A novel bispecific innate cell engager called AFM28 has demonstrated remarkable efficacy against leukemic stem and progenitor cells in acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS), according to new research published in Nature Communications. The therapeutic agent represents a significant advancement in targeting the most treatment-resistant cancer cell populations that drive disease relapse.
Targeting Therapy-Resistant Cancer Cells
AFM28 is designed as a tetravalent bispecific humanized IgG1-scFv fusion antibody that simultaneously binds to CD123 (the interleukin-3 receptor alpha chain) on leukemic cells and CD16A on natural killer (NK) cells. This dual targeting mechanism enables antibody-dependent cellular cytotoxicity (ADCC) against CD123+ malignant cells while leveraging the body's innate immune system.
The research team demonstrated that AFM28 showed high ADCC potency and efficacy irrespective of CD123 expression levels and mutational profiles, including mutations conferring poor prognosis such as TP53. Importantly, AFM28 enhanced NK cell cytotoxicity with higher efficacy and potency compared to an Fc-enhanced anti-CD123 IgG1 control antibody, particularly against cell lines with lower CD123 expression levels.
Superior Performance Against Primary Patient Samples
In studies using primary bone marrow samples from AML (n=10) and high-risk MDS (n=5) patients, AFM28 induced significant, concentration-dependent reduction of CD123+ blasts. The treatment resulted in 79% lysis (range 50-100%) of the CD123+ blast population in AML and 70% lysis (range 37-86%) in high-risk MDS samples.
Most notably, AFM28 achieved 98% depletion (range 95-99%) of leukemic stem and progenitor cells (LSPCs) in both AML and MDS patient samples. These LSPCs, phenotypically defined as CD34+ CD38− CD117+ cells, are considered the key drivers of disease relapse due to their therapy-resistant properties.
Preserving Healthy Blood Cell Production
A critical advantage of AFM28 is its selectivity for malignant cells over healthy hematopoietic progenitors. In colony-forming assays, AFM28 reduced colony numbers by 62% and 58% for AML and high-risk MDS samples respectively, while only causing a 25% reduction in healthy samples. This differential effect suggests that AFM28 may have less myelosuppressive activity against healthy hematopoietic progenitors compared to other CD123-targeting approaches.
Overcoming Treatment Resistance Mechanisms
The research revealed that AFM28 retained ADCC efficacy against CD123+ AML cell lines co-expressing CD64 and CD32, receptors that frequently impair the efficacy of therapeutic IgG1 antibodies. In contrast, the Fc-enhanced anti-CD123 IgG1 control antibody failed to mediate ADCC against CD64-high AML cell lines, highlighting AFM28's ability to overcome this resistance mechanism.
AFM28 also demonstrated the ability to block IL-3 receptor signaling, specifically inhibiting IL-3-dependent cell growth and abrogating STAT5 phosphorylation in IL-3-stimulated cells. This provides an additional mechanism of action independent of immune effector cell-mediated killing.
Promising Preclinical Safety and Efficacy
In xenograft mouse models, AFM28 administration reduced tumor growth in a dose-dependent manner and significantly prolonged survival compared to vehicle controls. The median increased life span was 66% for the 3 mg/kg dose, 50% for the 1 mg/kg dose, and 34% for the 0.3 mg/kg dose.
Safety studies in cynomolgus monkeys showed that repeated doses of up to 100 mg/kg/week were well tolerated without evidence of cytokine release syndrome risk factors. The treatment demonstrated expected pharmacodynamic activity through depletion of circulating CD123+ cells, including basophils and plasmacytoid dendritic cells.
Clinical Translation Underway
The promising preclinical data has led to the initiation of a phase 1 dose escalation study currently recruiting patients with relapsed or refractory AML (NCT05817058). This clinical trial will assess the safety and tolerability of AFM28 monotherapy in patients who have exhausted standard treatment options.
The research suggests that AFM28 may be particularly effective when combined with allogeneic NK cell therapy, potentially converting non-responders to responders. In autologous ex vivo studies using fresh AML patient blood samples, AFM28 demonstrated effective blast reduction in 50% of patients without additional NK cells, with this proportion increasing to 100% when combined with allogeneic NK cells.
Implications for Future Treatment
The development of AFM28 represents a significant advance in targeting leukemic stem cells, which are considered the root cause of treatment resistance and disease relapse in AML and MDS. By eliminating these critical cell populations while preserving healthy hematopoiesis, AFM28 may enable more durable remissions and improved long-term outcomes for patients with these challenging malignancies.
The bispecific innate cell engager approach also offers potential advantages over T cell-based therapies, including a lower risk of cytokine release syndrome and the ability to function independently of antigen presentation pathways that are frequently compromised in cancer cells.
