SGN-33 Modulates Cytokine and Chemokine Production by Activated Monocytes and Macrophages.

SGN-33, a humanized IgG1 anti-CD33 antibody that targets CD33, a sialoadhesion family member expressed on myeloid precursor cells, macrophages, and monocytes, is currently in Phase I clinical trials for acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). To further characterize and define the mechanism of action of SGN-33, we undertook a series of experiments in AML cell lines and in cultures of primary normal human monocytes and macrophages. SGN-33 has previously been shown to eliminate tumor cells through antibody-mediated effector functions including Antibody-Dependent Cellular Cytotoxicity (ADCC) and Complement Dependent Cytotoxicity (CDC). We have now demonstrated that SGN-33 binding also initiates Antibody-Dependent Cellular Phagocytosis (ADCP). Because CD33 has been hypothesized to mediate signal transduction, we explored the biochemical effects of SGN-33 binding on AML cell lines and normal monocytes and macrophages. SGN-33 binding stimulated tyrosine phosphorylation of CD33 and recruitment of SHP-1, with maximal effects between 5 to 30 minutes. Initiation of signaling by SGN-33 was not dependent upon antibody crosslinking. Furthermore, SGN-33 exposure significantly reduced the syntheses of pro-inflammatory cytokines (TNF-a, IL-6, IL-1b) and chemokines (RANTES, MCP-1, IL-8) by macrophages cultured in the presence of tumor cell conditioned media. Production of these cytokines by monocytes and macrophages activated in vitro by IFN-g and TGF-b were also blocked by SGN-33 binding (up to 100% reduction in TNF-a levels in vitro). These data demonstrate that the mechanisms of action of anti-CD33 immunotherapy may be broader than previously known. Because pro-inflammatory cytokines have been implicated in the growth and survival of tumor cells in patients, the capacity of SGN-33 to block these factors may yield clinical benefit. This hypothesis is currently being evaluated in clinical studies of SGN-33.