Combined Targeting of the MET and FGF Receptor Tyrosine Kinases Induces Sustained AML Cell Death by Preventing Compensatory Upregulation of HGF in Response to MET Kinase Inhibition

Abstract 1405

Despite improvements in the treatment of AML, high-risk disease including 8p11 myeloproliferative syndrome/stem cell leukemia remains largely refractory to current therapy, and is mostly fatal. Identification of effective therapeutic targets by using candidate gene approaches has been limited by the number and variety of genetic defects associated with AML. Based on the results of a genome-wide functional screen in AML using a retroviral library of short hairpin RNAs (shRNAs), we discovered that hepatocyte growth factor (HGF) is aberrantly expressed in about 40% of patients with AML, leading to activation of its receptor MET. Autocrine activation of MET is required for survival of AML cells, as genetic and pharmacologic inhibition of HGF or MET induces apoptosis and inhibits cell growth. However, AML cells treated for more than one week with the MET kinase inhibitor crizotinib develop resistance as a result of 13-fold upregulation of HGF expression, leading to restoration of MET pathway activity, and emphasizing autocrine HGF production as the basis for MET signaling addiction. Using simple thermodynamic models, we find that ligand-mediated receptor activation due to increased ligand expression effectively antagonizes the effects of targeted kinase inhibitors, causing complete restoration of aberrant signaling activity. Here we show that this mechanism of resistance can be overcome by combining MET kinase inhibition with depletion of HGF or targeting of pathways required for compensatory HGF upregulation. In the case of KG-1 cells derived from a patient with 8p11 myeloproliferative syndrome AML with chromosomal translocation mediated activation of FGFR1, we find that signaling downstream of the FGFR is required for the upregulation of HGF that occurs in response to MET kinase inhibition. Sustained MET inhibition due to the blockade of compensatory HGF upregulation leads to durable logarithmic cell kill as a result of combined treatment with the MET kinase inhibitor crizotinib and FGFR kinase inhibitor PD173074. In addition, we find that aberrant expression of HGF is highly correlated with expression of FGF2 (Pearson's correlation = 0.79, p < 0.001), with 10% of all patients exhibiting co-expression of HGF and FGF2. This co-expression leads to autocrine co-activation of MET and FGFR1, which can be effectively targeted using combined treatment with MET and FGFR kinase inhibitors. In all, these findings suggest a therapeutic strategy for patients with co-activation of MET and FGFR, including patients with 8p11 myeloproliferative syndrome AML, establish a paradigm of effective therapeutic targeting of aberrant HGF/MET signaling in AML, and emphasize the importance of targeting compensatory ligand-dependent receptor tyrosine kinase activation, as required for the development of rational combination therapy.