Table 1 Advantages and disadvantages of AML-associated antigens for antibody development (for a cartoon representation of each bispecific antibody format, please see Figure 1)

CD33

Expressed on the majority of myeloid blasts and leukemic stem cells (LSC)^{6, 117}

Not expressed or expressed at lower levels on normal HSCs^{6, 117}

Not expressed outside of the hematopoietic system¹¹⁸

High CD33 expression is associated with high-risk mutations and inversely associated with low-risk mutations^{6, 119}

High CD33 expression correlated with inferior disease features and outcome¹¹⁹

Expressed on immunosuppressive myeloid derived suppressor cells that are associated with extramedullary infiltration and detection of minimal residual disease¹²⁰

Not expressed on all myeloid blasts or myeloid stem cells¹¹⁷

Expressed on HSCs in some studies¹²¹

Expressed on activated natural-killer (NK) cells and T cells^{5, 9}

Modulation (decreased surface expression) of surface CD33 expression upon bivalent anti-CD33 antibody treatment^{10, 122}

Circulating CD33 might interfere with the anti-CD33 antibodies¹²³

Treatment-related myelosuppression (neutropenia and thrombocytopenia)¹²⁴

Relatively low abundance on cell surface¹²⁵

Tandem double and triple scFv (BiTE, ScBsTaFv, bsscFv, TandAb, BiKE, TriKE, sctb), chemical conjugates

CD123

Expressed on the majority of myeloid blasts and LSCs¹²⁶

Not expressed or expressed at lower levels on normal HSCs^{38, 121}

Absent on T cells¹²⁷

Associated with lower complete remission and poorer survival rates³⁹

Associates with higher proliferation and more resistance to apoptosis of AML cells³⁹

Expressed on HSCs^{121, 128}

Not expressed on all myeloid blasts or myeloid stem cells³⁹

A chimeric antigen receptor that was made based on CD123 antibody significantly reduced B cells, platelets and myeloid cells in an animal model¹²⁸

Tandem double and triple scFv (sctb), DART, BIf

WT1

Expressed on the majority of myeloid blasts and LSCs^{44, 129}

Is not expressed or expressed at extremely low levels in a small population of CD34(+)cells in bone marrow¹²⁹

Higher WT1 gene expression is associated with lower complete remission and decreased survival^{45, 130}

HLA-restricted expression of WT1 limits the application of each anti-WT1 antibody to one special HLA haplotype⁴⁶

Not expressed on all myeloid blasts or myeloid stem cells¹²⁹

Low cell surface density of HLA-WT1-peptide complexes⁴⁶

Expressed in some normal tissues¹³¹

Tandem double scFv (BiTE)

CD13

Expressed on the majority of myeloid blasts and LSCs¹³²

Expressed at higher levels on AML stem cells than on normal HSCs

Anti-CD13 monoclonal antibodies can induce apoptosis in AML cells

Expressed in some normal tissues and cells including monocytes, granulocytes, capillary endothelium, nephron convoluted tubules, bile ducts, pancreas, skin, small intestine and liver¹³³

Not expressed on all myeloid blasts¹³²

Expressed on HSCs¹²¹

Chemical conjugation (Fab′ fragments)

CD15

Expressed on the majority of AML cells¹³⁴

Expressed on some NK cells, T cells, monocytes, neutrophils, eosinophils and neurons^{52, 135}

Expressed on more than 50% of activated T cells¹³⁶

Chemical conjugation (Fab conjugated with whole IgM)

CD30

Associated with FLT-3-ITD mutations, leucocytosis and possibly poorer prognosis⁵⁷

Expressed on respiratory epithelial cells, glandular cells of gallbladder, colon, rectum and uterus^{137, 138}

Expressed on activated T cells⁵⁵

Not expressed on all AML cells^{56, 57}

Secreted upon cleavage of the extracellular domain^{58, 59}

T and Ab

CD45

Not internalized upon antibody ligation¹³⁹

Expressed on most AML cells¹⁴⁰

Not expressed outside hematopoietic system

Not expressed on LSCs¹⁴¹

Expressed on all hematopoietic cells except mature red blood cells and platelets⁶¹

CD47

Is a universal target in human cancers

CD47 upregulation on leukemic cells allows them to evade macrophage killing⁶⁴

Overexpressed on AML stem cells than on their normal hematopoietic counterparts⁶⁵

Overexpression of CD47 on AML cells is associated with shortened survival⁶⁵

Expressed on the majority of normal tissues⁶³

CD47 expression on normal tissues may generate an antigen sink preventing the therapeutic antibody to reach its target on AML cells⁶⁶

DVD-Ig

CLL1

Expressed on the majority of myeloid blasts and LSCs¹⁴²

Not expressed on normal tissues¹⁴²

Not expressed on normal hematopoietic stem cells^{67, 142}

Expressed on peripheral blood monocytes, dendritic cells and granulocytes¹⁴²

Relatively low abundance on cell surface¹⁴²

Antigen modulation upon ligation with anti CLL1 antibody¹⁴²

Not expressed on all AML cells¹⁴²

Chemical conjugation (Fab fragments)

FLT-3

Expressed on the majority of AML samples¹⁴³

Expression on LSCs is higher than on normal hematopoietic cells^{70, 73}

Expressed on LSCs¹⁴³

Expressed on hematopoietic stem and progenitor cells and on dendritic cells^{70, 71}

Not expressed on all AML cells¹⁴³

Tandem double scFv (BiTE), Fabsc

VEGF-A, Ang-2

VEGF-A and Ang-2 are overexpressed on the majority of AML bone marrow samples¹⁴⁴

Anti-angiogenic therapy might control disease in patients with relapsed AML¹⁴⁴

Expression of VEGF-A and Ang-2 on AML is associated with negative outcome^{74, 75, 144}

Not expressed on all AML cells¹⁴⁴

Are secreted from the cells so T-cell responses cannot be redirected against AML cells

CrossMab, chemical conjugation