Cytotoxic activity of gemtuzumab ozogamicin (Mylotarg) in acute myeloid leukemia correlates with the expression of protein kinase Syk

Acute myeloid leukemia (AML) cells express the cell surface antigen CD33 that, upon ligation with a monoclonal antibody (mAb), is a downregulator of cell growth in a Syk-dependent manner. An anti-CD33 mAb coupled to a toxin, gemtuzumab ozogamicin (GO), is used for the treatment of AML (Mylotarg). Therefore, we investigated whether the response of AML cells to GO treatment also depends on Syk expression. Forty primary AML samples (25 Syk-positive and 15 Syk-negative) were tested for their response to the anti-proliferative effects of GO and unmodified anti-CD33 mAb. A correlation between Syk expression and the response of leukemia cells to GO and anti-CD33 mAb was found. 'Blocking' of Syk by small interfering RNA resulted in unresponsiveness of AML cells to both GO and anti-CD33 mAb-mediated cytotoxicity. Syk upregulation by the de-methylating agent 5-azacytidine (5-aza) induced re-expression of Syk in some cases, resulting in enhanced GO and anti-CD33-mediated inhibition of leukemia cell growth. Thus, the cytotoxicity of both GO and anti-CD33 in primary AML samples was associated with Syk expression. 5-Aza restored Syk and increased the sensitivity of originally Syk-negative, non-responsive cells to CD33 ligation to levels of Syk-positive cells. These data have clinical significance for predicting response to GO and designing clinical trials.

Direct effect of bispecific anti-CD33 x anti-CD64 antibody on proliferation and signaling in myeloid cells

Bispecific anti-CD33 x anti-CD64 antibody (BsAb) directly inhibited proliferation and colony formation of human acute myeloid leukemia cell lines, without affecting the function of normal monocytes. Addition of BsAb to normal monocytes induced tyrosine phosphorylation of Cbl and Vav, association of these molecules with CD33, and downstream signaling. In leukemia cells that were insensitive to BsAb treatment, Vav and Cbl were constitutively phosphorylated and, therefore, constitutively associated with CD33. Direct growth inhibition is an additional mechanism by which BsAb may be useful in the therapy of AML.