity.

The new bispecific antibodies feature a complete Fc domain and form stable heterodimers, which are easy to produce. The Fc domain was modified to prevent binding to Fcγ receptors, reducing unintended T cell activation, while maintaining binding to human FcRn to ensure a long serum half-life.

A series of humanized anti-CD123 × anti-CD3 bispecific antibodies were created and their efficacy was tested using RTCC assays against the CD123+ AML cell lines KG-1a and TF-1. The most potent antibody, XmAb14045, was selected for further study. It has a high affinity for human CD123 and human CD3, and it demonstrated a low EC50 value, indicating strong T cell-mediated killing activity with minimal dosage.

XmAb14045 showed a significant serum half-life in mice and was tested in cynomolgus monkeys for its efficacy. Administered at varying doses, the antibody rapidly activated T cells and led to the depletion of over 99% of circulating CD123+ cells within an hour, especially at the higher doses. The counts of basophils and plasmacytoid dendritic cells (pDC), which serve as proxies for AML stem and blast cells, fell to baseline levels within four hours and remained low for several weeks.

The treatment also resulted in immediate and sustained activation of CD4+ and CD8+ T cells, as indicated by increased levels of activation markers CD25 and CD69. There was a rapid reduction in the number of circulating T cells, which returned to normal within a few days. In the bone marrow, CD123+ cells were depleted by over 95% at all doses, with no recovery observed within eight days post-treatment.

The study indicates that bispecific antibodies can effectively target and eliminate CD123+ cells in both the bloodstream and bone marrow. The changes in basophil and pDC numbers could serve as biomarkers for clinical effectiveness. The findings support the potential of anti-CD123 × anti-CD3 bispecific antibodies for clinical trials in AML patients.