Abstract 310: Mechanism of sorafenib resistance in acute myeloid leukemia

Activating mutations in FLT3 occur in up to 35% of patients with AML and correlate with poor prognosis. Therapy directed against FLT3 has been shown to induce response in patients with AML, but these responses are almost always transient. Dual PI3K/mTOR inhibitors have displayed promising results in the treatment of solid tumors, and of hematological cancers. In this report we describe that a dual PI3K/mTOR inhibitor is effective against sorafenib-responsive, and -resistant, AML cell lines both in vitro and in vivo. We generated two cell lines by sustained treatment with sorafenib. Parental cell lines carry the FLT3-ITD mutation and are highly responsive to FLT3 inhibitors, while sorafenib-resistant cell lines display resistance to multiple FLT3 inhibitors. Next generation sequencing did not show any significant difference in the mutational burden in between responsive and resistant cell lines. While next generation sequencing identified FLT3-D835Y with an allele-depth of 67:37 in a resistant cell line, Sanger sequencing and protein mass-spectroscopy did not identify any acquired mutations in the kinase domain of FLT3 in the resistant cells. Moreover, sorafenib treatment effectively blocked FLT3 activation in resistant cells, while it was unable to block colony formation or cell survival, suggesting that the resistant cells are no longer dependent on FLT3. Gene expression analysis of sorafenib-sensitive and -resistant cell lines, as well as of blasts from patients with sorafenib-resistant AML, suggested an enrichment of the PI3K/mTOR pathway that correlated with the resistant phenotype, which was further supported by phospho-specific-antibody array analysis. The selective PI3K/mTOR inhibitor, gedatolisib, efficiently blocked proliferation, colony and tumor formation of resistant cell lines as well as induces apoptosis. Taken together, our data suggest that aberrant activation of the PI3K/mTOR pathway results in FLT3-inhibitors-resistance and a dual specific PI3K/mTOR inhibitor is an effective treatment in both tyrosine kinase inhibitor sensitive and resistant AML. Citation Format: Oscar Lindblad, Eugenia Cordero, Alexandre Puissant, Lucy Macaulay, Nuzhat N. Kabir, Jianmin Sun, Karin Haraldsson, Ake Borg, Fredrik Levander, Kimberly Stegmaier, Kristian Pietras, Lars Ronnstrand, Julhash U. Kazi. Mechanism of sorafenib resistance in acute myeloid leukemia. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 310.