Synergistic Anti-Leukemic Effect of Combination Therapy with Anti-FLT3 Antibody IMC-EB10 and Methotrexate

FLT3 is a class III receptor tyrosine kinase overexpressed by blast cells in approximately 90% of acute myeloid leukemia and acute lymphoid leukemia (ALL) patients. Recent findings testing a small molecule FLT3 inhibitor (PKC412) in combination with eight conventional antileukemic agents, reported that methotrexate in particular did not combine well with PKC412 against leukemia cells (Furukawa, Y. et al., Leukemia 2007, 21: 1005–1014). IMC-EB10 is a fully human neutralizing antibody that binds FLT3 with high affinity (Kd 158 pM) and blocks FLT3 ligand binding to FLT3 (IC50 ~10 nM), leading to inhibition of MAPK, STAT5, and PI3K/Akt signaling in leukemia cells. Here we test for benefits of adding methotrexate to IMC-EB10. In an SEMK2 model of ALL we evaluated survival effects of treatments administered intraperitoneally, beginning 1 day after after intravenous injection of 5 × 10e5 cells/mouse. In this model the maximum tolerated dose (MTD) of methotrexate administered once per week for 3 weeks, was determined to be 100 mg/kg. At this MTD, Methotrexate increased median survival to 44 days, compared to 30 days in the control group (p<0.0001 versus control by Log Rank Test). IMC-EB10 at its maximally efficacious dose (10 mg/kg, ip, 2×/week) increased survival in the SEMK2 model to 60 days (p<0.0001 versus control). When these two regimens were combined, survival significantly increased to 83 days (p<0.0001 versus all other treatment groups). Furthermore, 4 of 12 combination treated mice survived to 168 days after SEMK2 cell injection, but none of the mice in the other groups survived to this point. In this regard these agents were able to achieve an effect in combination that they could not achieve as monotherapies. These data support the significant therapeutic potential of this combination strategy in the treatment of leukemia.