Novel anti-CD33 immunoconjugate demonstrating in vitro and in vivo activity against human acute myeloid leukemia (AML) cells.
Authors: Nadia C. Hebib, Fannie Larochelle, Bob Lutz, John Lambert, Walter Blattler, Denis Claude Roy
Journal: Cancer Research (2006)

Proc Amer Assoc Cancer Res, Volume 47, 2006

5558 

Acute myeloid leukemia (AML) is in dire need of novel therapeutic agents. Immunoconjugates combine the ability to directly target AML cells and selectively deliver potent cytotoxic agents. In this study, we evaluated the activity of a novel immunoconjugate against human AML cells using both in vitro and in vivo assays. This immunoconjugate (AVE9633) consists of huMY9-6, a humanized monoclonal antibody that binds CD33 with high affinity, covalently linked to the maytansinoid DM4, a potent antimicrotubular cytotoxic agent. Importantly, CD33 is a glycoprotein expressed by AML cells from >80% of patients but is absent from the surface of normal hematopoietic stem cells. When first tested against the human AML cell line HL-60, AVE9633 was found to exert in vitro cytotoxicity in a dose-dependent manner and a 5-day incubation with 10−7 M AVE9633 resulted in more than 3 logs depletion as measured in a limiting dilution assay. The addition of unconjugated anti-CD33 antibody abrogated the cytotoxic effect, but not the isotype control antibody, confirming its specificity for the CD33 antigen. When primary AML cells from 15 patients were exposed to AVE9633, a dose-dependent response was observed in both short-term and long-term (LTC) clonogenic cell assays. The mean percent elimination of AML colony-forming cells reached 70 and 99% at 10−8 M and 10−7 M, respectively. Importantly, no primary AML cells were found to be resistant to AVE9633. In contrast, greater than 10% of normal bone marrow progenitors (both short-and long-term) treated at the highest concentration (10−7 M) of immunoconjugate survived, thus preserving a significant proportion of progenitors that could contribute to hematopoietic reconstitution. We further examined the activity of AVE9633 in 2 NOD-SCID xenograft models: (1) NOD-SCID mice bearing CD33+ HL-60 or CD33− NALM-6 cell line xenografts; (2) NOD-SCID mice implanted with primary human AML cells. Intravenous administration of AVE9633 at weekly intervals for 4 doses to mice bearing HL-60 xenografts resulted in complete eradication of HL-60 cells in 60% of animals, compared to 100% lethality in animals receiving monoclonal antibody alone or PBS. However, AVE9633 had no therapeutic effect in mice injected with the CD33 negative acute lymphoblastic leukemia cell line NALM-6. In addition, NOD-SCID mice inoculated i.v. with 10-20 × 106 AML blasts from 3 different patients and treated weekly with AVE9633 for 4 doses demonstrated a drastic reduction in AML burden at both 2 and 4 months post AML cell administration. Normal bone marrow engraftment was not significantly suppressed by such treatment suggesting that the immunoconjugate spared normal progenitors. Our data therefore demonstrate that AVE9633 has significant in vitro and in vivo activity against human AML cells and supports the development of clinical trials with this new immunoconjugate.