Taking advantage of the extreme sensitivity for monoclonal antibodies while simultaneously coupling the immunoprotein to a cytotoxic payload is the foundation of antibody-drug conjugates (ADCs). However, developing the antibody portion of ADCs to target cell surface proteins specific to various cancer cells is challenging. Yet now, a group of Japanese researchers at the Tokyo Medical and Dental University (TMDU) have recently developed an ingenious strategy against chronic myelomonocytic leukemia (CMML) wherein the ADC effectively blocked malignant cell proliferation at the source. Findings from the new study were published recently in Frontiers in Immunology through an article titled, “An Antibody-Drug Conjugate That Selectively Targets Human Monocyte Progenitors for Anti-Cancer Therapy.”

Hematopoietic stem and progenitor cells (HSPCs) continually differentiate into the entire panoply of blood cells, as many as 500 billion per day in the average human. CMML results from genetic mutations in HSPCs and is characterized by increased monocytes and immature abnormal cells in the peripheral blood and bone marrow. This disordered hematopoiesis results in intractable anemia, infections, and bleeding disorders. Human stem cell transplantation is the only established cure, but this requires invasive preconditioning and risks Graft versus host disease (GvHD) and intractable infections, especially in the elderly age group affected. Conventional drugs may induce remission and reduce tumor burden but fluctuate between unresponsiveness and fatal marrow suppression.

HSPCs, being multipotent, can replenish all blood cell types and can self-renew. Targeting them would be a solution to cancers and other immune diseases, but this can also disrupt normal cell lines, resulting in red cell deficiency, causing anemias and white cell dysfunction, and increasing infection risks. Therefore, it is desirable to identify and specifically target monopotent progenitors, cells that are “committed” to produce the particular cell line.

A novel therapeutic strategy targeting cMoP for leukemias and tumors. [Department of Biodefense Research,TMDU]
“We had earlier identified monocyte progenitors and pre-monocytes which express the monocyte marker CD64,” explained lead study investigator Yuta Izumi, a doctoral candidate at TMDU. “We developed an ADC combining anti-CD64 antibody with a cytotoxic agent dimeric pyrrolobenzodiazepine (dPBD) that induces apoptosis of multiplying human monocyte-restricted progenitors but not of stable, mature monocytes.”

Co-first author Masashi Kanayama, PhD, assistant professor at TMDU, elaborated: “We found that anti-CD64-dPBD killed proliferating monocytic leukemia cells and blocked their generation from bone marrow progenitors in a patient-derived CMML xenograft experimental mouse model. Moreover, other types of hematopoietic cells, including HSPCs, neutrophils, lymphocytes, and platelets, were unaffected. Additionally, by depleting the source of monocytes, our ADC eliminated tumor-associated macrophages and significantly reduced tumor size in humanized mice bearing solid tumors.”

“Selectively targeting proliferating monocyte progenitors and leukemia cells with our double-barreled ADC strategy causes minimal collateral damage to other cell lineages,” concluded senior study investigator Toshiaki Ohteki, PhD, a professor at TMDU. “It is, therefore, a very promising therapeutic tool against monocytic leukemias, solid tumors, and monocyte-related inflammatory and auto-immune diseases.”