BP1223, a Novel T Cell Engager Targeting CD39 for Potent Antitumor Activity in Acute Myeloid Leukemia

Background: Acute myeloid leukemia (AML) is a challenging hematologic malignancy, particularly characterized by poor prognosis in patients with CD39-positive phenotypes that emerge following chemotherapy. CD39, an ectoenzyme involved in the hydrolysis of extracellular ATP to immunosuppressive adenosine, is known as a marker of AML and may be involved in disease progression. Recent studies suggest CD39 upregulation in AML cells promotes chemoresistance by activating mitochondrial oxidative phosphorylation (OXPHOS), a key energy-generating process in cancer cells. Inhibiting OXPHOS shows promise for AML treatment, particularly when combined with drugs like venetoclax. CD39 inhibition, through its link to OXPHOS regulation, might offer a synergistic therapeutic approach, especially for CD39-positive and chemotherapy-resistant AML. This study aims to evaluate the antitumor efficacy of BP1223, a bispecific antibody targeting CD39 and CD3, designed to redirect T cell cytotoxicity towards CD39-expressing leukemia cells, thereby overcoming immune evasion mechanisms in AML.

Methods: We conducted assays using cancer cell lines expressing CD39 and peripheral blood mononuclear cells (PBMCs) derived from healthy donors to evaluate T cell activation and cytotoxicity. Additionally, we assessed CD39 expression in blasts from AML patient bone marrow cells treated with venetoclax, a standard chemotherapeutic agent. Flow cytometry was used to quantify CD39 expression levels and phenotypic shifts in AML blasts. We investigated the depleting effect of BP1223 on blasts from multiple AML patient samples using both in vitro cytotoxicity assays and ex vivo bone marrow cultures. To assess safety, we investigated the impact of BP1223 on normal bone marrow cells, including hematopoietic stem cells and progenitor cells. We also examined on human umbilical vein endothelial cells (HUVECs), given the known expression of CD39 on endothelial cells.

Results: BP1223 induced robust T cell activation, characterized by increased PD-1 expression, and potent cytotoxicity against CD39-positive cancer cell lines, resulting in significant tumor cell lysis confirmed by flow cytometry. In ex vivo studies using clinical samples, venetoclax treatment enriched the AML blast population for CD39 positive cells by selectively eliminating CD39-negative cells. BP1223 effectively depleted these CD39-positive blasts ex vivo, and this effect was further augmented by the combination with venetoclax and azacitidine, resulting in nearly complete depletion at clinically relevant concentrations. Moreover, BP1223 also exerted bystander effects that eliminated CD39-negative blasts in co-culture systems, indicating the engagement of a broader anti-leukemic immune response. Normal bone marrow cells and HUVECs showed minimal cytotoxicity when exposed to BP1223, suggesting a favorable safety profile.

Conclusions: BP1223 demonstrates potent antitumor activity against both CD39-positive and -negative AML blasts, mediated through effective T cell engagement and activation. The bispecific antibody's ability to induce bystander cytotoxicity enhances its therapeutic potential, addressing both primary and resistant leukemic populations. The minimal impact on normal bone marrow cells and endothelial cells underscores BP1223’s high safety profile. These findings suggest that BP1223 could serve as a promising novel therapeutic option for AML, potentially improving outcomes in patients with CD39-expressing and chemotherapy-resistant disease.