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Acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) represent an unmet clinical need whose prognosis is still dismal. Alterations of immune response play a prominent role in AML/MDS pathogenesis, revealing novel options for immunotherapy. Among immune system regulators, CD47, immune checkpoints, and toll-like receptor 2 (TLR2) are major targets. Magrolimab antagonizes CD47, which is overexpressed by AML and MDS cells, thus inducing macrophage phagocytosis with clinical activity in AML/MDS. Sabatolimab, an inhibitor of T-cell immunoglobulin and mucin domain-containing protein 3 (TIM3), which disrupts its binding to galectin-9, has shown promising results in AML/MDS, enhancing the effector functions of lymphocytes and triggering tumor cell death. Several other surface molecules, namely CD33, CD123, CD45, and CD70, can be targeted with monoclonal antibodies (mAbs) that exert different mechanisms of action and include naked and conjugated antibodies, bispecific T-cell engagers, trispecific killer engagers, and fusion proteins linked to toxins.
TLR2 is a member of the Toll-like receptor family and is expressed on the surface of various cells, including hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HSPCs), and plays a fundamental role in pathogen recognition and in the activation of innate immunity [43]. Overexpression of TLR2 leads to upregulation of the IL-8 molecular pathway, which is often dysregulated in MDS patients [44][45]]. Antagonizing TLR2 with a mAb that interacts with its ligand-binding site may prevent heterodimerization of the receptor with TLR1 or TLR6, resulting in TLR2 pathway blockade [46].
Several clinical trials with mAbs (naked and conjugated) are currently ongoing in the frontline, relapsed/refractory (R/R), post allogeneic HSCT, and minimal residual disease (MRD)/maintenance setting, with the aim of improving the outcomes of AML patients. These mAbs may target immune regulatory molecules (CD47 and immune checkpoints) and other membrane antigens (CD 33, CD123, CD45, and CD70) (Table 1).
Recent studies have analyzed anti-CD33 BiTEs and trispecific killer engagers (TriKEs) in MDS. AMV564, a CD33XCD3 BiTE, was evaluated in a preclinical study, which showed its in vitro ability to reduce theMDSCcount and to increase the anti-PD1 antibody activity [67]. GTB-3550 is a CD33/CD16/IL15 TriKE, consisting of a fusion of two scFv, one against CD33 and one against CD16, bridged by an IL15 linker that promotes NK activation, with no significant reported toxicities [33]. A second-generation TriKE, GTB-3650, is under development, although clinical trials have not started yet. Overall, this evidence indicates that CD33 could be a possible target for MDS treatment, but further investigations are needed.
Preliminary results concerning the CD123XCD3 BiTE APVO436 are coming from an ongoing phase Ib trial (NCT03647800) [18]. This study reported an acceptable safety profile for the few enrolled MDS patients, all with R/R MDS after HMA failure. No severe AEs were reported and 50% achieved mCR [18]. Another strategy of CD123 targeting is represented by challenging CD123 with its ligand (IL3) fused to a toxin. In this setting, an ongoing phase Ib study (NCT03113643) has demonstrated the safety of tagraxofusp + AZA, reporting anemia, thrombocytopenia, and neutropenia as the most common grade ≥3 AEs [34].