Suppression of leukemia growth in vivo by a novel anti-FLT3 antibody-auristatin conjugate

Proc Amer Assoc Cancer Res, Volume 46, 2005 6181 The class III receptor tyrosine kinase FLT3 is an attractive therapeutic target as it is overexpressed in blasts of ∼90% of acute myelogenous leukemia (AML) and the majority of B-lymphoid leukemia patients. Internal tandem duplications (ITDs) in the juxtamembrane region and point mutations in the kinase domain of FLT3 are found in ∼37% of AML patients and are associated with a poor prognosis. We have recently developed a fully human, high affinity monoclonal antibody (EB10) which is readily internalized upon binding to FLT3 receptor on human leukemia cells. In the present study, a novel auristatin conjugate of the anti-FLT3 antibody (EB10-MMAF) was prepared using a dipeptide linker that allows for drug release inside the lysosomes of antigen-positive cells. The MMAF conjugates were stable in buffers and plasma. EB10-MMAF (drug/antibody ratio = 8) retained the binding affinity and internalizing capability of EB10. The conjugate was highly potent, and selectively inhibited the growth of FLT3-expressing leukemia cells with an IC50 of 0.19 nM and 0.08 nM for MV4;11 and BaF3-ITD cells (both positive for FLT3-ITD), 1.11 nM, 6.18 nM and 1.82 nM for REH, EOL-1, EM3 cells (all three positive for wild-type FLT3), and 135 nM for JM1 (negative for FLT3). An MMAF conjugate with a control antibody was not active in these cell lines (IC50s > 5.9 uM). Flow cytometric analysis with annexin V indicated that EB10-MMAF treatment induced apoptosis of leukemia cells in vitro . In vivo treatment with EB10-MMAF strongly inhibited leukemia growth and prolonged survival of mice in both EOL-1 (mean survival time ∼81.0 +/− 46.8 days for the 3 mg/kg group compared to 41.5+/−15 days for the untreated group) and BaF3-ITD (mean survival time ∼91.5 +/− 39.5 days for the 5 mg/kg group compared to 32.8 +/− 5.2 days for the untreated group) leukemia models. In summary, immunoconjugates composed of a fully human anti-FLT3 antibody and a potent auristatin drug may provide a valuable therapeutic approach for AML and other FLT3-positive leukemias. In summary, immunoconjugates composed of a fully human anti-FLT3 antibody and a potent auristatin drug may provide a valuable therapeutic approach for AML and other types of FLT3-positive leukemia.