Preclinical Development of AT1412, a Patient Derived CD9 Antibody That Does Not Induce Thrombosis for Treatment of B ALL

Despite recent advances in treatment of B-acute lymphoblastic leukemia (B-ALL) there is still a need for novel targeted therapies. The tetraspanin CD9 is expressed in 60-80% of B-ALL and correlates with adverse prognosis. Recently, the mouse CD9 antibody ALB6 was shown to induce leukemia rejection in NOD/SCID mice. However, clinical development of ALB6 and other CD9-targeting antibodies was hampered by their CD9 mediated induction of platelet aggregation. It is known that CD9 is still expressed on tumor cells after treatment with chemotherapy or blinatumomab (Leung, 2019; Linder, 2016).

AT1412 is a fully human antibody isolated from B cells of a patient that was cured from stage IV metastatic melanoma (Verdegaal, 2011). AT1412 targets CD9, without inducing platelet aggregation in vitro or thrombosis in cynomolgus monkeys after intravenous administration at therapeutic dose levels. By crystallography AT1412 was shown to bind a unique epitope preventing homodimerization of CD9, distinctly different from other CD9 antibodies.

AT1412 binds a majority of patient B-ALL samples, but not T-ALL and induces ADCC and ADCP of CD9 positive B-ALL primary cells and the level of cytotoxicity significantly correlated with that of AT1412 binding. In both NSG and immunodeficient mice harboring a human immune system (HIS mice) AT1412 demonstrated a strong, dose-dependent tumor rejection of B-ALL, most pronounced in the extramedullary sites. In HIS mice AT1412 treatment led to an accumulation of T cells and CD14+ myeloid cells at the tumor sites.

To support clinical development, pre-clinical safety of AT1412 was evaluated in cynomolgus monkeys. AT1412 demonstrated a half-life of 8.5 days, supporting 2-3 weekly administration in humans. Besides transient thrombocytopenia no other pathological deviations were observed. No effect on coagulation parameters, bruising or bleeding were observed macro- or microscopically. The thrombocytopenia is reversible, and its recovery accelerated in those animals developing anti-drug antibodies.

Taken together, we demonstrate that CD9 on B-ALL cells can be successfully targeted by AT1412. AT1412 targets a unique epitope and does not induce thrombosis. Pre-clinical safety assessment is supporting that AT1412 can be safely administered. A First in Human clinical study is scheduled to start early 2021 in human solid tumors to determine safety and efficacy. AT1412 efficacy will be evaluated in B-ALL in expansion cohorts.

Leung, K.T., Zhang, C., Chan, K.Y.Y., Li, K., Cheung, J.T.K., Ng, M.H.L., Zhang, X.-B., Sit, T., Lee, W.Y.W., Kang, W., et al. (2019). CD9 blockade suppresses disease progression of high-risk pediatric B-cell precursor acute lymphoblastic leukemia and enhances chemosensitivity. Leukemia.

Linder, K., Gandhiraj, D., Hanmantgad, M., Seiter, K., and Liu, D. (2016). Complete remission after single agent blinatumomab in a patient with pre-B acute lymphoid leukemia relapsed and refractory to three prior regimens: HyperCVAD, high dose cytarabine mitoxantrone and CLAG. Exp. Hematol. Oncol. 5, 5-8.

Verdegaal, E.M.E., Visser, M., Ramwadhdoebé, T.H., van der Minne, C.E., van Steijn, J. a Q.M.J., Kapiteijn, E., Haanen, J.B. a G., van der Burg, S.H., Nortier, J.W.R., and Osanto, S. (2011). Successful treatment of metastatic melanoma by adoptive transfer of blood-derived polyclonal tumor-specific CD4+ and CD8+ T cells in combination with low-dose interferon-alpha. Cancer Immunol. Immunother. 60, 953-963.